Learning Python, 6th Edition

Chapter 17. Scopes

The preceding chapter introduced basic function definitions and calls. As we saw, Python’s core function model is simple to use, but even simple function examples quickly lead to questions about the meaning of variables in code. This chapter moves on to present the details behind Python’s scopes—the places where variables are defined and looked up. Like module files, scopes help prevent same-name clashes across a program’s code: names defined in one program unit don’t interfere with names in another.

As you’ll learn here, the place where a name is assigned in your code is crucial to determining what the name means. You’ll also find that scope usage can have a major impact on program maintenance effort; overuse of globals, for example, is a generally bad idea. On the plus side, you’ll also learn that scopes can provide a way to retain state information between function calls, and they offer an alternative to classes in some roles.

Python Scopes Basics

Now that we’re starting to write our own functions, we need to get more formal about what names mean in Python. When we 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. And when we talk about names in relation to code, namespaces correspond to scopes: the location of a name’s assignment in your source code determines the scope of the name’s visibility in your program.

Just about everything related to names, including scope classification, ...