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List extend vs append

🧠 Predict the output Topic: lists Difficulty: easy Free

What this problem practises

Lists are mutable and assigned by reference. That one fact explains most list bugs: aliasing between what look like "copies", append() and sort() returning None, and slice boundaries that disagree with the index arithmetic.

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: easy — the fix or the trick is normally a single line, and a careful read should be enough.

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:

a = [1, 2]
b = [3, 4]
a.extend(b)
print(a)
print(len(a))

Understand the rule

This PyDebug exercise focuses on list extend vs append. 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 list extend vs append.
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 list extend vs append 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.

Solve it in the interactive editor →

🔮 Learning path: Output Prediction

Problem 11 of 18 in this path. Work them in order — each one builds on the rule before it.

← Checking membership in a list Next in path: List slicing with step →

Keep going

Output-prediction strategies — how to predict tricky snippets without running them.
Browse every lists problem in this topic.
Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.