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9. Classes ¶

Classes provide a means of bundling data and functionality together. Creating a new class creates a new type of object, allowing new instances of that type to be made. Each class instance can have attributes attached to it for maintaining its state. Class instances can also have methods (defined by its class) for modifying its state.

Compared with other programming languages, Python’s class mechanism adds classes with a minimum of new syntax and semantics. It is a mixture of the class mechanisms found in C++ and Modula-3. Python classes provide all the standard features of Object Oriented Programming: the class inheritance mechanism allows multiple base classes, a derived class can override any methods of its base class or classes, and a method can call the method of a base class with the same name. Objects can contain arbitrary amounts and kinds of data. As is true for modules, classes partake of the dynamic nature of Python: they are created at runtime, and can be modified further after creation.

In C++ terminology, normally class members (including the data members) are public (except see below Private Variables ), and all member functions are virtual . As in Modula-3, there are no shorthands for referencing the object’s members from its methods: the method function is declared with an explicit first argument representing the object, which is provided implicitly by the call. As in Smalltalk, classes themselves are objects. This provides semantics for importing and renaming. Unlike C++ and Modula-3, built-in types can be used as base classes for extension by the user. Also, like in C++, most built-in operators with special syntax (arithmetic operators, subscripting etc.) can be redefined for class instances.

(Lacking universally accepted terminology to talk about classes, I will make occasional use of Smalltalk and C++ terms. I would use Modula-3 terms, since its object-oriented semantics are closer to those of Python than C++, but I expect that few readers have heard of it.)

9.1. A Word About Names and Objects ¶

Objects have individuality, and multiple names (in multiple scopes) can be bound to the same object. This is known as aliasing in other languages. This is usually not appreciated on a first glance at Python, and can be safely ignored when dealing with immutable basic types (numbers, strings, tuples). However, aliasing has a possibly surprising effect on the semantics of Python code involving mutable objects such as lists, dictionaries, and most other types. This is usually used to the benefit of the program, since aliases behave like pointers in some respects. For example, passing an object is cheap since only a pointer is passed by the implementation; and if a function modifies an object passed as an argument, the caller will see the change — this eliminates the need for two different argument passing mechanisms as in Pascal.

9.2. Python Scopes and Namespaces ¶

Before introducing classes, I first have to tell you something about Python’s scope rules. Class definitions play some neat tricks with namespaces, and you need to know how scopes and namespaces work to fully understand what’s going on. Incidentally, knowledge about this subject is useful for any advanced Python programmer.

Let’s begin with some definitions.

A namespace is a mapping from names to objects. Most namespaces are currently implemented as Python dictionaries, but that’s normally not noticeable in any way (except for performance), and it may change in the future. Examples of namespaces are: the set of built-in names (containing functions such as abs() , and built-in exception names); the global names in a module; and the local names in a function invocation. In a sense the set of attributes of an object also form a namespace. The important thing to know about namespaces is that there is absolutely no relation between names in different namespaces; for instance, two different modules may both define a function maximize without confusion — users of the modules must prefix it with the module name.

By the way, I use the word attribute for any name following a dot — for example, in the expression z.real , real is an attribute of the object z . Strictly speaking, references to names in modules are attribute references: in the expression modname.funcname , modname is a module object and funcname is an attribute of it. In this case there happens to be a straightforward mapping between the module’s attributes and the global names defined in the module: they share the same namespace! [ 1 ]

Attributes may be read-only or writable. In the latter case, assignment to attributes is possible. Module attributes are writable: you can write modname.the_answer = 42 . Writable attributes may also be deleted with the del statement. For example, del modname.the_answer will remove the attribute the_answer from the object named by modname .

Namespaces are created at different moments and have different lifetimes. The namespace containing the built-in names is created when the Python interpreter starts up, and is never deleted. The global namespace for a module is created when the module definition is read in; normally, module namespaces also last until the interpreter quits. The statements executed by the top-level invocation of the interpreter, either read from a script file or interactively, are considered part of a module called __main__ , so they have their own global namespace. (The built-in names actually also live in a module; this is called builtins .)

The local namespace for a function is created when the function is called, and deleted when the function returns or raises an exception that is not handled within the function. (Actually, forgetting would be a better way to describe what actually happens.) Of course, recursive invocations each have their own local namespace.

A scope is a textual region of a Python program where a namespace is directly accessible. “Directly accessible” here means that an unqualified reference to a name attempts to find the name in the namespace.

Although scopes are determined statically, they are used dynamically. At any time during execution, there are 3 or 4 nested scopes whose namespaces are directly accessible:

the innermost scope, which is searched first, contains the local names

the scopes of any enclosing functions, which are searched starting with the nearest enclosing scope, contain non-local, but also non-global names

the next-to-last scope contains the current module’s global names

the outermost scope (searched last) is the namespace containing built-in names

If a name is declared global, then all references and assignments go directly to the next-to-last scope containing the module’s global names. To rebind variables found outside of the innermost scope, the nonlocal statement can be used; if not declared nonlocal, those variables are read-only (an attempt to write to such a variable will simply create a new local variable in the innermost scope, leaving the identically named outer variable unchanged).

Usually, the local scope references the local names of the (textually) current function. Outside functions, the local scope references the same namespace as the global scope: the module’s namespace. Class definitions place yet another namespace in the local scope.

It is important to realize that scopes are determined textually: the global scope of a function defined in a module is that module’s namespace, no matter from where or by what alias the function is called. On the other hand, the actual search for names is done dynamically, at run time — however, the language definition is evolving towards static name resolution, at “compile” time, so don’t rely on dynamic name resolution! (In fact, local variables are already determined statically.)

A special quirk of Python is that – if no global or nonlocal statement is in effect – assignments to names always go into the innermost scope. Assignments do not copy data — they just bind names to objects. The same is true for deletions: the statement del x removes the binding of x from the namespace referenced by the local scope. In fact, all operations that introduce new names use the local scope: in particular, import statements and function definitions bind the module or function name in the local scope.

The global statement can be used to indicate that particular variables live in the global scope and should be rebound there; the nonlocal statement indicates that particular variables live in an enclosing scope and should be rebound there.

9.2.1. Scopes and Namespaces Example ¶

This is an example demonstrating how to reference the different scopes and namespaces, and how global and nonlocal affect variable binding:

The output of the example code is:

Note how the local assignment (which is default) didn’t change scope_test 's binding of spam . The nonlocal assignment changed scope_test 's binding of spam , and the global assignment changed the module-level binding.

You can also see that there was no previous binding for spam before the global assignment.

9.3. A First Look at Classes ¶

Classes introduce a little bit of new syntax, three new object types, and some new semantics.

9.3.1. Class Definition Syntax ¶

The simplest form of class definition looks like this:

Class definitions, like function definitions ( def statements) must be executed before they have any effect. (You could conceivably place a class definition in a branch of an if statement, or inside a function.)

In practice, the statements inside a class definition will usually be function definitions, but other statements are allowed, and sometimes useful — we’ll come back to this later. The function definitions inside a class normally have a peculiar form of argument list, dictated by the calling conventions for methods — again, this is explained later.

When a class definition is entered, a new namespace is created, and used as the local scope — thus, all assignments to local variables go into this new namespace. In particular, function definitions bind the name of the new function here.

When a class definition is left normally (via the end), a class object is created. This is basically a wrapper around the contents of the namespace created by the class definition; we’ll learn more about class objects in the next section. The original local scope (the one in effect just before the class definition was entered) is reinstated, and the class object is bound here to the class name given in the class definition header ( ClassName in the example).

