4.2 Scope and return values

Scope means where a variable is visible. Return values let functions send results back so the rest of the program can use them.

Scope: local vs global

Variables created inside a function are local: they only exist in that function.

def set_score():
    score = 100
    print(score)  # 100

set_score()
print(score)  # Error! score doesn't exist here

Variables defined outside functions (at the top of the file) are global and can be read inside functions. But if you assign to that name inside a function, you create a new local variable unless you use global (which we’ll avoid for now). So: prefer passing values in and returning values out instead of global variables.

Passing data in, getting data out

• Parameters pass data in.
• Return passes data out.

Example:

def add(a, b):
    total = a + b
    return total

x = add(3, 5)   # x is 8
print(add(10, 2))  # 12

You can return more than one value: return score, lives and then s, l = get_state().

Returning early

You can use return in the middle of a function to exit early:

def is_positive(n):
    if n <= 0:
        return False
    return True

A quick debugging tip

When something goes wrong:

1. Add print(...) to see what values variables have.
2. Check your indentation — wrong indent can cause logic errors or “unexpected indent” messages.
3. Read the error message: it often says which line and what went wrong (e.g. “name ‘x’ is not defined” means you used a variable that doesn’t exist in that scope).

Mini project: Helper functions

Write a small game or quiz that uses at least two functions that return values, e.g.:

• roll_dice() returns a random number 1–6.
• ask_question(question, answer) returns True if the user’s input matches the answer, else False.

Use the return values in your main loop to update score or state.

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Next: 5.1 Pygame setup and the game loop — open a window and run a game loop.