Module 3 Lesson 5: Variable Scope

In Python, not all variables are accessible from everywhere. Some variables only exist inside a specific function, while others are available to the entire program. This concept is called Scope.

Lesson Overview

In this lesson, we will cover:

• Local Scope: Variables that live inside a function.
• Global Scope: Variables that live in the "open air."
• The LEGB Rule: How Python looks for variables.
• The global Keyword: Why you should use it sparingly.

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1. Local Scope ("Inside the House")

A variable created inside a function is Local to that function. It is born when the function starts and dies when the function ends.

def my_function():
    x = 10 # Local Variable
    print(f"Inside the function, x is {x}")

my_function()
# print(x) # This will cause an ERROR! 'x' doesn't exist out here.

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2. Global Scope ("On the Street")

A variable created in the main body of your Python file is Global. Any function can read it.

score = 100 # Global Variable

def show_score():
    print(f"The global score is {score}")

show_score() # Works perfectly

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3. The Shadowing Problem

What happens if you have a local variable with the same name as a global one? The local one "shadows" (hides) the global one.

message = "Global Message"

def show_message():
    message = "Local Message" # This is a NEW local variable
    print(message)

show_message() # Prints "Local Message"
print(message)  # Prints "Global Message" (still safe!)

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4. The global Keyword (Use with Caution)

If you want to change a global variable from inside a function, you must tell Python explicitly using the global keyword.

count = 0

def increment():
    global count # Tell Python we mean the global one
    count += 1

increment()
print(count) # Output: 1

Pro Tip: In professional coding, we try to avoid global variables whenever possible. It’s better to pass data in as Parameters and get data out as Return Values.

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Practice Exercise: The Bank Balance

Create a file named bank_scope.py.

1. Create a global variable balance = 500.
2. Write a function deposit(amount) that adds to the global balance.
3. Write a function withdraw(amount) that subtracts from the global balance.
4. Write a function show_balance() that simply prints the current balance.
5. Perform a few transactions and print the results.

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Quick Knowledge Check

1. What is a "Local Variable"?
2. Can a function read a Global Variable without the global keyword?
3. What happens to a local variable once the function finishes running?
4. Why is using too many global variables considered a "bad practice"?

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Key Takeaways

• Variable scope determines where a variable can be seen or used.
• Local variables are private to their functions.
• Global variables are accessible everywhere.
• Use global to modify global variables (but try to avoid it!).

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What’s Next?

We understand the mechanics, but how do we build programs that are truly reusable? In Lesson 6, we’ll look at Writing Reusable Code and the best practices for structuring your functions!