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sorted() with a key function
🧠 Predict the output Topic: sorting Difficulty: medium Free
What this problem practises
The two sorting APIs are not interchangeable: sorted() returns a brand-new list while list.sort() sorts in place and returns None. Key functions compare the values they extract, and comparing mixed types raises TypeError.
This is an output-prediction problem: read the snippet carefully and write down exactly what it prints before anything runs. These questions are interview staples because they test whether you know the language rules, or merely recognise their syntax.
Difficulty: medium — expect to hold two interacting rules in your head at once, such as a boundary combined with a mutation.
The challenge
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 snippet
The interactive editor pre-fills exactly this code — read it, then submit your prediction:
words = ["banana", "kiwi", "apple", "fig"]
print(sorted(words, key=len))Understand the rule
This PyDebug exercise focuses on sorted with a key function. It is a small reproducible Python debugging task: identify the input and expected state, then predict each statement before changing code. When output differs, inspect types, boundaries, mutation, and control flow instead of guessing.
How to trace it
- Read the prompt and write the expected output for sorted with a key function.
- Run the code mentally, recording values after each loop or function call.
- Compare actual and expected states and locate the first divergence.
- Make the smallest fix, then test a normal and a boundary case.
Common wrong fixes
- Changing several lines at once and hiding the original cause.
- Checking only the happy path and missing zero, empty, or boundary input.
- Confusing printed representation with the stored value.
Rule to remember
Treat sorted with a key function as a focused experiment. A reliable solution explains the output, handles its edge case, and can be verified with a small test in the browser editor.
🔮 Learning path: Output Prediction
Problem 17 of 18 in this path. Work them in order — each one builds on the rule before it.
← Dictionary comprehension with conditionNext in path: while loop with a condition update →
More sorting problems
Keep going
- Output-prediction strategies — how to predict tricky snippets without running them.
- Browse every sorting problem in this topic.
- Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.