Python Interactive Guide - Step 3 Functions (5) - Scope
- This course, Python Interactive Guide, is designed to help you learn the basics of Python programming through hands-on, interactive examples.
- The “Style Guide” sections introduce clean coding practices, mainly based on PEP8.
- You can run and experiment with every code example.
Feel free to try things out - reloading the page will reset everything.
This is a continuation of “Step 3 Functions.”
3.4. Scope
In Python, the accessibility of variables, functions, and other identifiers is determined by where they are defined in your code. This accessible range is called a scope.
3.4.1. LEGB Rule
When an identifier appears in the code, Python searches for its definition in the following order, and uses the first match it finds.
This rule is called the LEGB rule, named after the initials of the four scopes it searches.
- Local: The immediate scope within the current function
- Enclosing: The scope of any containing functions (for nested functions)
- Global: The scope of the current file
- Built-in: The scope containing Python’s built-in objects, such as the
printfunction,lenfunction, and constants likeTrue
Built-in functions and constants in Python (e.g.,
print, len, True)
Identifiers defined in the current file
Identifiers such as local variables of the outer function
Identifiers such as variables defined within the current function
→ The search begins in this scope and extends outward to broader scopes
Local Scope
Variables and other identifiers defined within a function have a “local scope” that exists only within that function:
Enclosing Scope
When a function is defined inside another function, the local scope of the outer function becomes the “enclosing scope” for the inner function.
Python checks the enclosing scope after checking the local scope.
Global Scope
Variables and other identifiers defined at the file level have a “global scope” and can be accessed from anywhere within that file.
Python checks the global scope after the local and enclosing scopes.
Built-in Scope
Python has a “built-in scope” containing built-in functions like len(), print(), min(),
and built-in constants such as True, False, and None.
This is the last scope Python checks when looking for an identifier.
Try to predict the output of the following code before running it:
Example Answer
3.4.2. global Declaration
As we’ve seen, it’s possible to reference global variables from inside a function.
However, by default, you cannot modify the values of global variables inside functions.
By default, when you perform an assignment operation inside a function, you create and assign to a local variable:
You can use the global keyword to declare that a variable is an identifier from the global scope:
- By declaring a variable as
global, you explicitly indicate that it’s a global variable and can modify its value - You can declare multiple variables at once by separating them with commas
There are missing parts in the following code.
Modify the program by using the global keyword appropriately.
Example Answer
As programs become more complex, using global variables can cause unexpected problems.
Keep the following in mind:
- Avoid global variables when possible - Use function arguments and return values instead
- Use them as read-only - Only use them for configuration values and other data that doesn’t change
- Use descriptive names - Use names that clearly indicate the purpose of the variable
3.4.3. nonlocal Declaration
You can use the nonlocal keyword to declare that a variable is an identifier from the enclosing scope:
- By declaring a variable as
nonlocal, you explicitly indicate that it’s a variable from the enclosing scope and can modify its value - You can declare multiple variables at once by separating them with commas
3.4.4. When External Variables Are Referenced
In Python, when referencing variables from outer scopes (outside the local scope) within a function, their values are evaluated at the time of function call, not function definition.
As shown above, the latest value of external variables at execution time is used.
If you want to preserve the value at definition time, passing it as a default argument is a convenient approach.
Summary
In this section, we learned about “scope” which indicates the accessible range of identifiers:
- LEGB Rule: The order of reference for identifiers (Local → Enclosing → Global → Built-in)
globalDeclaration: How to modify global variables from inside functions using theglobalkeywordnonlocalDeclaration: How to modify variables of outer functions from inner functions using thenonlocalkeyword- When External Variables Are Referenced: External variables are evaluated at execution time, not definition time
Understanding scope is very important when creating complex programs in Python.
By minimizing the use of global variables and utilizing function parameters and return values as much as possible, you can create more maintainable code.