difference between method and function

Question

Some list operations are reached by a dot notation

C.append(e)

while other operations requires the list object as an argument to a function, as in

len(C)

Why this happens? Are there any rules whether functionality regarding an object is reached through the first way (method) or the second one (function)? Thanks.

• Andrew Li: Because you need a list to append, append is an instance method because you need an instance for it to work, but to find length you only need to pass an argument. len is a global builtin.
• WhatsThePoint: usually ones with brackets return a value
• user2201041: In the case of len, I believe it just calls the object's __len__ method anyway. When in doubt, consult the documentation.
• unutbu: Related: The advantages of having static function like len(), max(), and min() over inherited method calls.

Answer 1

There is no difference for many built-in functions. The global functions like len, iter, str are calls to object methods __len__, __iter__, __str__ etc.

See Basic customization and Emulating container types in Python reference:

object.__len__(self)

Called to implement the built-in function len(). Should return the length of the object, an integer>= 0. ...

Advantage of such soluttion is that an object can override these special functions just by overriding corresponding "double-underscore" methods, e.g __len__.

Although append does not have a built-in, since it is a less universal operation than len, str etc.