Weird behavior with lexical scope

[mrstevegross]

I ran into a weird behavior with lexical scope in Python. I'm hoping
someone on this forum can explain it to me.

Here's the situation: I have an Outer class. In the Outer class, I
define a nested class 'Inner' with a simple constructor. Outer's
constructor creates an instance of Inner. The code looks like this:

=========
class Outer:
  class Inner:
    def __init__(self):
      pass
  def __init__ (self):
    a = Inner()
Outer()
=========

However, the above code doesn't work. The creation of Inner() fails.
The error message looks like this:

  File "/tmp/foo.py", line 12, in <module>
    Outer()
  File "/tmp/foo.py", line 10, in __init__
    a = Inner()
NameError: global name 'Inner' is not defined

This surprises me! Since the construction of Inner takes place within
the lexical scope 'Outer', I assumed the interpreter would search the
Outer scope and find the 'Inner' symbol. But it doesn't! If I change:
  a = Inner()
to
  a = Outer.Inner()

it works fine, though.

So, can anyone explain to me how Python looks up symbols? It doesn't
seem to be searching the scopes I expected...

Thanks,
--Steve