Question

>>> import __builtin__
>>> class MyClass:
...     def __init__(self):
...             self.a = 2
...             __builtin__.a = self.a
...
>>> myclass = MyClass()
>>> myclass.a = 4
>>> a = 2
>>> print a
2
>>> print myclass.a
4

I would expect a and myclass.a to have the same value. Why don’t they?

PS. Why you would use __builtin__ like this is another question.

Answer 1

Python does not have references. Assigning a value e.g. using = rebinds the name.

If you want such behaviour you need a mutable object, e.g. a list with one element:

>>> import __builtin__
>>> class MyClass:
...     def __init__(self):
...         self.a = [2]
...         __builtin__.a = self.a
...
>>> myclass = MyClass()
>>> myclass.a[0] = 4
>>> a[0] = 2
>>> print a
[2]
>>> print myclass.a
[2]
>>>

However, reconsider if you really want to put stuff like this into __builtins__.