Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the type errors pattern
Totals that concatenate, sorts that put 10 before 9, comparisons that refuse to run.
What went wrong: In Python 3, filter() returns a lazy iterator that does not evaluate length or support len().
The fix: Use a list comprehension to produce a concrete list: len([u for u in users if u.get('active')])
Remember: In Python 3, filter() and map() return iterators without len(); materialize into a list to check length.
The pattern behind it: type errors.
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