poc_isoparse.py

from datetime import datetime, timedelta, timezone

def strptime_z(dt_str, fmt_str):
    # Hard-coded so this demo works, not an actual proposed strptime_z
    tzi = None
    if fmt_str.endswith('%:z'):
        tz_str = dt_str[-6:]
        h, m = map(int, tz_str.split(':'))
        tzi = timezone(offset=timedelta(hours=h, minutes=m))
        fmt_str = fmt_str[:-len('%:z')]
        dt_str = dt_str[:-6]

    return datetime.strptime(dt_str, fmt_str).replace(tzinfo=tzi)


def isoparse(dt_str, sep='T'):
    base_fmt = "%Y-%m-%d"

    len_str = len(dt_str)
    if len_str > 10:
        base_fmt += sep

    if len_str == 10:
        tail = ''
    elif len_str == 13:
        tail = '%H'                     # hours, no tzinfo
    elif len_str == 16:
        tail = '%H:%M'                  # minutes, no tzinfo
    elif len_str == 19:
        if dt_str[-6] in '-+':
            tail = '%H%:z'              # hours, with tzinfo
        else:
            tail = '%H:%M:%S'           # seconds, no tzinfo
    elif len_str == 22:
        tail = '%H:%M%:z'              # minutes, with tzinfo
    elif len_str in {23, 26}:
        tail = '%H:%M:%S.%f'            # milliseconds/microseconds, no tzinfo
    elif len_str== 25:
        tail = '%H:%M:%S%:z'            # seconds, with tzinfo
    elif len_str in {29, 32}:
        tail = '%H:%M:%S.%f%:z'         # milliseconds/microseconds, with tzinfo
    else:
        raise ValueError('Invalid isoformat string')

    return strptime_z(dt_str, base_fmt + tail)


if __name__ == "__main__":
    dt = datetime(2017, 4, 1, 14, 24, 32, tzinfo=tz.gettz('US/Eastern'))

    timespecs = ['hours', 'minutes',
                 'seconds', 'milliseconds', 'microseconds']
    tzinfos = [timezone(timedelta(hours=-4)), None]
    seps = ('T', ' ')

    outputs = []
    for tzi in tzinfos:
        for ii, timespec in enumerate(timespecs):
            for sep in seps:
                dt_tz = dt.replace(tzinfo=tzi)
                dt_str = dt_tz.isoformat(timespec=timespec, sep=sep)
                dt_comp = dt_tz
                if ii < timespecs.index('milliseconds'):
                    dt_comp = dt_comp.replace(microsecond=0)
                if ii < timespecs.index('seconds'):
                    dt_comp = dt_comp.replace(second=0)
                if ii < timespecs.index('minutes'):
                    dt_comp = dt_comp.replace(minute=0)

                assert dt_comp == isoparse(dt_str, sep=sep)