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Calling a function before defining it

🐛 Fix the bug Topic: functions Difficulty: easy 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: easy — the fix or the trick is normally a single line, and a careful read should be enough.

The challenge

This should print a greeting, but throws a NameError. Fix the order.

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 'greet' 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:

greet("Sam")

def greet(name):
    print(f"Hello, {name}!")

Understand the rule

Python executes a module from top to bottom. A function name must be bound before the call runs. A def statement creates that binding when execution reaches it; the fact that the definition appears later in the file does not help an earlier call.

How to trace it

  1. Read the file in execution order rather than scanning for a definition anywhere below.
  2. At the call site, ask whether the name already exists in the current scope.
  3. Move the definition above the call, or place the call in a main guard after all definitions.
  4. Check imports separately because an imported name must also be available before use.

Common wrong fixes

Rule to remember

Definitions are executable statements. Put reusable definitions first and start the program after them, commonly with if __name__ == '__main__'.

Solve it in the interactive editor →

🧩 Learning path: Functions

Problem 1 of 12 in this path. Work them in order — each one builds on the rule before it.

↩ Path overviewNext in path: Calling a function with too few arguments →

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