How to initialise a class variable with __init__()?

Question

As I understand, a class property can be used in a sense as the static variable for Python. I have a class and I would like to set this static variable during the first run of __init__(), i.e. when the first object is being created and to keep it like that. Not to update it each time. What should I do exactly? Even setting a flag doesn't help.

class A():
    foo=None
    foo_set_flag=False
    def __init__(self,arg_foo):
        if not self.foo_set_flag:
            self.foo=arg_foo
            foo_set_flag=True
a=A(12)
b=A(13)
print a.foo
print b.foo

which gives:

12
13

and of course I expect

12
12

Answer 1

Use ClassName.attribute_name to access class attribute:

class A:
    foo = None
    foo_set_flag = False

    def __init__(self, arg_foo):
        if not A.foo_set_flag:
            A.foo = arg_foo
            A.foo_set_flag = True

a=A(12)
b=A(13)
print a.foo # -> 12
print b.foo # -> 12

self.foo = ... creates a new instance attribute. Reading self.foo will access class attribute only if there is no instance attribute foo, otherwise it access the instnace variable foo.