9.3.2. Class Objects ¶

Class objects support two kinds of operations: attribute references and instantiation.

Attribute references use the standard syntax used for all attribute references in Python: obj.name . Valid attribute names are all the names that were in the class’s namespace when the class object was created. So, if the class definition looked like this:

then MyClass.i and MyClass.f are valid attribute references, returning an integer and a function object, respectively. Class attributes can also be assigned to, so you can change the value of MyClass.i by assignment. __doc__ is also a valid attribute, returning the docstring belonging to the class: "A simple example class" .

Class instantiation uses function notation. Just pretend that the class object is a parameterless function that returns a new instance of the class. For example (assuming the above class):

creates a new instance of the class and assigns this object to the local variable x .

The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named __init__() , like this:

When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly created class instance. So in this example, a new, initialized instance can be obtained by:

Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__() . For example,

9.3.3. Instance Objects ¶

Now what can we do with instance objects? The only operations understood by instance objects are attribute references. There are two kinds of valid attribute names: data attributes and methods.

data attributes correspond to “instance variables” in Smalltalk, and to “data members” in C++. Data attributes need not be declared; like local variables, they spring into existence when they are first assigned to. For example, if x is the instance of MyClass created above, the following piece of code will print the value 16 , without leaving a trace:

The other kind of instance attribute reference is a method . A method is a function that “belongs to” an object. (In Python, the term method is not unique to class instances: other object types can have methods as well. For example, list objects have methods called append, insert, remove, sort, and so on. However, in the following discussion, we’ll use the term method exclusively to mean methods of class instance objects, unless explicitly stated otherwise.)

Valid method names of an instance object depend on its class. By definition, all attributes of a class that are function objects define corresponding methods of its instances. So in our example, x.f is a valid method reference, since MyClass.f is a function, but x.i is not, since MyClass.i is not. But x.f is not the same thing as MyClass.f — it is a method object , not a function object.

9.3.4. Method Objects ¶

Usually, a method is called right after it is bound:

In the MyClass example, this will return the string 'hello world' . However, it is not necessary to call a method right away: x.f is a method object, and can be stored away and called at a later time. For example:

will continue to print hello world until the end of time.

What exactly happens when a method is called? You may have noticed that x.f() was called without an argument above, even though the function definition for f() specified an argument. What happened to the argument? Surely Python raises an exception when a function that requires an argument is called without any — even if the argument isn’t actually used…

Actually, you may have guessed the answer: the special thing about methods is that the instance object is passed as the first argument of the function. In our example, the call x.f() is exactly equivalent to MyClass.f(x) . In general, calling a method with a list of n arguments is equivalent to calling the corresponding function with an argument list that is created by inserting the method’s instance object before the first argument.

In general, methods work as follows. When a non-data attribute of an instance is referenced, the instance’s class is searched. If the name denotes a valid class attribute that is a function object, references to both the instance object and the function object are packed into a method object. When the method object is called with an argument list, a new argument list is constructed from the instance object and the argument list, and the function object is called with this new argument list.

9.3.5. Class and Instance Variables ¶

Generally speaking, instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class:

As discussed in A Word About Names and Objects , shared data can have possibly surprising effects with involving mutable objects such as lists and dictionaries. For example, the tricks list in the following code should not be used as a class variable because just a single list would be shared by all Dog instances:

Correct design of the class should use an instance variable instead:

9.4. Random Remarks ¶

If the same attribute name occurs in both an instance and in a class, then attribute lookup prioritizes the instance:

Data attributes may be referenced by methods as well as by ordinary users (“clients”) of an object. In other words, classes are not usable to implement pure abstract data types. In fact, nothing in Python makes it possible to enforce data hiding — it is all based upon convention. (On the other hand, the Python implementation, written in C, can completely hide implementation details and control access to an object if necessary; this can be used by extensions to Python written in C.)

Clients should use data attributes with care — clients may mess up invariants maintained by the methods by stamping on their data attributes. Note that clients may add data attributes of their own to an instance object without affecting the validity of the methods, as long as name conflicts are avoided — again, a naming convention can save a lot of headaches here.

There is no shorthand for referencing data attributes (or other methods!) from within methods. I find that this actually increases the readability of methods: there is no chance of confusing local variables and instance variables when glancing through a method.

Often, the first argument of a method is called self . This is nothing more than a convention: the name self has absolutely no special meaning to Python. Note, however, that by not following the convention your code may be less readable to other Python programmers, and it is also conceivable that a class browser program might be written that relies upon such a convention.

Any function object that is a class attribute defines a method for instances of that class. It is not necessary that the function definition is textually enclosed in the class definition: assigning a function object to a local variable in the class is also ok. For example:

Now f , g and h are all attributes of class C that refer to function objects, and consequently they are all methods of instances of C — h being exactly equivalent to g . Note that this practice usually only serves to confuse the reader of a program.

Methods may call other methods by using method attributes of the self argument:

Methods may reference global names in the same way as ordinary functions. The global scope associated with a method is the module containing its definition. (A class is never used as a global scope.) While one rarely encounters a good reason for using global data in a method, there are many legitimate uses of the global scope: for one thing, functions and modules imported into the global scope can be used by methods, as well as functions and classes defined in it. Usually, the class containing the method is itself defined in this global scope, and in the next section we’ll find some good reasons why a method would want to reference its own class.

Each value is an object, and therefore has a class (also called its type ). It is stored as object.__class__ .

9.5. Inheritance ¶

Of course, a language feature would not be worthy of the name “class” without supporting inheritance. The syntax for a derived class definition looks like this:

The name BaseClassName must be defined in a namespace accessible from the scope containing the derived class definition. In place of a base class name, other arbitrary expressions are also allowed. This can be useful, for example, when the base class is defined in another module:

Execution of a derived class definition proceeds the same as for a base class. When the class object is constructed, the base class is remembered. This is used for resolving attribute references: if a requested attribute is not found in the class, the search proceeds to look in the base class. This rule is applied recursively if the base class itself is derived from some other class.

There’s nothing special about instantiation of derived classes: DerivedClassName() creates a new instance of the class. Method references are resolved as follows: the corresponding class attribute is searched, descending down the chain of base classes if necessary, and the method reference is valid if this yields a function object.

Derived classes may override methods of their base classes. Because methods have no special privileges when calling other methods of the same object, a method of a base class that calls another method defined in the same base class may end up calling a method of a derived class that overrides it. (For C++ programmers: all methods in Python are effectively virtual .)

An overriding method in a derived class may in fact want to extend rather than simply replace the base class method of the same name. There is a simple way to call the base class method directly: just call BaseClassName.methodname(self, arguments) . This is occasionally useful to clients as well. (Note that this only works if the base class is accessible as BaseClassName in the global scope.)

Python has two built-in functions that work with inheritance:

Use isinstance() to check an instance’s type: isinstance(obj, int) will be True only if obj.__class__ is int or some class derived from int .

Use issubclass() to check class inheritance: issubclass(bool, int) is True since bool is a subclass of int . However, issubclass(float, int) is False since float is not a subclass of int .

9.5.1. Multiple Inheritance ¶

Python supports a form of multiple inheritance as well. A class definition with multiple base classes looks like this:

For most purposes, in the simplest cases, you can think of the search for attributes inherited from a parent class as depth-first, left-to-right, not searching twice in the same class where there is an overlap in the hierarchy. Thus, if an attribute is not found in DerivedClassName , it is searched for in Base1 , then (recursively) in the base classes of Base1 , and if it was not found there, it was searched for in Base2 , and so on.

