Short description of the scoping rules

What exactly are the Python scoping rules?

If I have some code:

code1
class Foo:
   code2
   def spam.....
      code3
      for code4..:
       code5
       x()

Where is x found? Some possible choices include the list below:

1. In the enclosing source file
2. In the class namespace
3. In the function definition
4. In the for loop index variable
5. Inside the for loop

Also there is the context during execution, when the function spam is passed somewhere else. And maybe lambda functions pass a bit differently?

There must be a simple reference or algorithm somewhere. It's a confusing world for intermediate Python programmers.