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Learning Python, 3rd Edition by Mark Lutz

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Chapter 16. Scopes and Arguments

Chapter 15 introduced basic function definitions and calls. As we saw, Python’s basic function model is simple to use. This chapter presents the details behind Python’s scopes—the places where variables are defined and looked up—and behind argument passing—the way that objects are sent to functions as inputs.

Scope Rules

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 code determines the scope of the name’s visibility to your code.

Just about everything related to names, including scope classification, happens at assignment time in Python. As we’ve seen, names in Python spring into existence when they are first assigned values, and they must be assigned before they are used. Because names are not declared ahead of time, Python uses the location of the assignment of a name to associate it with (i.e., bind it to) a particular namespace. In other words, the place where you assign a name in your source code determines the namespace it will live in, and hence its scope of visibility.

Besides packaging code, functions add an extra namespace layer to your programs—by default, all names assigned ...

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