Python argparse: Create timedelta object from argument?

Question

I am attempting to use argparse to convert an argument into a timedelta object. My program reads in strings supplied by the user and converts them to various datetime objects for later usage. I cannot get the filter_length argument to process correctly though. My code:

import datetime
import time
import argparse

def mkdate(datestring):
    return datetime.datetime.strptime(datestring, '%Y-%m-%d').date()

def mktime(timestring):
    return datetime.datetime.strptime(timestring, '%I:%M%p').time()

def mkdelta(deltatuple):
    return datetime.timedelta(deltatuple)

parser = argparse.ArgumentParser()
parser.add_argument('start_date', type=mkdate, nargs=1)
parser.add_argument('start_time', type=mktime, nargs=1, )
parser.add_argument('filter_length', type=mkdelta, nargs=1, default=datetime.timedelta(1))#default filter length is 1 day.

I run the program, passing 1 as the timedelta value (I only want it to be one day):

> python program.py 2012-09-16 11:00am 1

But I get the following error:

>>> program.py: error: argument filter_length: invalid mkdelta value: '1'

I don't understand why the value is invalid. If I call the mkdelta function on its own, like this:

mkdelta(1)
print mkdelta(1)

It returns:

datetime.timedelta(1)
1 day, 0:00:00

This is exactly the value that I'm looking for. Can someone help me figure out how to do this conversion properly using argparse?

Answer 1

Notice the quotes around '1' in your error message? You pass a string to mkdelta, whereas in your test code, you pass an integer.

Answer 2

Your function doesn't handle a string argument, which is what argparse is handing it; call int() on it:

def mkdelta(deltatuple):
    return datetime.timedelta(int(deltatuple))

If you need to support more than days, you'll have to find a way to parse the argument passed in into timedelta arguments.

You could, for example, support d, h, m or s postfixes to denote days, hours, minutes or seconds:

_units = dict(d=60*60*24, h=60*60, m=60, s=1)
def mkdelta(deltavalue):
    seconds = 0
    defaultunit = unit = _units['d']  # default to days
    value = ''
    for ch in list(str(deltavalue).strip()):
        if ch.isdigit():
            value += ch
            continue
        if ch in _units:
            unit = _units[ch]
            if value:
                seconds += unit * int(value)
                value = ''
                unit = defaultunit
            continue
        if ch in ' \t':
            # skip whitespace
            continue
        raise ValueError('Invalid time delta: %s' % deltavalue)
    if value:
        seconds = unit * int(value)
    return datetime.timedelta(seconds=seconds)

Now your mkdelta method accepts more complete deltas, and even integers still:

>>> mkdelta('1d')
datetime.timedelta(1)
>>> mkdelta('10s')
datetime.timedelta(0, 10)
>>> mkdelta('5d 10h 3m 10s')
datetime.timedelta(5, 36190)
>>> mkdelta(5)
datetime.timedelta(5)
>>> mkdelta('1')
datetime.timedelta(1)

The default unit is days.

Answer 3

You could use a custom action to collect all the remaining args and parse them into a timedelta.

This will allow you to write CLI commands such as

% test.py 2012-09-16 11:00am 2 3 4 5
datetime.timedelta(2, 3, 5004)   # args.filter_length

You could also provide optional arguments for --days, --seconds, etc, so you can write CLI commands such as

% test.py 2012-09-16 11:00am --weeks 6 --days 0
datetime.timedelta(42)           # args.filter_length

% test.py 2012-09-16 11:00am --weeks 6.5 --days 0
datetime.timedelta(45, 43200)

---

import datetime as dt
import argparse

def mkdate(datestring):
    return dt.datetime.strptime(datestring, '%Y-%m-%d').date()

def mktime(timestring):
    return dt.datetime.strptime(timestring, '%I:%M%p').time()

class TimeDeltaAction(argparse.Action):
    def __call__(self, parser, args, values, option_string = None):
        # print '{n} {v} {o}'.format(n = args, v = values, o = option_string)
        setattr(args, self.dest, dt.timedelta(*map(float, values)))

parser = argparse.ArgumentParser()
parser.add_argument('start_date', type = mkdate)
parser.add_argument('start_time', type = mktime)
parser.add_argument('--days', type = float, default = 1)
parser.add_argument('--seconds', type = float, default = 0)
parser.add_argument('--microseconds', type = float, default = 0)
parser.add_argument('--milliseconds', type = float, default = 0)
parser.add_argument('--minutes', type = float, default = 0)
parser.add_argument('--hours', type = float, default = 0)
parser.add_argument('--weeks', type = float, default = 0)
parser.add_argument('filter_length', nargs = '*', action = TimeDeltaAction)

args = parser.parse_args()
if not args.filter_length:
    args.filter_length = dt.timedelta(
        args.days, args.seconds, args.microseconds, args.milliseconds,
        args.minutes, args.hours, args.weeks)
print(repr(args.filter_length))