To me, Alexander's example
  >>> calendar.isleap("%d")
  False
is a buggy result. So I would reclassify the issue.

The rationale for not checking input types is that bad types result in an error, but that does not happen here due to a design decision that some consider clever and some buggy, and some both. (Guido himself would like to deprecate it.)

I am in favor of the 'year & 3 == 0' fix so that any input string (and indeed, anything that does not implement both % and & operators) raises an error. Intermediate to expert programmers should know or learn about bit masking. Add a comment to explain the reason -- and a test case.