Question

Python’s iterators are great and all, but sometimes I really do want a C-style for loop – not a foreach loop. For example, I have a start date and an end date and I want to do something for every day in that range. I can do this with a while loop, of course:

    current = start
    while current <= finish:
        do_stuff(current)
        current += timedelta(1)

This works, but it’s 3 lines instead of 1 (in C or C-based languages) and I often find myself forgetting to write the incrementing line, especially if the loop body is quite complex. Is there a more elegant and less error-prone way of doing this in Python?

Answer 1

The elegant and Pythonic way to do it is to encapsulate the idea of a range of dates in its own generator, then use that generator in your code:

import datetime

def daterange(start, end, delta):
    """ Just like `range`, but for dates! """
    current = start
    while current < end:
        yield current
        current += delta

start = datetime.datetime.now()
end = start + datetime.timedelta(days=20)

for d in daterange(start, end, datetime.timedelta(days=1)):
    print d

prints:

2009-12-22 20:12:41.245000
2009-12-23 20:12:41.245000
2009-12-24 20:12:41.245000
2009-12-25 20:12:41.245000
2009-12-26 20:12:41.245000
2009-12-27 20:12:41.245000
2009-12-28 20:12:41.245000
2009-12-29 20:12:41.245000
2009-12-30 20:12:41.245000
2009-12-31 20:12:41.245000
2010-01-01 20:12:41.245000
2010-01-02 20:12:41.245000
2010-01-03 20:12:41.245000
2010-01-04 20:12:41.245000
2010-01-05 20:12:41.245000
2010-01-06 20:12:41.245000
2010-01-07 20:12:41.245000
2010-01-08 20:12:41.245000
2010-01-09 20:12:41.245000
2010-01-10 20:12:41.245000

This is similar to the answer about range, except that the built-in range won’t work with datetimes, so we have to create our own, but at least we can do it just once in an encapsulated way.