python static fields of own type

Question

I notice python won't let you add an instance of a class to itself as a static member at class definition.

>>> class Foo:
...     A = Foo()
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in Foo
NameError: name 'Foo' is not defined

However either of the following work:

>>> class Foo:
...     pass
... 
>>> class Foo:
...     A = Foo()
... 
>>> Foo.A
<__main__.Foo instance at 0x100854440>

or

>>> class Foo:
...     pass
... 
>>> Foo.A = Foo()
>>> 
>>> Foo.A
<__main__.Foo instance at 0x105843440>

I can't find any enlightening code examples or explanations. Why does python treat the first case differently? Where is A going in each of the two subsequent cases?

Answer 1

Your first example doesn't work because you haven't created the class Foo yet. You're in the process of doing so (hence the NameError)

Your second example works because you have a class called Foo(). You override it, but you still keep a copy of it. Take a look at this:

>>> class Foo:
...     def __init__(self):
...             print 'hi'
... 
>>> class Foo:
...     A = Foo()
... 
hi
>>> Foo.A
<__main__.Foo instance at 0x101019950>
>>> Foo.A.__init__
<bound method Foo.__init__ of <__main__.Foo instance at 0x101019950>>

A is an attribute that has the value of a class you overrode.

As for your third example, you're simply making an attribute of a class that is an instance of the class.