In fact, it is slightly more complex than that; the method resolution order changes dynamically to support cooperative calls to super() . This approach is known in some other multiple-inheritance languages as call-next-method and is more powerful than the super call found in single-inheritance languages.

Dynamic ordering is necessary because all cases of multiple inheritance exhibit one or more diamond relationships (where at least one of the parent classes can be accessed through multiple paths from the bottommost class). For example, all classes inherit from object , so any case of multiple inheritance provides more than one path to reach object . To keep the base classes from being accessed more than once, the dynamic algorithm linearizes the search order in a way that preserves the left-to-right ordering specified in each class, that calls each parent only once, and that is monotonic (meaning that a class can be subclassed without affecting the precedence order of its parents). Taken together, these properties make it possible to design reliable and extensible classes with multiple inheritance. For more detail, see https://www.python.org/download/releases/2.3/mro/ .

9.6. Private Variables ¶

“Private” instance variables that cannot be accessed except from inside an object don’t exist in Python. However, there is a convention that is followed by most Python code: a name prefixed with an underscore (e.g. _spam ) should be treated as a non-public part of the API (whether it is a function, a method or a data member). It should be considered an implementation detail and subject to change without notice.

Since there is a valid use-case for class-private members (namely to avoid name clashes of names with names defined by subclasses), there is limited support for such a mechanism, called name mangling . Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam , where classname is the current class name with leading underscore(s) stripped. This mangling is done without regard to the syntactic position of the identifier, as long as it occurs within the definition of a class.

Name mangling is helpful for letting subclasses override methods without breaking intraclass method calls. For example:

The above example would work even if MappingSubclass were to introduce a __update identifier since it is replaced with _Mapping__update in the Mapping class and _MappingSubclass__update in the MappingSubclass class respectively.

Note that the mangling rules are designed mostly to avoid accidents; it still is possible to access or modify a variable that is considered private. This can even be useful in special circumstances, such as in the debugger.

Notice that code passed to exec() or eval() does not consider the classname of the invoking class to be the current class; this is similar to the effect of the global statement, the effect of which is likewise restricted to code that is byte-compiled together. The same restriction applies to getattr() , setattr() and delattr() , as well as when referencing __dict__ directly.

9.7. Odds and Ends ¶

Sometimes it is useful to have a data type similar to the Pascal “record” or C “struct”, bundling together a few named data items. The idiomatic approach is to use dataclasses for this purpose:

A piece of Python code that expects a particular abstract data type can often be passed a class that emulates the methods of that data type instead. For instance, if you have a function that formats some data from a file object, you can define a class with methods read() and readline() that get the data from a string buffer instead, and pass it as an argument.

Instance method objects have attributes, too: m.__self__ is the instance object with the method m() , and m.__func__ is the function object corresponding to the method.

9.8. Iterators ¶

By now you have probably noticed that most container objects can be looped over using a for statement:

This style of access is clear, concise, and convenient. The use of iterators pervades and unifies Python. Behind the scenes, the for statement calls iter() on the container object. The function returns an iterator object that defines the method __next__() which accesses elements in the container one at a time. When there are no more elements, __next__() raises a StopIteration exception which tells the for loop to terminate. You can call the __next__() method using the next() built-in function; this example shows how it all works:

Having seen the mechanics behind the iterator protocol, it is easy to add iterator behavior to your classes. Define an __iter__() method which returns an object with a __next__() method. If the class defines __next__() , then __iter__() can just return self :

9.9. Generators ¶

Generators are a simple and powerful tool for creating iterators. They are written like regular functions but use the yield statement whenever they want to return data. Each time next() is called on it, the generator resumes where it left off (it remembers all the data values and which statement was last executed). An example shows that generators can be trivially easy to create:

Anything that can be done with generators can also be done with class-based iterators as described in the previous section. What makes generators so compact is that the __iter__() and __next__() methods are created automatically.

Another key feature is that the local variables and execution state are automatically saved between calls. This made the function easier to write and much more clear than an approach using instance variables like self.index and self.data .

In addition to automatic method creation and saving program state, when generators terminate, they automatically raise StopIteration . In combination, these features make it easy to create iterators with no more effort than writing a regular function.

9.10. Generator Expressions ¶

Some simple generators can be coded succinctly as expressions using a syntax similar to list comprehensions but with parentheses instead of square brackets. These expressions are designed for situations where the generator is used right away by an enclosing function. Generator expressions are more compact but less versatile than full generator definitions and tend to be more memory friendly than equivalent list comprehensions.

9.1. A Word About Names and Objects
9.2.1. Scopes and Namespaces Example
9.3.1. Class Definition Syntax
9.3.2. Class Objects
9.3.3. Instance Objects
9.3.4. Method Objects
9.3.5. Class and Instance Variables
9.4. Random Remarks
9.5.1. Multiple Inheritance
9.6. Private Variables
9.7. Odds and Ends
9.8. Iterators
9.9. Generators
9.10. Generator Expressions

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8. Errors and Exceptions

10. Brief Tour of the Standard Library

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21.12 — Overloading the assignment operator

The copy assignment operator (operator=) is used to copy values from one object to another already existing object .

assignment operators

Assignment operators, what is “self assignment”.

Self assignment is when someone assigns an object to itself. For example,

Obviously no one ever explicitly does a self assignment like the above, but since more than one pointer or reference can point to the same object (aliasing), it is possible to have self assignment without knowing it:

This is only valid for copy assignment. Self-assignment is not valid for move assignment.

Why should I worry about “self assignment”?

If you don’t worry about self assignment , you’ll expose your users to some very subtle bugs that have very subtle and often disastrous symptoms. For example, the following class will cause a complete disaster in the case of self-assignment:

If someone assigns a Fred object to itself, line #1 deletes both this->p_ and f.p_ since *this and f are the same object. But line #2 uses *f.p_ , which is no longer a valid object. This will likely cause a major disaster.

The bottom line is that you the author of class Fred are responsible to make sure self-assignment on a Fred object is innocuous . Do not assume that users won’t ever do that to your objects. It is your fault if your object crashes when it gets a self-assignment.

Aside: the above Fred::operator= (const Fred&) has a second problem: If an exception is thrown while evaluating new Wilma(*f.p_) (e.g., an out-of-memory exception or an exception in Wilma ’s copy constructor ), this->p_ will be a dangling pointer — it will point to memory that is no longer valid. This can be solved by allocating the new objects before deleting the old objects.

Okay, okay, already; I’ll handle self-assignment. How do I do it?

You should worry about self assignment every time you create a class . This does not mean that you need to add extra code to all your classes: as long as your objects gracefully handle self assignment, it doesn’t matter whether you had to add extra code or not.

We will illustrate the two cases using the assignment operator in the previous FAQ :

If self-assignment can be handled without any extra code, don’t add any extra code. But do add a comment so others will know that your assignment operator gracefully handles self-assignment:

Example 1a:

Example 1b:

If you need to add extra code to your assignment operator, here’s a simple and effective technique:

Or equivalently:

By the way: the goal is not to make self-assignment fast. If you don’t need to explicitly test for self-assignment, for example, if your code works correctly (even if slowly) in the case of self-assignment, then do not put an if test in your assignment operator just to make the self-assignment case fast. The reason is simple: self-assignment is almost always rare, so it merely needs to be correct - it does not need to be efficient. Adding the unnecessary if statement would make a rare case faster by adding an extra conditional-branch to the normal case, punishing the many to benefit the few.

In this case, however, you should add a comment at the top of your assignment operator indicating that the rest of the code makes self-assignment is benign, and that is why you didn’t explicitly test for it. That way future maintainers will know to make sure self-assignment stays benign, or if not, they will need to add the if test.

