python private attribute

Question

class A():

    def __init__(self):
        self.__var = 5

    def get_var(self):
        return self.__var

    def set_var(self, value):
        self.__var = value

    var = property(get_var, set_var)

a = A()
a.var = 10
print a.var == a._A__var

Can anyone explain why result is False?

Santa · Accepted Answer · 2010-05-25 09:04:03Z

4

The property decorator only works on new-style classes. In Python 2.x, you have to extend the object class:

class A(object):

    def __init__(self):
        self.__var = 5

    def get_var(self):
        return self.__var

    def set_var(self, value):
        self.__var = value

    var = property(get_var, set_var)

Without the behavior of the new-style class, the assignment a.var = 10 just binds a new value (10) to a new member attribute a.var.

answered May 25, 2010 at 9:04

Santa

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4 Comments

Denis Dubinin Over a year ago

Thank you. But i have another question. Why if i just do print A().var i got 5 Seems like get method work with property decorator.

Santa Over a year ago

Because A() creates a new object of type A. And the constructor method initializes the __var member attribute to be 5. The subsequent property access to .var queries the newly created A object, and returns 5.

riza Over a year ago

In Python 3, you don't have to do like this.

Santa Over a year ago

That is why I prefixed my answer with "In Python 2.x, ...." All user-defined classes in Python 3.x are new-style by default.

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