I would just add an exception to the isleap() function.

def isleap(year):
    """Return 1 for leap years, 0 for non-leap years."""
    try:
        return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
    except TypeError:
        # somehow inform the user that the argument 'year' must be an integer