pythonType errorEasy

PIN builder crashes on a list of digits

Time: 0:00

BugFind the bug and fix the code

What it should do

build_pin(digits) joins a list of digits into a PIN. Example: build_pin([1, 2, 3, 4]) → "1234".

What happens instead

build_pin([1, 2, 3, 4]) crashes: TypeError: sequence item 0: expected str instance, int found.

def build_pin(digits):
    return "".join(digits)

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

⏱️0:00

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Hint 3

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This bug belongs to the type errors pattern

Totals that concatenate, sorts that put 10 before 9, comparisons that refuse to run.

Learn the type errors pattern Step through your own code

Spoiler: why this bug happens(try it first; the hints get warmer one at a time)

What went wrong: join() only works with text. The digits are numbers.

The fix: return "".join(str(d) for d in digits)

Remember: convert numbers to text before join().

The pattern behind it: type errors.

More type errors to practise

Seen this error in your own code?

TypeError: sequence item 0: expected str instance, int found

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