I’m creating a derived class; should my assignment operators call my base class’s assignment operators?

Yes (if you need to define assignment operators in the first place).

If you define your own assignment operators, the compiler will not automatically call your base class’s assignment operators for you. Unless your base class’s assignment operators themselves are broken, you should call them explicitly from your derived class’s assignment operators (again, assuming you create them in the first place).

However if you do not create your own assignment operators, the ones that the compiler create for you will automatically call your base class’s assignment operators.

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Python Classes and Objects

In the last tutorial, we learned about Python OOP . We know that Python also supports the concept of objects and classes.

An object is simply a collection of data ( variables ) and methods ( functions ). Similarly, a class is a blueprint for that object.

Before we learn about objects, let's first learn about classes in Python.

Python Classes

A class is considered a blueprint of objects.

We can think of the class as a sketch (prototype) of a house. It contains all the details about the floors, doors, windows, etc.

Based on these descriptions, we build the house; the house is the object.

Since many houses can be made from the same description, we can create many objects from a class.

Define Python Class

We use the class keyword to create a class in Python. For example,

Here, we have created a class named ClassName .

Let's see an example,

Bike - the name of the class
name/gear - variables inside the class with default values "" and 0 respectively.

Note : The variables inside a class are called attributes.

Python Objects

An object is called an instance of a class.

Suppose Bike is a class then we can create objects like bike1 , bike2 , etc from the class.

Here's the syntax to create an object.

Here, bike1 is the object of the class. Now, we can use this object to access the class attributes.

Access Class Attributes Using Objects

We use the . notation to access the attributes of a class. For example,

Here, we have used bike1.name and bike1.gear to change and access the value of name and gear attributes, respectively.

Example 1: Python Class and Objects

In the above example, we have defined the class named Bike with two attributes: name and gear .

We have also created an object bike1 of the class Bike .

Finally, we have accessed and modified the properties of an object using the . notation.

Create Multiple Objects of Python Class

We can also create multiple objects from a single class. For example,

In the above example, we have created two objects employee1 and employee2 of the Employee class.

Python Methods

We can also define a function inside a Python class. A Python function defined inside a class is called a method .

In the above example, we have created a class named Room with:

Attributes : length and breadth
Method : calculate_area()

Here, we have created an object named study_room from the Room class.

We then used the object to assign values to attributes: length and breadth .

Notice that we have also used the object to call the method inside the class,

Here, we have used the . notation to call the method. Finally, the statement inside the method is executed.

Python Constructors

Earlier we assigned a default value to a class attribute,

However, we can also initialize values using the constructors. For example,

Here, __init__() is the constructor function that is called whenever a new object of that class is instantiated.

The constructor above initializes the value of the name attribute.

We have used the self.name to refer to the name attribute of the bike1 object.

If we use a constructor to initialize values inside a class, we need to pass the corresponding value during the object creation of the class.

Here, "Mountain Bike" is passed to the name parameter of __init__() .

Python inheritance
Python classmethod()

Introduction

Video: Python Classes and Objects

Sorry about that.

C++ Assignment Operator Overloading

Prerequisite: Operator Overloading

The assignment operator,”=”, is the operator used for Assignment. It copies the right value into the left value. Assignment Operators are predefined to operate only on built-in Data types.

Assignment operator overloading is binary operator overloading.
Overloading assignment operator in C++ copies all values of one object to another object.
Only a non-static member function should be used to overload the assignment operator.

We can’t directly use the Assignment Operator on objects. The simple explanation for this is that the Assignment Operator is predefined to operate only on built-in Data types. As the class and objects are user-defined data types, so the compiler generates an error.

here, a and b are of type integer, which is a built-in data type. Assignment Operator can be used directly on built-in data types.

c1 and c2 are variables of type “class C”. Here compiler will generate an error as we are trying to use an Assignment Operator on user-defined data types.

The above example can be done by implementing methods or functions inside the class, but we choose operator overloading instead. The reason for this is, operator overloading gives the functionality to use the operator directly which makes code easy to understand, and even code size decreases because of it. Also, operator overloading does not affect the normal working of the operator but provides extra functionality to it.

Now, if the user wants to use the assignment operator “=” to assign the value of the class variable to another class variable then the user has to redefine the meaning of the assignment operator “=”. Redefining the meaning of operators really does not change their original meaning, instead, they have been given additional meaning along with their existing ones.

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The Writing Center • University of North Carolina at Chapel Hill

Understanding Assignments

What this handout is about.

The first step in any successful college writing venture is reading the assignment. While this sounds like a simple task, it can be a tough one. This handout will help you unravel your assignment and begin to craft an effective response. Much of the following advice will involve translating typical assignment terms and practices into meaningful clues to the type of writing your instructor expects. See our short video for more tips.

Basic beginnings

Regardless of the assignment, department, or instructor, adopting these two habits will serve you well :

Read the assignment carefully as soon as you receive it. Do not put this task off—reading the assignment at the beginning will save you time, stress, and problems later. An assignment can look pretty straightforward at first, particularly if the instructor has provided lots of information. That does not mean it will not take time and effort to complete; you may even have to learn a new skill to complete the assignment.
Ask the instructor about anything you do not understand. Do not hesitate to approach your instructor. Instructors would prefer to set you straight before you hand the paper in. That’s also when you will find their feedback most useful.

Assignment formats

Many assignments follow a basic format. Assignments often begin with an overview of the topic, include a central verb or verbs that describe the task, and offer some additional suggestions, questions, or prompts to get you started.

An Overview of Some Kind

The instructor might set the stage with some general discussion of the subject of the assignment, introduce the topic, or remind you of something pertinent that you have discussed in class. For example:

“Throughout history, gerbils have played a key role in politics,” or “In the last few weeks of class, we have focused on the evening wear of the housefly …”

The Task of the Assignment

Pay attention; this part tells you what to do when you write the paper. Look for the key verb or verbs in the sentence. Words like analyze, summarize, or compare direct you to think about your topic in a certain way. Also pay attention to words such as how, what, when, where, and why; these words guide your attention toward specific information. (See the section in this handout titled “Key Terms” for more information.)

“Analyze the effect that gerbils had on the Russian Revolution”, or “Suggest an interpretation of housefly undergarments that differs from Darwin’s.”

Additional Material to Think about

Here you will find some questions to use as springboards as you begin to think about the topic. Instructors usually include these questions as suggestions rather than requirements. Do not feel compelled to answer every question unless the instructor asks you to do so. Pay attention to the order of the questions. Sometimes they suggest the thinking process your instructor imagines you will need to follow to begin thinking about the topic.

“You may wish to consider the differing views held by Communist gerbils vs. Monarchist gerbils, or Can there be such a thing as ‘the housefly garment industry’ or is it just a home-based craft?”

These are the instructor’s comments about writing expectations:

“Be concise”, “Write effectively”, or “Argue furiously.”

Technical Details

These instructions usually indicate format rules or guidelines.

“Your paper must be typed in Palatino font on gray paper and must not exceed 600 pages. It is due on the anniversary of Mao Tse-tung’s death.”

The assignment’s parts may not appear in exactly this order, and each part may be very long or really short. Nonetheless, being aware of this standard pattern can help you understand what your instructor wants you to do.

Interpreting the assignment

Ask yourself a few basic questions as you read and jot down the answers on the assignment sheet:

Why did your instructor ask you to do this particular task?

Who is your audience.

What kind of evidence do you need to support your ideas?

