Learning Python, 5th Edition

Chapter 17. Scopes

Chapter 16 introduced basic function definitions and calls. As we saw, Python’s core function model is simple to use, but even simple function examples quickly led us to questions about the meaning of variables in our 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 name clashes across your program’s code: names defined in one program unit don’t interfere with names in another.

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

Python Scope Basics

Now that you’re ready to start writing your own functions, we need to get 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. When we talk about the search for a name’s value in relation to code, the term scope refers to a namespace: that is, the location of a name’s assignment in your source code determines the scope of the name’s visibility to your code.

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