from datetime import datetime
import pytz


def with_gmt_offset(timezones, now=None):
    """
    Given a list of timezones (either strings of timezone objects),
    return a list of choices with
        * values equal to what was passed in
        * display strings formated with GMT offsets and without
          underscores. For example: "GMT-05:00 America/New York"
        * sorted by their timezone offset
    """
    now = now or datetime.utcnow()
    _choices = []
    for tz in timezones:
        tz_str = str(tz)
        delta = pytz.timezone(tz_str).utcoffset(now)
        display = "GMT{sign}{gmt_diff} {timezone}".format(
            sign='+' if delta == abs(delta) else '-',
            gmt_diff=str(abs(delta)).zfill(8)[:-3],
            timezone=tz_str.replace('_', ' ')
        )
        _choices.append((delta, tz, display))
    _choices.sort(key=lambda x: x[0])
    choices = [(one, two) for zero, one, two in _choices]
    return choices