What kind of writing style is acceptable?

What are the absolute rules of the paper?

Try to look at the question from the point of view of the instructor. Recognize that your instructor has a reason for giving you this assignment and for giving it to you at a particular point in the semester. In every assignment, the instructor has a challenge for you. This challenge could be anything from demonstrating an ability to think clearly to demonstrating an ability to use the library. See the assignment not as a vague suggestion of what to do but as an opportunity to show that you can handle the course material as directed. Paper assignments give you more than a topic to discuss—they ask you to do something with the topic. Keep reminding yourself of that. Be careful to avoid the other extreme as well: do not read more into the assignment than what is there.

Of course, your instructor has given you an assignment so that he or she will be able to assess your understanding of the course material and give you an appropriate grade. But there is more to it than that. Your instructor has tried to design a learning experience of some kind. Your instructor wants you to think about something in a particular way for a particular reason. If you read the course description at the beginning of your syllabus, review the assigned readings, and consider the assignment itself, you may begin to see the plan, purpose, or approach to the subject matter that your instructor has created for you. If you still aren’t sure of the assignment’s goals, try asking the instructor. For help with this, see our handout on getting feedback .

Given your instructor’s efforts, it helps to answer the question: What is my purpose in completing this assignment? Is it to gather research from a variety of outside sources and present a coherent picture? Is it to take material I have been learning in class and apply it to a new situation? Is it to prove a point one way or another? Key words from the assignment can help you figure this out. Look for key terms in the form of active verbs that tell you what to do.

Key Terms: Finding Those Active Verbs

Here are some common key words and definitions to help you think about assignment terms:

Information words Ask you to demonstrate what you know about the subject, such as who, what, when, where, how, and why.

define —give the subject’s meaning (according to someone or something). Sometimes you have to give more than one view on the subject’s meaning
describe —provide details about the subject by answering question words (such as who, what, when, where, how, and why); you might also give details related to the five senses (what you see, hear, feel, taste, and smell)
explain —give reasons why or examples of how something happened
illustrate —give descriptive examples of the subject and show how each is connected with the subject
summarize —briefly list the important ideas you learned about the subject
trace —outline how something has changed or developed from an earlier time to its current form
research —gather material from outside sources about the subject, often with the implication or requirement that you will analyze what you have found

Relation words Ask you to demonstrate how things are connected.

compare —show how two or more things are similar (and, sometimes, different)
contrast —show how two or more things are dissimilar
apply—use details that you’ve been given to demonstrate how an idea, theory, or concept works in a particular situation
cause —show how one event or series of events made something else happen
relate —show or describe the connections between things

Interpretation words Ask you to defend ideas of your own about the subject. Do not see these words as requesting opinion alone (unless the assignment specifically says so), but as requiring opinion that is supported by concrete evidence. Remember examples, principles, definitions, or concepts from class or research and use them in your interpretation.

assess —summarize your opinion of the subject and measure it against something
prove, justify —give reasons or examples to demonstrate how or why something is the truth
evaluate, respond —state your opinion of the subject as good, bad, or some combination of the two, with examples and reasons
support —give reasons or evidence for something you believe (be sure to state clearly what it is that you believe)
synthesize —put two or more things together that have not been put together in class or in your readings before; do not just summarize one and then the other and say that they are similar or different—you must provide a reason for putting them together that runs all the way through the paper
analyze —determine how individual parts create or relate to the whole, figure out how something works, what it might mean, or why it is important
argue —take a side and defend it with evidence against the other side

More Clues to Your Purpose As you read the assignment, think about what the teacher does in class:

What kinds of textbooks or coursepack did your instructor choose for the course—ones that provide background information, explain theories or perspectives, or argue a point of view?
In lecture, does your instructor ask your opinion, try to prove her point of view, or use keywords that show up again in the assignment?
What kinds of assignments are typical in this discipline? Social science classes often expect more research. Humanities classes thrive on interpretation and analysis.
How do the assignments, readings, and lectures work together in the course? Instructors spend time designing courses, sometimes even arguing with their peers about the most effective course materials. Figuring out the overall design to the course will help you understand what each assignment is meant to achieve.

Now, what about your reader? Most undergraduates think of their audience as the instructor. True, your instructor is a good person to keep in mind as you write. But for the purposes of a good paper, think of your audience as someone like your roommate: smart enough to understand a clear, logical argument, but not someone who already knows exactly what is going on in your particular paper. Remember, even if the instructor knows everything there is to know about your paper topic, he or she still has to read your paper and assess your understanding. In other words, teach the material to your reader.

Aiming a paper at your audience happens in two ways: you make decisions about the tone and the level of information you want to convey.

Tone means the “voice” of your paper. Should you be chatty, formal, or objective? Usually you will find some happy medium—you do not want to alienate your reader by sounding condescending or superior, but you do not want to, um, like, totally wig on the man, you know? Eschew ostentatious erudition: some students think the way to sound academic is to use big words. Be careful—you can sound ridiculous, especially if you use the wrong big words.
The level of information you use depends on who you think your audience is. If you imagine your audience as your instructor and she already knows everything you have to say, you may find yourself leaving out key information that can cause your argument to be unconvincing and illogical. But you do not have to explain every single word or issue. If you are telling your roommate what happened on your favorite science fiction TV show last night, you do not say, “First a dark-haired white man of average height, wearing a suit and carrying a flashlight, walked into the room. Then a purple alien with fifteen arms and at least three eyes turned around. Then the man smiled slightly. In the background, you could hear a clock ticking. The room was fairly dark and had at least two windows that I saw.” You also do not say, “This guy found some aliens. The end.” Find some balance of useful details that support your main point.

You’ll find a much more detailed discussion of these concepts in our handout on audience .

The Grim Truth

With a few exceptions (including some lab and ethnography reports), you are probably being asked to make an argument. You must convince your audience. It is easy to forget this aim when you are researching and writing; as you become involved in your subject matter, you may become enmeshed in the details and focus on learning or simply telling the information you have found. You need to do more than just repeat what you have read. Your writing should have a point, and you should be able to say it in a sentence. Sometimes instructors call this sentence a “thesis” or a “claim.”

So, if your instructor tells you to write about some aspect of oral hygiene, you do not want to just list: “First, you brush your teeth with a soft brush and some peanut butter. Then, you floss with unwaxed, bologna-flavored string. Finally, gargle with bourbon.” Instead, you could say, “Of all the oral cleaning methods, sandblasting removes the most plaque. Therefore it should be recommended by the American Dental Association.” Or, “From an aesthetic perspective, moldy teeth can be quite charming. However, their joys are short-lived.”

Convincing the reader of your argument is the goal of academic writing. It doesn’t have to say “argument” anywhere in the assignment for you to need one. Look at the assignment and think about what kind of argument you could make about it instead of just seeing it as a checklist of information you have to present. For help with understanding the role of argument in academic writing, see our handout on argument .

What kind of evidence do you need?

There are many kinds of evidence, and what type of evidence will work for your assignment can depend on several factors–the discipline, the parameters of the assignment, and your instructor’s preference. Should you use statistics? Historical examples? Do you need to conduct your own experiment? Can you rely on personal experience? See our handout on evidence for suggestions on how to use evidence appropriately.

Make sure you are clear about this part of the assignment, because your use of evidence will be crucial in writing a successful paper. You are not just learning how to argue; you are learning how to argue with specific types of materials and ideas. Ask your instructor what counts as acceptable evidence. You can also ask a librarian for help. No matter what kind of evidence you use, be sure to cite it correctly—see the UNC Libraries citation tutorial .

