util.py

import datetime
import string


class DateParser(object):
    """Parse various convenient date phrases"""
    DATE_FORMAT = '%Y-%m-%dT%H:%M:%SZ'

    @staticmethod
    def _parse_timedelta(n, unit):
        """
        Parse a timedelta in the given units (single chars or short strings)

        Returns a timedelta
        """
        mappings = {
            's': 'seconds',
            'm': 'minutes',
            'h': 'hours',
            'd': 'days',
            'w': 'weeks'
        }
        if unit not in mappings:
            raise ValueError('Unexpected unit: %s' % unit)

        data = {}
        data[mappings[unit]] = n
        return datetime.timedelta(**data)

    def _parse_token(self, s):
        """
        Parse the string into a datetime or timedelta, and remainder

        :param s: The string to parse

        :returns: A datetime or timedelta representing the first token in the
                  string, and the unparsed remainder of the string
        :rtype: ([datetime or timedelta], str)
        """
        if s is None:
            return None, None

        s = s.strip()
        if s == '':
            return None, None

        # If it starts with + or -, it'll be a timedelta
        if s[0] in ['+', '-']:
            mult = 1 if s[0] == '+' else -1
            s = s[1:].lstrip()
            phrase = s.split(' ')[0]
            s = s[len(phrase):]

            n = ''

            for c in phrase:
                if c not in string.digits:
                    break

                n += c

            unit = phrase[len(n):]

            try:
                n = int(n)
            except ValueError:
                raise ValueError(
                    'Expected a number after + or - but found %s' % n
                )

            return self._parse_timedelta(n, unit) * mult, s

        now = datetime.datetime.now()
        today = datetime.datetime(
            year=now.year,
            month=now.month,
            day=now.day
        )

        frag = s.split(' ')[0]
        s = s[len(frag):]

        if frag.lower() == 'now':
            return now, s

        if frag.lower() == 'today':
            return today, s

        if frag.lower() == 'yesterday':
            return today - datetime.timedelta(days=1), s

        if frag.lower() == 'tomorrow':
            return today + datetime.timedelta(days=1), s

        return datetime.datetime.strptime(frag, self.DATE_FORMAT), s

    def parse(self, s):
        value, remainder = self._parse_token(s)

        while remainder:
            next_token, remainder = self._parse_token(remainder)
            value += next_token

        return value