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Title: Understanding Python's Global Variables: Why Can't I Set a Global Variable?
Introduction:
Python provides a variety of variable scopes, including global, local, and nonlocal. Understanding how these scopes work is crucial for writing efficient and bug-free Python code. One common question that arises is, "Why can't I set a global variable in Python?" In this tutorial, we will explore Python's variable scopes and explain why setting global variables might not work as expected.
In Python, a global variable is a variable declared at the top level of a module or defined outside of any function or class. Global variables can be accessed and modified from any part of your code. However, setting global variables directly can lead to unexpected results due to the concept of variable scopes in Python.
Python defines several variable scopes, including:
Python uses the LEGB (Local, Enclosing, Global, Built-in) rule to determine the scope in which a variable is found. When you try to access a variable, Python searches for it in the order of these scopes, stopping when it finds the first occurrence.
To modify a global variable within a local scope, you must use the global keyword. This tells Python to treat the variable as global, allowing you to change its value. Here's an example:
In this example, we use the global keyword inside the function to specify that global_var is a global variable. Without the global keyword, Python would create a new local variable instead of modifying the global one.
The reason you might experience issues when setting global variables is because, by default, Python treats a variable as local if you try to assign a value to it within a function. For example:
In this case, Python creates a new local variable called global_var within the set_global_variable function scope, and it does not modify the global variable of the same name.