A function is a callable object in Python, i.e. can be called using the call operator. However other objects can also emulate a function by implementing __call__method.
def a(): pass # a() is an example of function print a print type(a)
C:/Users/TutorialsPoint/~.py <function a at 0x0000000005765C18> <type 'function'>
A method is a special class of function, one that can be bound or unbound.
class C: def c(self): pass print C.c # example of unbound method print type(C.c) print C().c # example of bound method print type(C().c) print C.c()
Of course, an unbound method cannot be called without passing as argument.
<function a at 0xb741b5a4> <type 'function'> <unbound method C.c> <type 'instancemethod'> <bound method C.c of <__main__.C instance at 0xb71ade0c>> <type 'instancemethod'> Traceback (most recent call last): File "~.py", line 11, in <module> print C.c() TypeError: unbound method c() must be called with C instance as first argument (got nothing instead)
In Python, there is not much difference between a bound method, a function or a callable object (that is an object that implements __call__ method), or a class constructor. They all look the same, they just have different naming conventions and may look vastly different though under the hood.
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
>>> d = A().a #this is a bound method of A() >>> d() # this is a function
It also means that even though there is a fundamental difference between len(...) and str(...) (str is a type constructor), we won't notice the difference until we go a little deeper:
>>>len <built-in function len> >>> str <type 'str'>