Variable Scope

When you create a variable, it does not automatically become available everywhere in your program. The scope of a variable describes the region of code where that variable exists and can be used. Think of scope like a building with different rooms — a key that opens one room does not necessarily open all the others.

Global and Local Scope

There are two main types of scope you need to understand at this stage:

• Global scope — a variable created outside any function. It can be read from almost anywhere in the program.
• Local scope — a variable created inside a function. It only exists while that function is running, and is destroyed when the function finishes.

player_name = "Alex"        # global variable

def greet_player():
    message = "Welcome!"    # local variable
    print(player_name)      # OK — can read the global
    print(message)          # OK — local variable exists here

greet_player()
print(player_name)          # OK — still accessible globally
print(message)              # ERROR — message does not exist here

Why Scope Matters

Scope keeps different parts of a program from accidentally interfering with each other. If every variable were global, a function deep in your code could silently overwrite a value used somewhere else — a nightmare to debug.

Consider this example:

score = 0               # global score

def calculate_bonus(points):
    score = points * 2  # this creates a NEW local variable
    return score

result = calculate_bonus(10)
print(result)   # 20
print(score)    # 0 — the global score is unchanged

The score inside calculate_bonus is a completely separate variable that shadows the global one temporarily. The global score is never touched.

Modifying a Global Variable Inside a Function

If you genuinely need a function to change a global variable, Python requires you to declare it with the global keyword:

high_score = 0

def update_high_score(new_score):
    global high_score
    if new_score > high_score:
        high_score = new_score

update_high_score(45)
print(high_score)   # 45

Scope in a Nutshell

A Practical Example

def calculate_area(width, height):
    area = width * height   # local to this function
    return area

def calculate_perimeter(width, height):
    perimeter = 2 * (width + height)  # different local variable
    return perimeter

w = 5
h = 3
print(calculate_area(w, h))         # 15
print(calculate_perimeter(w, h))    # 16

Both functions have their own local variable. Neither can accidentally read or change the other's internal values, which makes both functions safe and independent.