You cannot always tell from the assignment just what sort of writing style your instructor expects. The instructor may be really laid back in class but still expect you to sound formal in writing. Or the instructor may be fairly formal in class and ask you to write a reflection paper where you need to use “I” and speak from your own experience.

Try to avoid false associations of a particular field with a style (“art historians like wacky creativity,” or “political scientists are boring and just give facts”) and look instead to the types of readings you have been given in class. No one expects you to write like Plato—just use the readings as a guide for what is standard or preferable to your instructor. When in doubt, ask your instructor about the level of formality she or he expects.

No matter what field you are writing for or what facts you are including, if you do not write so that your reader can understand your main idea, you have wasted your time. So make clarity your main goal. For specific help with style, see our handout on style .

Technical details about the assignment

The technical information you are given in an assignment always seems like the easy part. This section can actually give you lots of little hints about approaching the task. Find out if elements such as page length and citation format (see the UNC Libraries citation tutorial ) are negotiable. Some professors do not have strong preferences as long as you are consistent and fully answer the assignment. Some professors are very specific and will deduct big points for deviations.

Usually, the page length tells you something important: The instructor thinks the size of the paper is appropriate to the assignment’s parameters. In plain English, your instructor is telling you how many pages it should take for you to answer the question as fully as you are expected to. So if an assignment is two pages long, you cannot pad your paper with examples or reword your main idea several times. Hit your one point early, defend it with the clearest example, and finish quickly. If an assignment is ten pages long, you can be more complex in your main points and examples—and if you can only produce five pages for that assignment, you need to see someone for help—as soon as possible.

Tricks that don’t work

Your instructors are not fooled when you:

spend more time on the cover page than the essay —graphics, cool binders, and cute titles are no replacement for a well-written paper.
use huge fonts, wide margins, or extra spacing to pad the page length —these tricks are immediately obvious to the eye. Most instructors use the same word processor you do. They know what’s possible. Such tactics are especially damning when the instructor has a stack of 60 papers to grade and yours is the only one that low-flying airplane pilots could read.
use a paper from another class that covered “sort of similar” material . Again, the instructor has a particular task for you to fulfill in the assignment that usually relates to course material and lectures. Your other paper may not cover this material, and turning in the same paper for more than one course may constitute an Honor Code violation . Ask the instructor—it can’t hurt.
get all wacky and “creative” before you answer the question . Showing that you are able to think beyond the boundaries of a simple assignment can be good, but you must do what the assignment calls for first. Again, check with your instructor. A humorous tone can be refreshing for someone grading a stack of papers, but it will not get you a good grade if you have not fulfilled the task.

Critical reading of assignments leads to skills in other types of reading and writing. If you get good at figuring out what the real goals of assignments are, you are going to be better at understanding the goals of all of your classes and fields of study.

You may reproduce it for non-commercial use if you use the entire handout and attribute the source: The Writing Center, University of North Carolina at Chapel Hill

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cppreference.com

Copy assignment operator.

A copy assignment operator is a non-template non-static member function with the name operator = that can be called with an argument of the same class type and copies the content of the argument without mutating the argument.

[ edit ] Syntax

For the formal copy assignment operator syntax, see function declaration . The syntax list below only demonstrates a subset of all valid copy assignment operator syntaxes.

[ edit ] Explanation

The copy assignment operator is called whenever selected by overload resolution , e.g. when an object appears on the left side of an assignment expression.

[ edit ] Implicitly-declared copy assignment operator

If no user-defined copy assignment operators are provided for a class type, the compiler will always declare one as an inline public member of the class. This implicitly-declared copy assignment operator has the form T & T :: operator = ( const T & ) if all of the following is true:

each direct base B of T has a copy assignment operator whose parameters are B or const B & or const volatile B & ;
each non-static data member M of T of class type or array of class type has a copy assignment operator whose parameters are M or const M & or const volatile M & .

Otherwise the implicitly-declared copy assignment operator is declared as T & T :: operator = ( T & ) .

Due to these rules, the implicitly-declared copy assignment operator cannot bind to a volatile lvalue argument.

A class can have multiple copy assignment operators, e.g. both T & T :: operator = ( T & ) and T & T :: operator = ( T ) . If some user-defined copy assignment operators are present, the user may still force the generation of the implicitly declared copy assignment operator with the keyword default . (since C++11)

The implicitly-declared (or defaulted on its first declaration) copy assignment operator has an exception specification as described in dynamic exception specification (until C++17) noexcept specification (since C++17)

Because the copy assignment operator is always declared for any class, the base class assignment operator is always hidden. If a using-declaration is used to bring in the assignment operator from the base class, and its argument type could be the same as the argument type of the implicit assignment operator of the derived class, the using-declaration is also hidden by the implicit declaration.

[ edit ] Implicitly-defined copy assignment operator

If the implicitly-declared copy assignment operator is neither deleted nor trivial, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used or needed for constant evaluation (since C++14) . For union types, the implicitly-defined copy assignment copies the object representation (as by std::memmove ). For non-union class types, the operator performs member-wise copy assignment of the object's direct bases and non-static data members, in their initialization order, using built-in assignment for the scalars, memberwise copy-assignment for arrays, and copy assignment operator for class types (called non-virtually).

[ edit ] Deleted copy assignment operator

An implicitly-declared or explicitly-defaulted (since C++11) copy assignment operator for class T is undefined (until C++11) defined as deleted (since C++11) if any of the following conditions is satisfied:

T has a non-static data member of a const-qualified non-class type (or possibly multi-dimensional array thereof).
T has a non-static data member of a reference type.
T has a potentially constructed subobject of class type M (or possibly multi-dimensional array thereof) such that the overload resolution as applied to find M 's copy assignment operator
does not result in a usable candidate, or
in the case of the subobject being a variant member , selects a non-trivial function.

[ edit ] Trivial copy assignment operator

The copy assignment operator for class T is trivial if all of the following is true:

it is not user-provided (meaning, it is implicitly-defined or defaulted);
T has no virtual member functions;
T has no virtual base classes;
the copy assignment operator selected for every direct base of T is trivial;
the copy assignment operator selected for every non-static class type (or array of class type) member of T is trivial.

A trivial copy assignment operator makes a copy of the object representation as if by std::memmove . All data types compatible with the C language (POD types) are trivially copy-assignable.

[ edit ] Eligible copy assignment operator

Triviality of eligible copy assignment operators determines whether the class is a trivially copyable type .

[ edit ] Notes

If both copy and move assignment operators are provided, overload resolution selects the move assignment if the argument is an rvalue (either a prvalue such as a nameless temporary or an xvalue such as the result of std::move ), and selects the copy assignment if the argument is an lvalue (named object or a function/operator returning lvalue reference). If only the copy assignment is provided, all argument categories select it (as long as it takes its argument by value or as reference to const, since rvalues can bind to const references), which makes copy assignment the fallback for move assignment, when move is unavailable.

It is unspecified whether virtual base class subobjects that are accessible through more than one path in the inheritance lattice, are assigned more than once by the implicitly-defined copy assignment operator (same applies to move assignment ).

See assignment operator overloading for additional detail on the expected behavior of a user-defined copy-assignment operator.

[ edit ] Example

[ edit ] defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[ edit ] See also

converting constructor
copy constructor
copy elision
default constructor
aggregate initialization
constant initialization
copy initialization
default initialization
direct initialization
initializer list
list initialization
reference initialization
value initialization
zero initialization
move assignment
move constructor
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Creating Class Assignment Data

Instructors use the Class Assignment component to create, view, or edit their class assignments. They can create each assignment, including its assignment category, maximum points, and due date. Instructors can even define an extended due date for an assignment or specify whether a student sees the assignment grade when accessing the student's view of Gradebook. When accessing the Class Assignment component, the system prompts instructors with choices that help them identify the class that they want to edit.

