Instance methods in Python

class Request:
    def send():
        print('Sent')

And you can call the send() function via the Request class like this:

The send() is a function object, which is an instance of the function class as shown in the following output:

print(Request.send)

Output:

<function Request.send at 0x00000276F9E00310>

The type of the send is function:

print(type(Request.send))

Output:

<class 'function'>

The following creates a new instance of the Request class:

http_request = Request()

If you display the http_request.send, it’ll return a bound method object:

print(http_request.send)

Output:

<bound method Request.send of <__main__.Request object at 0x00000104B6C3D580>>

So the http_request.send is not a function like Request.send. The following checks if the Request.send is the same object as http_request.send. It’ll returns False as expected:

print(type(Request.send) is type(http_request.send))

The reason is that the type of the Request.send is function while the type of the http_request.send is method, as shown below:

print(type(http_request.send))  # <class 'method'>
print(type(Request.send))  # <class 'function'>

So when you define a function inside a class, it’s purely a function. However, when you access that function via an object, the function becomes a method.

Therefore, a method is a function that is bound to an instance of a class.

If you call the send() function via the http_request object, you’ll get a TypeError as follows:

http_request.send()

Error

TypeError: send() takes 0 positional arguments but 1 was given

Because the http_request.send is a method that is bound to the http_request object, Python always implicitly passes the object to the method as the first argument.

The following redefines the Request class where the send function accepts a list of arguments:

class Request:
    def send(*args):
        print('Sent', args)

The following calls the send function from the Request class:

Request.send()

Output

Sent ()

The send() function doesn’t receive any arguments.

However, if you call the send() function from an instance of the Request class, the args is not empty:

http_request.send()

Output

Sent (<__main__.Request object at 0x000001374AF4D580>,)

In this case, the send() method receives an object which is the http_request, which is the object that it is bound to.

The following illustrates that the object that calls the send() method is the one that Python implicitly passes into the method as the first argument:

print(hex(id(http_request)))
http_request.send()

Output

0x1ee3a74d580
Sent (<__main__.Request object at 0x000001EE3A74D580>,)

The http_request object is the same as the one Python passes to the send() method as the first argument because they have the same memory address. In other words, you can access the instance of the class as the first argument inside the send() method:

The following method call:

http_request.send()

is equivalent to the following function call:

Request.send(http_request)

For this reason, a method of an object always has the object as the first argument. By convention, it is called self:

class Request:
    def send(self):
        print('Sent', self)

If you have worked with other programming languages such as Java or C#, the self is the same as the this object.