Scope Rules

Before we started writing functions, all the code we wrote was at the top level of a module (i.e., not nested in a def), so the names we used either lived in the module itself or were built-ins predefined by Python (e.g., open). Functions provide nested namespaces (scopes) that localize the names they use, such that names inside a function won’t clash with those outside it (in a module or another function). Again, functions define a local scope, and modules define a global scope. The two scopes are related as follows:

 

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  • The enclosing module is a global scope. Each module is a global scope—that is, a namespace in which variables created (assigned) at the top level of the module file live. Global variables become attributes of a module object to the outside world but can be used as simple variables within a module file.
  • The global scope spans a single file only. Don’t be fooled by the word “global” here—names at the top level of a file are only global to code within that single file. There is really no notion of a single, all-encompassing global file-based scope in Python. Instead, names are partitioned into modules, and you must always import a module explicitly if you want to be able to use the names its file defines. When you hear “global” in Python, think “module.”
  • Each call to a function creates a new local scope. Every time you call a function, you create a new local scope—that is, a namespace in which the names created inside that function will usually live. You can think of each def statement (and lambda expression) as defining a new local scope, but because Python allows functions to call themselves to loop (an advanced technique known as recursion), the local scope in fact technically corresponds to a function call—in other words, each call creates a new local namespace. Recursion is useful when processing structures whose shapes can’t be predicted ahead of time.
  • Assigned names are local unless declared global or nonlocal. By default, all the names assigned inside a function definition are put in the local scope (the namespace associated with the function call). If you need to assign a name that lives at the top level of the module enclosing the function, you can do so by declaring it in a global statement inside the function. If you need to assign a name that lives in an enclosing def, as of Python 3.0 you can do so by declaring it in a nonlocal statement.
  • All other names are enclosing function locals, globals, or built-ins. Names not assigned a value in the function definition are assumed to be enclosing scope locals (in an enclosing def), globals (in the enclosing module’s namespace), or built-ins (in the predefined __builtin__ module Python provides).

There are a few subtleties to note here. First, keep in mind that code typed at the interactive command prompt follows these same rules. You may not know it yet, but code run interactively is really entered into a built-in module called __main__; this module works just like a module file, but results are echoed as you go. Because of this, interactively created names live in a module, too, and thus follow the normal scope rules: they are global to the interactive session. You’ll learn more about modules in the next part of this book.

Also note that any type of assignment within a function classifies a name as local. This includes = statements, module names in import, function names in def, function argument names, and so on. If you assign a name in any way within a def, it will become a local to that function.

Conversely, in-place changes to objects do not classify names as locals; only actual name assignments do. For instance, if the name L is assigned to a list at the top level of a module, a statement L = X within a function will classify L as a local, but L.append(X) will not. In the latter case, we are changing the list object that L references, not L itself—L is found in the global scope as usual, and Python happily modifies it without requiring a global (or nonlocal) declaration. As usual, it helps to keep the distinction between names and objects clear: changing an object is not an assignment to a name.