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Calling a function with too few arguments
🐛 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 area of a rectangle, but 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: TypeError: area() missing 1 required positional argument: 'width'. 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(length, width):
return length * width
print(area(5))Understand the rule
TypeError: missing 1 required positional argument means the call supplied fewer values than the def line demands. Python checks arity at call time and its message does the accounting for you — it names the exact missing parameter. A closely related trap: calling a method on the class instead of an instance leaves self unfilled and produces the same error.
How to trace it
- Read the missing parameter's name in the message; it is quoted directly.
- Compare the call against the def line, matching positionally left to right.
- If the missing name is self, the call is on the class (Dog.bark()) instead of an instance (d.bark()).
- Decide the fix: pass the argument, or give the parameter a default in the signature if it is genuinely optional.
Common wrong fixes
- Adding a default value to silence the error when the function cannot actually do its job without the argument.
- Editing the function to drop the parameter while other call sites still pass it — creating the opposite TypeError.
- Ignoring the quoted name and guessing which argument is missing instead of reading it.
Rule to remember
Python's arity errors name the exact missing parameter — read it, then look at the call site. Defaults are for genuinely optional values, not for making error messages disappear.
🧩 Learning path: Functions
Problem 2 of 12 in this path. Work them in order — each one builds on the rule before it.
← Calling a function before defining itNext in path: Next problem →
More functions problems
Keep going
- Python debugging guide — a systematic method for finding bugs like this one fast.
- Browse every functions problem in this topic.
- Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.