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.

Actually, a concise rule for Python Scope resolution, from Learning Python, 3rd. Ed.. (These rules are specific to variable names, not attributes. If you reference it without a period, these rules apply.)

LEGB Rule

• Local — Names assigned in any way within a function (def or lambda), and not declared global in that function

• Enclosing-function — Names assigned in the local scope of any and all statically enclosing functions (def or lambda), from inner to outer

• Global (module) — Names assigned at the top-level of a module file, or by executing a global statement in a def within the file

• Built-in (Python) — Names preassigned in the built-in names module: open, range, SyntaxError, etc

So, in the case of

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

The for loop does not have its own namespace. In LEGB order, the scopes would be

• L: Local in def spam (in code3, code4, and code5)
• E: Any enclosing functions (if the whole example were in another def)
• G: Were there any x declared globally in the module (in code1)?
• B: Any builtin x in Python.

x will never be found in code2 (even in cases where you might expect it would, see Antti’s answer or here).