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Sorting a list with mixed types

🐛 Fix the bug 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 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 sort a list of scores, but it crashes because of one bad entry. 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: '<' not supported between instances of 'str' and 'int'. 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:

scores = [88, 92, "75", 60]
sorted_scores = sorted(scores)
print(sorted_scores)

Understand the rule

Python 3 refuses to guess how a string compares to a number: sorted([3, 'one', 2]) raises TypeError: '<' not supported between instances of 'str' and 'int'. (Python 2 allowed it with arbitrary results — the change was deliberate.) The repair is deciding what the data should be: convert everything to one type, or sort with a key that imposes an order.

How to trace it

  1. Read the two type names in the message; they identify the incompatible pair the sort tried to compare.
  2. Inspect the list and ask why the types are mixed — often a number arrived as text from input() or a CSV.
  3. If they are all logically numbers, convert first: sorted(int(x) for x in items).
  4. If mixing is legitimate, provide a key that makes them comparable, e.g. key=str, and accept the ordering that implies.

Common wrong fixes

Rule to remember

Mixed str/int lists cannot be ordered — fix the data at its source or state an explicit key. The TypeError names both offending types, which points straight to the stray value.

Solve it in the interactive editor →

🔢 Learning path: Variables and Data Types

Problem 13 of 13 in this path. Work them in order — each one builds on the rule before it.

← Previous problem🏁 Path complete — review it →

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