How Does Python's LEGB Rule Determine Variable Scope?

Python's LEGB Rule: Demystifying Scoping Rules

Python's scoping rules, often a source of confusion, can be readily apprehended through the LEGB rule.

LEGB Rule

The LEGB rule provides a clear algorithm for determining the availability of variables within a given scope:

• L (Local): Variables defined within a function or lambda expression.
• E (Enclosing-function): Variables defined in enclosing functions, from inner to outer.
• G (Global): Variables defined at the module level or declared global within a function.
• B (Built-in): Pre-assigned variables from Python's built-in names module.

Example

Consider the following code:

class Foo:
    def spam():
        for var in []:
            x()

Using the LEGB rule, we can determine the scope of the variable x:

• L: None. x is not defined locally within the spam function.
• E: None. The spam function is not nested within another function.
• G: Yes. If x is defined at the module level, it will be available within the spam function.
• B: Yes. If x is a built-in variable (e.g., a Python function), it will be accessible.

In this example, x would be found if it were defined at the module level or is a Python built-in.

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