day_generator.py

from datetime import timedelta, date
from copy import copy

# Takes either string in form "2014-01-01" or date, and return days with day_step in between
# If start is after end and day_step is not specified, it will change it to -1 automatically, otherwise
# day_step defaults to 1 day
def day_generator(start_day=None, end_day=None, day_step=None):
    if start_day is None:
        start_day = date.today() - timedelta(day=1)
    elif type(start_day) == str:
        year, month, day = [ int(x) for x in start_day.split('-') ]
        start_day = date(year, month, day)

    if end_day is None:
        end_day = start_day
    elif type(end_day) == str:
        year, month, day = [ int(x) for x in end_day.split('-') ]
        end_day = date(year, month, day)

    current_day = copy(start_day)
    
    if day_step is None:
        if end_day < start_day:
            day_step = -1
        else:
            day_step = 1
    
    range_start = min(start_day, end_day)
    range_end = max(start_day, end_day)
    
    while current_day >= range_start and current_day <= range_end:
        yield current_day
        current_day += timedelta(days=day_step)