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Name Resolution: The LEGB Rule
If the prior section sounds confusing, it really boils down to three simple rules. With a def statement:
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- Name references search at most four scopes: local, then enclosing functions (if any), then global, then built-in.
- Name assignments create or change local names by default.
- global and nonlocal declarations map assigned names to enclosing module and function scopes.
In other words, all names assigned inside a function def statement (or a lambda, an expression we’ll meet later) are locals by default. Functions can freely use names assigned in syntactically enclosing functions and the global scope, but they must declare such nonlocals and globals in order to change them.
Python’s name-resolution scheme is sometimes called the LEGB rule, after the scope names:
- When you use an unqualified name inside a function, Python searches up to four scopes—the local (L) scope, then the local scopes of any enclosing (E) defs and lambdas, then the global (G) scope, and then the built-in (B) scope—and stops at the first place the name is found. If the name is not found during this search, Python reports an error. As we learned in Chapter 6, names must be assigned before they can be used.
- When you assign a name in a function (instead of just referring to it in an expression), Python always creates or changes the name in the local scope, unless it’s declared to be global or nonlocal in that function.
- When you assign a name outside any function (i.e., at the top level of a module file, or at the interactive prompt), the local scope is the same as the global scope—the module’s namespace.
Figure 17-1 illustrates Python’s four scopes. Note that the second scope lookup layer, E—the scopes of enclosing defs or lambdas—can technically correspond to more than one lookup layer. This case only comes into play when you nest functions within functions, and it is addressed by the nonlocal statement.[36]
Figure 17-1. The LEGB scope lookup rule. When a variable is referenced, Python searches for it in this order: in the local scope, in any enclosing functions’ local scopes, in the global scope, and finally in the built-in scope. The first occurrence wins. The place in your code where a variable is assigned usually determines its scope. In Python 3, nonlocal declarations can also force names to be mapped to enclosing function scopes, whether assigned or not.
Also keep in mind that these rules apply only to simple variable names (e.g., spam). In Parts V and VI, we’ll see that qualified attribute names (e.g., object.spam) live in particular objects and follow a completely different set of lookup rules than those covered here. References to attribute names following periods (.) search one or more objects, not scopes, and may invoke something called “inheritance”; more on this in Part VI of this book.
