Parsing a date in python without using a default

Question

I\'m using python\'s dateutil.parser tool to parse some dates I\'m getting from a third party feed. It allows specifying a default date, which itself defaults

Answer 1

simple-date does this for you (it does try multiple formats, internally, but not as many as you might think, because the patterns it uses extend python's date patterns with optional parts, like regexps).

see https://github.com/andrewcooke/simple-date - but only python 3.2 and up (sorry).

it's more lenient than what you want by default:

>>> for date in ('2011-10-12', '2011-10', '2011', '10-12', '2011-10-12T11:45:30', '10-12 11:45', ''):
...   print(date)
...   try: print(SimpleDate(date).naive.datetime)
...   except: print('nope')
... 
2011-10-12
2011-10-12 00:00:00
2011-10
2011-10-01 00:00:00
2011
2011-01-01 00:00:00
10-12
nope
2011-10-12T11:45:30
2011-10-12 11:45:30
10-12 11:45
nope

nope

but you could specify your own format. for example:

>>> from simpledate import SimpleDateParser, invert
>>> parser = SimpleDateParser(invert('Y-m-d(%T| )?(H:M(:S)?)?'))
>>> for date in ('2011-10-12', '2011-10', '2011', '10-12', '2011-10-12T11:45:30', '10-12 11:45', ''):
...   print(date)
...   try: print(SimpleDate(date, date_parser=parser).naive.datetime)
...   except: print('nope')
... 
2011-10-12
2011-10-12 00:00:00
2011-10
nope
2011
nope
10-12
nope
2011-10-12T11:45:30
2011-10-12 11:45:30
10-12 11:45
nope

nope

ps the invert() just switches the presence of % which otherwise become a real mess when specifying complex date patterns. so here only the literal T character needs a % prefix (in standard python date formatting it would be the only alpha-numeric character without a prefix)

Answer 2

I ran into the exact same problem with dateutil, I wrote this function and figured I would post it for posterity's sake. Basically using the underlying _parse method like @ILYA Khlopotov suggests:

from dateutil.parser import parser
import datetime
from StringIO import StringIO

_CURRENT_YEAR = datetime.datetime.now().year
def is_good_date(date):
    try:
        parsed_date = parser._parse(parser(), StringIO(date))
    except:
        return None
    if not parsed_date: return None
    if not parsed_date.year: return None
    if parsed_date.year < 1890 or parsed_date.year > _CURRENT_YEAR: return None
    if not parsed_date.month: return None
    if parsed_date.month < 1 or parsed_date.month > 12: return None
    if not parsed_date.day: return None
    if parsed_date.day < 1 or parsed_date.day > 31: return None
    return parsed_date

The returned object isn't adatetime instance, but it has the .year, .month, and, .day attributes, which was good enough for my needs. I suppose you could easily convert it to a datetime instance.

Answer 3

This is probably a "hack", but it looks like dateutil looks at very few attributes out of the default you pass in. You could provide a 'fake' datetime that explodes in the desired way.

>>> import datetime
>>> import dateutil.parser
>>> class NoDefaultDate(object):
...     def replace(self, **fields):
...         if any(f not in fields for f in ('year', 'month', 'day')):
...             return None
...         return datetime.datetime(2000, 1, 1).replace(**fields)
>>> def wrap_parse(v):
...     _actual = dateutil.parser.parse(v, default=NoDefaultDate())
...     return _actual.date() if _actual is not None else None
>>> cases = (
...   ('2011-10-12', datetime.date(2011, 10, 12)),
...   ('2011-10', None),
...   ('2011', None),
...   ('10-12', None),
...   ('2011-10-12T11:45:30', datetime.date(2011, 10, 12)),
...   ('10-12 11:45', None),
...   ('', None),
...   )
>>> all(wrap_parse(test) == expected for test, expected in cases)
True

Answer 4

Depending on your domain following solution might work:

DEFAULT_DATE = datetime.datetime(datetime.MINYEAR, 1, 1)

def parse_no_default(dt_str):    
    dt = parser.parse(dt_str, default=DEFAULT_DATE).date()
    if dt != DEFAULT_DATE:
       return dt
    else:
       return None

Another approach would be to monkey patch parser class (this is very hackiesh, so I wouldn't recommend it if you have other options):

import dateutil.parser as parser
def parse(self, timestr, default=None,
          ignoretz=False, tzinfos=None,
          **kwargs):
    return self._parse(timestr, **kwargs)
parser.parser.parse = parse

You can use it as follows:

>>> ffffd = parser.parser().parse('2011-01-02', None)
>>> ffffd
_result(year=2011, month=01, day=02)
>>> ffffd = parser.parser().parse('2011', None)
>>> ffffd
_result(year=2011)

By checking which members available in result (ffffd) you could determine when return None. When all fields available you can convert ffffd into datetime object:

# ffffd might have following fields:
# "year", "month", "day", "weekday",
# "hour", "minute", "second", "microsecond",
# "tzname", "tzoffset"
datetime.datetime(ffffd.year, ffffd.month, ffffd.day)