Scoping rules

A scope defines the visibility of a name within a block, where a block is a piece of Python code executed as a unit. For simplicity, this would be a module, a function body, and a class definition. A name refers to text bound to an object.

For more information about names, see /tag names

A module is the source code file itself, and encompasses all blocks defined within it. Therefore if a variable is defined at the module level (top-level code block), it is a global variable and can be accessed anywhere in the module as long as the block in which it's referenced is executed after it was defined.

Alternatively if a variable is defined within a function block for example, it is a local variable. It is not accessible at the module level, as that would be outside its scope. This is the purpose of the return statement, as it hands an object back to the scope of its caller. Conversely if a function was defined inside the previously mentioned block, it would have access to that variable, because it is within the first function's scope.

>>> def outer():
...     foo = 'bar'     # local variable to outer
...     def inner():
...         print(foo)  # has access to foo from scope of outer
...     return inner    # brings inner to scope of caller
>>> inner = outer()  # get inner function
>>> inner()  # prints variable foo without issue
Official Documentation
1. Program structure, name binding and resolution
2. global statement
3. nonlocal statement