To define class assignments:

Search and identify the class for which you want to create assignments.

Describe the class assignments and define the class assignments on the Assignments page.

Create weight values for each category on the Category Weight page.

Create grading scales for the class on the Grading Scale page.

This section lists prerequisites and discusses how to:

Create, define and cluster assignments by category.

Set weight values.

Define marks and grades for grading schemes.

Prerequisites

Verify that the following prerequisites are met before you create class assignment data:

To define class assignments, instructors must have grade roster access of Approve or Post for the class.

Note: Instructors with Grade access can only enter grades and modify existing assignments.

To select a class within a term, first create assignment categories on the Gradebook Category page.

To set weight values, first define assignment categories.

To define marks and grades for class grading schemes, first define grading schemes and grading bases.

Pages Used to Create Class Assignment Data

Creating, defining, and clustering assignments by category.

Access the Class Assignments page (click the Go to Class Assignments icon on the Gradebook - Select a Class page).

Image: Assignments page (1 of 2)

This example illustrates the fields and controls on the Assignments page (1 of 2). You can find definitions for the fields and controls later on this page.

Image: Assignments page (2 of 2)

This example illustrates the fields and controls on the Assignments page (2 of 2). You can find definitions for the fields and controls later on this page.

Note: You cannot delete an assignment that is required by the institution or that has been graded.

Setting Weight Values

Access the Category Weight page (click the Go to Class Assignments icon on the Gradebook - Select a Class page).

Image: Category Weight page

This example illustrates the fields and controls on the Category Weight page. You can find definitions for the fields and controls later on this page.

Defining Marks and Grades for Grading Schemes

Access the Grading Scale page (click the Go to Class Assignments icon on the Gradebook - Select a Class page).

Image: Grading Scale page

This example illustrates the fields and controls on the Grading Scale page. You can find definitions for the fields and controls later on this page.

Note: If no assignments, category weights, or grading scales exist, the system populates the fields for those items with the values on the Course Assignments - Grading Scale page. Also, the system creates grading scheme and grading scale records to match the grading scheme and grading scale on the Class Association page, in conjunction with any values at the course level. If the grading basis is optional, the system creates grading basis records to match the linked grading bases.

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Classes are a template for creating objects. They encapsulate data with code to work on that data. Classes in JS are built on prototypes but also have some syntax and semantics that are unique to classes.

For more examples and explanations, see the Using classes guide.

Description

Defining classes.

Classes are in fact "special functions ", and just as you can define function expressions and function declarations , a class can be defined in two ways: a class expression or a class declaration .

Like function expressions, class expressions may be anonymous, or have a name that's different from the variable that it's assigned to. However, unlike function declarations, class declarations have the same temporal dead zone restrictions as let or const and behave as if they are not hoisted .

The body of a class is the part that is in curly braces {} . This is where you define class members, such as methods or constructor.

The body of a class is executed in strict mode even without the "use strict" directive.

A class element can be characterized by three aspects:

Kind: Getter, setter, method, or field
Location: Static or instance
Visibility: Public or private

Together, they add up to 16 possible combinations. To divide the reference more logically and avoid overlapping content, the different elements are introduced in detail in different pages:

Public instance method

Public instance getter

Public instance setter

Public instance field

Public static method, getter, setter, and field

Everything that's private

Note: Private properties have the restriction that all property names declared in the same class must be unique. All other public properties do not have this restriction — you can have multiple public properties with the same name, and the last one overwrites the others. This is the same behavior as in object initializers .

In addition, there are two special class element syntaxes: constructor and static initialization blocks , with their own references.

Constructor

The constructor method is a special method for creating and initializing an object created with a class. There can only be one special method with the name "constructor" in a class — a SyntaxError is thrown if the class contains more than one occurrence of a constructor method.

A constructor can use the super keyword to call the constructor of the super class.

You can create instance properties inside the constructor:

Alternatively, if your instance properties' values do not depend on the constructor's arguments, you can define them as class fields .

Static initialization blocks

Static initialization blocks allow flexible initialization of static properties , including the evaluation of statements during initialization, while granting access to the private scope.

Multiple static blocks can be declared, and these can be interleaved with the declaration of static fields and methods (all static items are evaluated in declaration order).

Methods are defined on the prototype of each class instance and are shared by all instances. Methods can be plain functions, async functions, generator functions, or async generator functions. For more information, see method definitions .

Static methods and fields

The static keyword defines a static method or field for a class. Static properties (fields and methods) are defined on the class itself instead of each instance. Static methods are often used to create utility functions for an application, whereas static fields are useful for caches, fixed-configuration, or any other data that doesn't need to be replicated across instances.

Field declarations

With the class field declaration syntax, the constructor example can be written as:

Class fields are similar to object properties, not variables, so we don't use keywords such as const to declare them. In JavaScript, private properties use a special identifier syntax, so modifier keywords like public and private should not be used either.

As seen above, the fields can be declared with or without a default value. Fields without default values default to undefined . By declaring fields up-front, class definitions become more self-documenting, and the fields are always present, which help with optimizations.

See public class fields for more information.

Private properties

Using private fields, the definition can be refined as below.

It's an error to reference private fields from outside of the class; they can only be read or written within the class body. By defining things that are not visible outside of the class, you ensure that your classes' users can't depend on internals, which may change from version to version.

Private fields can only be declared up-front in a field declaration. They cannot be created later through assigning to them, the way that normal properties can.

For more information, see private properties .

Inheritance

The extends keyword is used in class declarations or class expressions to create a class as a child of another constructor (either a class or a function).

If there is a constructor present in the subclass, it needs to first call super() before using this . The super keyword can also be used to call corresponding methods of super class.

Evaluation order

When a class declaration or class expression is evaluated, its various components are evaluated in the following order:

The extends clause, if present, is first evaluated. It must evaluate to a valid constructor function or null , or a TypeError is thrown.
The constructor method is extracted, substituted with a default implementation if constructor is not present. However, because the constructor definition is only a method definition, this step is not observable.
The class elements' property keys are evaluated in the order of declaration. If the property key is computed, the computed expression is evaluated, with the this value set to the this value surrounding the class (not the class itself). None of the property values are evaluated yet.
Methods and accessors are installed in the order of declaration. Instance methods and accessors are installed on the prototype property of the current class, and static methods and accessors are installed on the class itself. Private instance methods and accessors are saved to be installed on the instance directly later. This step is not observable.
The class is now initialized with the prototype specified by extends and implementation specified by constructor . For all steps above, if an evaluated expression tries to access the name of the class, a ReferenceError is thrown because the class is not initialized yet.
For each instance field (public or private), its initializer expression is saved. The initializer is evaluated during instance creation, at the start of the constructor (for base classes) or immediately before the super() call returns (for derived classes).
For each static field (public or private), its initializer is evaluated with this set to the class itself, and the property is created on the class.
Static initialization blocks are evaluated with this set to the class itself.
The class is now fully initialized and can be used as a constructor function.

For how instances are created, see the constructor reference.

Binding this with instance and static methods

When a static or instance method is called without a value for this , such as by assigning the method to a variable and then calling it, the this value will be undefined inside the method. This behavior is the same even if the "use strict" directive isn't present, because code within the class body is always executed in strict mode.

