Scope, State, and Closures in Python
Understanding scope is crucial for writing bug-free Python code. Scope defines the “visibility” of a variable—where in your program you can access it. Closely related is a variable’s lifetime, which is how long it exists in memory.
These concepts come together in a powerful feature called closures, where a function “remembers” the environment in which it was created.
The LEGB Rule for Name Resolution
When you use a variable name, Python has a strict order of places it looks for that name. This is known as the LEGB Rule.
1. Local
The first place Python looks is inside the current function’s local scope. These are variables defined directly within the function.
2. Enclosing
If the name isn’t found locally, Python looks in the scopes of any enclosing functions, from the innermost to the outermost. This is relevant for nested functions.
3. Global
If the name still isn’t found, Python checks the module-level (global) scope. These are variables defined at the top level of your script.
4. Built-in
Finally, if the name is nowhere to be found, Python checks the built-in scope, which contains all the names that are always available, like print(), len(), and str().
If Python fails to find the name in any of these scopes, it raises a NameError.
Pyground
Given a nested function structure with a variable `x` defined at multiple levels, predict the output.
Expected Output:
Inside inner_function, x is: I am Local Inside outer_function, x is: I am Enclosing In the main script, x is: I am Global
Output:
Modifying Variables in Different Scopes
By default, you can only read from outer scopes. If you assign a value to a variable inside a function, Python assumes it’s a new local variable. To modify variables in outer scopes, you need special keywords.
Local Scope
Variables created inside a function are local to it. Their lifetime is tied to the function call; they are created when the function starts and destroyed when it returns.
Pyground
What happens if you try to access a local variable from outside its function?
Expected Output:
I exist only inside this function.