class ParentClass(object):
def __init__(self):
self.__x = 1
self.y = 10
def PRINT(self):
print (self.__x, self.y)
class ChildClass(ParentClass):
def __init__(self):
super(ChildClass, self).__init__()
self.__x = 2
self.y = 20
c = ChildClass()
c.PRINT()
Why is the output (1, 20)? I know how I got 20, but shouldn't it be 2 instead of 1?
Just to expand slightly on poke's answer...
From the official Python tutorial
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. This mangling is done without regard to the syntactic position of the identifier, as long as it occurs within the definition of a class.
So if we add this line to the end of your code:
print c._ChildClass__x, c._ParentClass__x,
it will print
(1, 20)
2 1
Members that start with two underscores are “private”. Although Python does not have a real way to restrict access to members, it does some name mangling to give them a more complicated name so they stay more or less private.
So in your case, ParentClass has a __x field that is used in the PRINT method. And ChildClass has a separate, independent __x field that is not used anywhere. So for the printing, only the parent’s __x is used.
To fix this, simply change the two underscores to a single underscore, to mark the name as “internal” but not private to the type.
ParentClass()._ParentClass__x.
For 'private' variable (__ as prefix) in a class, name of it has been changed. You can use the new name to call variable accordingly. Like this:
#!/usr/bin/python
# -*- coding: utf-8 -*-
class ParentClass(object):
def __init__(self):
self.__x = 1
self.y = 10
def PRINT(self):
print (self.__x, self.y)
print (self._ParentClass__x, self.y)
print (self._ChildClass__x, self.y)
class ChildClass(ParentClass):
def __init__(self):
super(ChildClass, self).__init__()
self.__x = 2
self.y = 20
c = ChildClass()
c.PRINT()
OUTPUT:
(1, 20)
(1, 20)
(2, 20)
.PRINT() method is a little scary. If we do e.g., p = ParentClass(); p.PRINT() we get AttributeError: 'ParentClass' object has no attribute '_ChildClass__x'