The tests named “rejects span longer than string length” and “rejects empty string and nonzero span” in the problem specifications use the error message “span must be smaller than string length”.
It conflicts with the first test, which is called “finds the largest product if span equals length”. Something like “span cannot be larger than string length” would be more accurate because span and string length can be equal.
I am not sure if it is going to be accepted because this issue has already been raised in a Python topic; however, I raise the discussion again because the problem specification is involved.