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join() and split()

🧠 Predict the output Topic: strings Difficulty: medium Free

What this problem practises

Python strings are immutable sequences. "Fixing" a string always produces a new one, so the return value has to be captured; slicing rules apply, and every method — upper(), strip(), split(), join() — behaves differently from the in-place mutation you might be picturing.

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:

sentence = "I love Python programming"
words = sentence.split()
print(len(words))
print("-".join(words))

Understand the rule

This PyDebug exercise focuses on join and split. 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 join and split.
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 join and split 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 6 of 18 in this path. Work them in order — each one builds on the rule before it.

← String startswith and endswith Next in path: zip() combining two lists →

Keep going

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