python predefined function (__ symbol in python)

Question

Hello there i am very new to python (and i am from java background ) my question is does the predefined functions such as length, size or init (constructor of a class) starts with

__function__()

is this standard python syntax that every predefined should starts with this symbol ? ( for example touple.__len__() )? if not then whats the purpose of use this symbol i am asking this quesion because this symbol confuse me a lot in python.

Take a look here, http://docs.python.org/2/reference/datamodel.html — , Nov 23 '13 at 15:56
This is documented in http://docs.python.org/2/reference/lexical_analysis.html#reserved-classes-of-identifiers and http://docs.python.org/2/reference/datamodel.html#special-method-names — Martijn Pieters, Nov 23 '13 at 16:08

tawmas · Accepted Answer · 2013-11-23T16:50:19.463

3

In python, method names which start and end in a double underscore have special meaning and are, by convention, reserved (there is nothing preventing you from creating your own "special" names, but you should rather not). For example, they are used to implement operator overloading.

You can find a nice introduction in A Guide to Python's Magic Methods. You can also see the Data model - Special method names section of the language reference.

You should also note that methods with an initial double underscore and no final double underscore are treated specially by the interpreter (they are name-mangled so that you get a "private-ish" method). This has nothing to do with methods starting and ending with a double underscore.

edited Nov 23 '13 at 16:50

answered Nov 23 '13 at 15:58

tawmas

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They aren't reserved, but some such names have a special effect on the object they are of; they may modify the actions of operators on the object. – Ramchandra Apte Nov 23 '13 at 16:01
1

They are "reserved" by convention. You should not create names with a double leading and ending underscore. But, just like a lot of other aspects of Python, nothing in the language is enforcing it. – tawmas Nov 23 '13 at 16:05
1

You should add that to the answer then. – Ramchandra Apte Nov 23 '13 at 16:06
I followed your suggestion. – tawmas Nov 23 '13 at 16:15

score 1 · Answer 2 · answered Nov 23 '13 at 16:15

Something that begins and ends with a double underscore or often called a dunder, is a magic function or method. Usually, they make objects work with operators or base functions (like len or statements like del, so basically functions or statements of importance).

So, for example:

__add__ is for the + operator
__mul__ is for * operator.

So, if you have two objects of the same type say Foo, then Foo('a') + Foo('b'), would call Foo('a').__add__(Foo('b')).

They behave as reserved words and have some protection. So, for example:

class Foo(object):

    def __init__(self):
        self.x = 10

    def __len__(self):
        return "Cheese"


if __name__ == '__main__':
    a = Foo()
    print a.__len__()

would print out Cheese to the console. However, this would give you a TypeError:

if __name__ == '__main__':
    a = Foo()
    print len(a)

score -2 · Answer 3 · answered Nov 23 '13 at 15:58

the __ and _ symbol is for "private" functions

usually you do not use function name with _. functions that start with _ are usually python's and are being used so dont override them or something if you dont know what you are doing

if you want to make a function "private" you use _. e.g : def _myFunc()

python predefined function (__ symbol in python)

3 Answers3