Скачать или смотреть Understanding Python Function Scope: Why Global Variables Work and Local Ones Don’t

Discover why your Python function only modifies global variables and learn efficient strategies to manage variable scope without relying on globals.
---
This video is based on the question https://stackoverflow.com/q/70221447/ asked by the user 'Miggle Sizzle' ( https://stackoverflow.com/u/13090383/ ) and on the answer https://stackoverflow.com/a/70221554/ provided by the user 'Derek O' ( https://stackoverflow.com/u/5327068/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: python function only works when value is global

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Python Function Scope: Why Global Variables Work and Local Ones Don’t

When writing Python functions, you may encounter unexpected behavior when trying to modify variables. This situation often leads to confusion, especially when your function appears to work perfectly with global variables but fails to operate correctly when using local variables. Let's explore this common issue and learn how to effectively manage variable scope in Python.

The Problem

In your Python code, you might have noticed that your function togglesize() operates correctly only when the variable is_small is declared as a global variable. Here’s a brief look at the relevant code snippets:

Using Global Variables

[[See Video to Reveal this Text or Code Snippet]]

Using Local Variables

[[See Video to Reveal this Text or Code Snippet]]

Observations

In the version using the global keyword, you can successfully toggle the value of is_small. However, in the local version, the function always resets is_small to True and fails to toggle it effectively. The main question is: Why does this happen?

Understanding Variable Scope

Global vs. Local Scope

Global Variables: These are accessible throughout the entire program. By declaring is_small as a global variable, any changes made within togglesize() directly affect the global is_small, allowing the function to toggle the value as expected.

Local Variables: When you define is_small inside togglesize(), it is treated as a local variable. This means any modifications to is_small do not affect the variable outside of the function. Each time togglesize() is called, is_small starts off with the same value (in this case, True).

Why Local Version Doesn’t Work

In the local function, although you can toggle is_small within the function, the changes do not persist once the function exits. Therefore, every time you call togglesize(), is_small is reset, preventing the toggle effect.

Solution: Passing Variables as Arguments

Instead of relying on global variables, a cleaner approach is to modify the function such that it accepts is_small as an argument and returns the new value. Here’s how you can do it:

Updated Function Definition

[[See Video to Reveal this Text or Code Snippet]]

Calling the Function

You can use the function like this:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Solution

Function Argument: By passing is_small into the function, you ensure that any modifications apply not just locally but also when you receive the return value.

Return Value: The function will now return the modified value of is_small, allowing it to reflect any changes you have made.

Conclusion

Understanding the scope of variables in Python is crucial for writing effective and predictable code. By using global variables, you may achieve results, but it is often better practice to pass variables as arguments and return their updated values. This approach leads to clearer, cleaner code that adheres to Python’s design philosophy of readability.

Now, you can effectively manage variable states within your functions without relying on global variables, making your code more maintainable and robust!