If we rewrite the above using traditional function-based syntax in non–strict mode, then this method calls are automatically bound to globalThis . In strict mode, the value of this remains as undefined .

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Synonyms of assignment

as in lesson
as in appointment
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Thesaurus Definition of assignment

Synonyms & Similar Words

responsibility
undertaking
requirement
designation
appointment
authorization
installment
installation
destination
emplacement
investiture
singling (out)

Antonyms & Near Antonyms

dethronement

Synonym Chooser

How does the noun assignment contrast with its synonyms?

Some common synonyms of assignment are chore , duty , job , stint , and task . While all these words mean "a piece of work to be done," assignment implies a definite limited task assigned by one in authority.

When is it sensible to use chore instead of assignment ?

While the synonyms chore and assignment are close in meaning, chore implies a minor routine activity necessary for maintaining a household or farm.

When is duty a more appropriate choice than assignment ?

Although the words duty and assignment have much in common, duty implies an obligation to perform or responsibility for performance.

When might job be a better fit than assignment ?

The synonyms job and assignment are sometimes interchangeable, but job applies to a piece of work voluntarily performed; it may sometimes suggest difficulty or importance.

When could stint be used to replace assignment ?

In some situations, the words stint and assignment are roughly equivalent. However, stint implies a carefully allotted or measured quantity of assigned work or service.

When can task be used instead of assignment ?

The meanings of task and assignment largely overlap; however, task implies work imposed by a person in authority or an employer or by circumstance.

Thesaurus Entries Near assignment

assignments

Cite this Entry

“Assignment.” Merriam-Webster.com Thesaurus , Merriam-Webster, https://www.merriam-webster.com/thesaurus/assignment. Accessed 8 Apr. 2024.

More from Merriam-Webster on assignment

Nglish: Translation of assignment for Spanish Speakers

Britannica English: Translation of assignment for Arabic Speakers

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IMAGES

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COMMENTS

c++
ClassName = Other.ClassName; return *this; } This is the general convention used when overloading operator=. The return statement allows chaining of assignments (like a = b = c) and passing the parameter by const reference avoids copying Other on its way into the function call. edited Dec 22, 2010 at 13:54.
9. Classes
When a class definition is entered, a new namespace is created, and used as the local scope — thus, all assignments to local variables go into this new namespace. In particular, function definitions bind the name of the new function here. When a class definition is left normally (via the end), a class object is created. This is basically a ...
Assignment operators
for assignments to class type objects, the right operand could be an initializer list only when the assignment is defined by a user-defined assignment operator. removed user-defined assignment constraint. CWG 1538. C++11. E1 ={E2} was equivalent to E1 = T(E2) ( T is the type of E1 ), this introduced a C-style cast. it is equivalent to E1 = T{E2}
Python Classes: The Power of Object-Oriented Programming
In this code snippet, you define Circle using the class keyword. Inside the class, you write two methods. The .__init__() method has a special meaning in Python classes. This method is known as the object initializer because it defines and sets the initial values for your attributes. You'll learn more about this method in the Instance Attributes section.
Classes and Objects in Java
A class in Java is a set of objects which shares common characteristics/ behavior and common properties/ attributes. It is a user-defined blueprint or prototype from which objects are created. For example, Student is a class while a particular student named Ravi is an object. Properties of Java Classes. Class is not a real-world entity.
21.12
21.12 — Overloading the assignment operator. Alex November 27, 2023. The copy assignment operator (operator=) is used to copy values from one object to another already existing object. As of C++11, C++ also supports "Move assignment". We discuss move assignment in lesson 22.3 -- Move constructors and move assignment .
Assignment Operators
If you define your own assignment operators, the compiler will not automatically call your base class's assignment operators for you. Unless your base class's assignment operators themselves are broken, you should call them explicitly from your derived class's assignment operators (again, assuming you create them in the first place).
Python's Assignment Operator: Write Robust Assignments
Like with a function, when you call the class's constructor with the appropriate arguments to create an instance, Python assigns the arguments to the parameters defined in the class initializer. Another example of implicit assignments is the current instance of a class, which in Python is called self by convention.
Python Classes and Objects (With Examples)
In the above example, we have defined the class named Bike with two attributes: name and gear. We have also created an object bike1 of the class Bike. Finally, we have accessed and modified the properties of an object using the . notation. Create Multiple Objects of Python Class.
C++ Assignment Operator Overloading
The assignment operator,"=", is the operator used for Assignment. It copies the right value into the left value. Assignment Operators are predefined to operate only on built-in Data types. Assignment operator overloading is binary operator overloading. Overloading assignment operator in C++ copies all values of one object to another object.
Understanding Assignments
What this handout is about. The first step in any successful college writing venture is reading the assignment. While this sounds like a simple task, it can be a tough one. This handout will help you unravel your assignment and begin to craft an effective response. Much of the following advice will involve translating typical assignment terms ...
Copy assignment operator
5,6) Definition of a copy assignment operator outside of class definition (the class must contain a declaration (1) ). 6) The copy assignment operator is explicitly-defaulted. The copy assignment operator is called whenever selected by overload resolution, e.g. when an object appears on the left side of an assignment expression.
C++ assignment operator in derived class
The copy assignment operator of the derived class that is implicitly declared by the compiler hides assignment operators of the base class. Use using declaration in the derived class the following way. using A::operator =; B():A(){}; virtual ~B(){}; virtual void doneit(){myWrite();} Another approach is to redeclare the virtual assignment ...
Creating Class Assignment Data
Pages Used to Create Class Assignment Data. Click the icon next to a class on the My Schedule page. Create, define, and cluster assignments by category. Click the Category Weight link on the Class Assignments page. Set weight values for each class assignment category. Click the Grading Scale link on the Class Assignments page.
Classes
The body of a class is the part that is in curly braces {}. This is where you define class members, such as methods or constructor. The body of a class is executed in strict mode even without the "use strict" directive. A class element can be characterized by three aspects: Kind: Getter, setter, method, or field. Location: Static or instance.
Class Definition in Java
Class Definition in Java. In object-oriented programming, a class is a basic building block. It can be defined as template that describes the data and behaviour associated with the class instantiation. Instantiating is a class is to create an object (variable) of that class that can be used to access the member variables and methods of the class.
Assigning a class variable in class definition versus at class
The difference is that you default-initialize the member variables and then assign values to them in the first case. In the second case you value-initialize them - which is preferred. A third option, that also value-initializes them, is to use the member-initializer list: class Foo. {. private: int x; double y;
ASSIGNMENT Synonyms: 97 Similar and Opposite Words
Synonyms for ASSIGNMENT: task, job, duty, project, mission, chore, responsibility, function; Antonyms of ASSIGNMENT: dismissal, discharge, firing, expulsion ...
class assignment Definition
More Definitions of class assignment. class assignment means an assignment issued under Section 96.1. Sample 1. Based on 1 documents. class assignment means a declaration issued by the Authority pursuant to this Act granting a class of persons, subject to certain rules, rights to use . Sample 1.
Overloading assignment operator in C#
There is already a special instance of overloading = in place that the designers deemed ok: property setters. Let X be a property of foo. In foo.X = 3, the = symbol is replaced by the compiler by a call to foo.set_X(3). You can already define a public static T op_Assign(ref T assigned, T assignee) method. All that is left is for the compiler to ...
How to make python class support item assignment?
To avoid inheritance from dict, you can make a class inherit from MutableMapping, and then provide methods for __setitem__ and __getitem__. Additionally, the class will need to support methods for __delitem__, __iter__, __len__, and (optionally) other inherited mixin methods, like pop. The documentation has more info on the details.