Difference between methods and functions, in Python compared to C++

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A function is a callable object in Python, i.e. can be called using the call operator (though other objects can emulate a function by implementing __call__). For example:

>>> def a(): pass
>>> a
<function a at 0x107063aa0>
>>> type(a)
<type 'function'>

A method is a special class of function, one that can be bound or unbound.

>>> class A:
...   def a(self): pass
>>> A.a
<unbound method A.a>
>>> type(A.a)
<type 'instancemethod'>

>>> A().a
<bound method A.a of <__main__.A instance at 0x107070d88>>
>>> type(A().a)
<type 'instancemethod'>

Of course, an unbound method cannot be called (at least not directly without passing an instance as an argument):

>>> A.a()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method a() must be called with A instance as first argument (got nothing instead)

In Python, in most cases, you won’t notice the difference between a bound method, a function, or a callable object (i.e. an object that implements __call__), or a class constructor. They all look the same, they just have different naming conventions. Under the hood, the objects may look vastly different though.

This means that a bound method can be used as a function, this is one of the many small things that makes Python so powerful

>>> b = A().a
>>> b()

It also means that even though there is a fundamental difference between len(...) and str(...) (the latter is a type constructor), you won’t notice the difference until you dig a little deeper:

>>> len
<built-in function len>
>>> str
<type 'str'>

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