Learning Python

Scope Rules in Functions

Now that we’ve stepped up to writing our own functions, we need to get a bit more formal about what names mean in Python. When you use a name in a program, Python creates, changes, or looks up the name in what is known as a namespace—a place where names live. As we’ve seen, names in Python spring into existence when they are assigned a value. Because names aren’t declared ahead of time, Python uses the assignment of a name to associate it with a particular namespace. Besides packaging code, functions add an extra namespace layer to your programs—by default, names assigned inside a function are associated with that function’s namespace, and no other.

Here’s how this works. Before you started writing functions, all code was written at the top-level of a module, so the names either lived in the module itself, or were built-ins that Python predefines (e.g., open).^[29] Functions provide a nested namespace (sometimes called a scope), which localizes the names they use, such that names inside the function won’t clash with those outside (in a module or other function). We usually say that functions define a local scope, and modules define a global scope. The two scopes are related as follows:

The enclosing module is a global scope

Each module is a global scope—a namespace where variables created (assigned) at the top level of a module file live.

Each call to a function is a new local scope

Every time you call a function, you create a new local scope—a namespace where ...