Python generator to construct range of dates

date-range.py

from datetime import datetime, timedelta

def date_range(start, end, step=7, date_format="%m-%d-%Y"):
    """
    Creates generator with a range of dates.
    The dates occur every 7th day (default).
    
    :param start: the start date of the date range
    :param end: the end date of the date range
    :param step: the step size of the dates
    :param date_format: the string format of the dates inputted and returned
    """
    start = datetime.strptime(str(start), date_format)
    end = datetime.strptime(str(end), date_format)
    num_days = (end_date - start_date).days
    
    for d in xrange(0, num_days + step, step):
        date_i = start + timedelta(days=d)
        yield date_i.strftime(date_format)

Thank you!