Variable with double underscore cannot be used in class?

Question

Consider the following code:

def __g_fun():
    pass

__a = 3
a=3

class Test(object):
    def __init__(self):
        global a,__a
        print "locals:",locals()
        print "global:",globals()
        self.aa = __a
    def fun(self):
        return __g_fun()


t=Test()
t.fun()

Output:

locals: {'self': <__main__.Test object at 0x7f53580d50d0>}

global: {'__g_fun': <function __g_fun at 0x7f53580c52a8>, 'a': 3, '__a': 3, '__builtins__': <module '__builtin__' (built-in)>, '__file__': 'test.py', '__package__': None, 'Test': <class '__main__.Test'>, '__name__': '__main__', '__doc__': None}

Traceback (most recent call last):
  File "test.py", line 19, in <module>
    t=Test()
  File "test.py", line 11, in __init__
    self.aa = __a
NameError: global name '_Test__a' is not defined

Is it the case that variable with double underscore cannot be used in class?

Asked By: sven

||

Source

Answer 1

Indeed – a double underline prefix inside a class code is treated in a special way by the Python compiler –
at compile time, such variables are name mangled to include the class name as a prefix –
so that the name __a anywhere inside the class Test will be changed to _Test__a. (remembering that “compile time” is often transparent to the user, and can be taken as “at the time the program is run”)

That is a feature meant to allow one to have names that are assessed in methods of a class, and not by its subclasses (not in an automatic form) – a feature that in some other languages is carried out by the “private” member modifier.

Check the Python class documentation at: https://docs.python.org/2/tutorial/classes.html#private-variables-and-class-local-references

Answered By: jsbueno

Answer 2

The Python 3 docs say:

Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped.

Answered By: Nico Schlömer

Variable with double underscore cannot be used in class?

Question:

Answers: