When to Use the Global Keyword for Global Variables?

Function Scope and Global Variables

When using functions in programming, it's important to understand the concept of scope. Functions create their own private namespace, separate from the global scope. This means that variables declared within a function are only accessible within that function.

Problem: Function Not Modifying Global Variable

Consider the following code:

done = False

def function():
    for loop:
        code
        if not comply:
            done = True

while done == False:
    function()

The intention of this code is to exit the while loop when done is set to True within the function(). However, the code doesn't exit the loop after function() sets done to True.

Why the Problem Occurs

The problem arises because the variable done in the function() is a local variable, created within the function's namespace. It's separate from the global variable done that's defined outside the function. When the function() assigns a new value to done, it's actually modifying the local variable, not the global one.

Solution: Using global

To fix this issue, you need to use the global keyword to explicitly reference the global variable within the function. Here's the corrected code:

def function():
    global done
    for loop:
        code
        if not comply:
            done = True

By using global, you ensure that the done variable inside the function refers to the same variable that's defined in the global scope. This allows you to modify the global variable from within the function.

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