Method Example

To clarify these concepts, let’s turn to an example. Suppose we define the following class:

class NextClass:                            # Define class
    def printer(self, text):                # Define method
        self.message = text                 # Change instance
        print(self.message)                 # Access instance

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The name printer references a function object; because it’s assigned in the class statement’s scope, it becomes a class object attribute and is inherited by every instance made from the class. Normally, because methods like printer are designed to process instances, we call them through instances:

>>> x = NextClass()                         # Make instance

>>> x.printer('instance call')              # Call its method
instance call

>>> x.message                               # Instance changed
'instance call'

When we call the method by qualifying an instance like this, printer is first located by inheritance, and then its self argument is automatically assigned the instance object (x); the text argument gets the string passed at the call ('instance call'). Notice that because Python automatically passes the first argument to self for us, we only actually have to pass in one argument. Inside printer, the name self is used to access or set per-instance data because it refers back to the instance currently being processed.

Methods may be called in one of two ways—through an instance, or through the class itself. For example, we can also call printer by going through the class name, provided we pass an instance to the self argument explicitly:

>>> NextClass.printer(x, 'class call')      # Direct class call
class call

>>> x.message                               # Instance changed again
'class call'

Calls routed through the instance and the class have the exact same effect, as long as we pass the same instance object ourselves in the class form. By default, in fact, you get an error message if you try to call a method without any instance:

>>> NextClass.printer('bad call')
TypeError: unbound method printer() must be called with NextClass instance...