← Back

Home › Problems › Using a variable before it's defined

Using a variable before it's defined

🐛 Fix the bug Topic: functions Difficulty: medium Free

What this problem practises

Function bugs are mostly about names and values: positional argument order, default parameters evaluated once at definition time, local versus global scope, and the implicit None returned whenever a function has no return.

This is a fix-the-bug problem: a short program is given in its broken state, and the task is to repair the code so it produces the required output. The interactive editor runs your fixed version in the browser and checks it for you — no setup, no local interpreter.

Difficulty: medium — expect to hold two interacting rules in your head at once, such as a boundary combined with a mutation.

The challenge

This should calculate and print the area of a rectangle, but it crashes. Fix it.

The code runs in PyDebug’s in-browser compiler. Open the problem, run the snippet, watch the exact error or output, and keep iterating until it is right. Signed-in solves count toward XP and your daily streak.

The error it raises

Run the broken snippet and Python stops with this exact error: NameError: name 'total' is not defined. Match it to the Python errors guide, then repair the line that produced it — knowing the message is half the fix.

The snippet

The interactive editor pre-fills exactly this code, broken and ready to repair:

def area():
    print(length * width)

length = 5
width = 3
area()
print(total)

Understand the rule

This bug is about statement order: the code reads a variable on a line that runs before the assignment that creates it. Python executes a script strictly top to bottom, and a function body runs not where it is written but where it is called — so an assignment placed later in the file has simply not happened yet when the earlier line asks for the value.

How to trace it

  1. Number the lines in execution order, which for straight-line code is top to bottom.
  2. Mark the line where the variable is first assigned and the line where it is first read.
  3. If the read comes first, move the assignment above it — or move the reading code into a function called later.
  4. Re-run and confirm the same name is not also shadowed inside a function scope.

Common wrong fixes

Rule to remember

Assignment must execute before use. Python has no hoisting: the position of a line in the file matters only through when it actually runs.

Solve it in the interactive editor →

🔢 Learning path: Variables and Data Types

Problem 2 of 13 in this path. Work them in order — each one builds on the rule before it.

← Previous problemNext in path: Next problem →

More functions problems

Keep going