datetime to string, easy; string to datetime... oh my, my!

Going from a datetime object to a string in Python: 

import datetime
str(datetime.datetime.now())

>>> '2008-10-10 16:40:25.126049'

String to datetime: datetime.datetime.from_string('2008-10-10 16:40:25.126049')?

Nope, you need a bit more code courtesy of Kelly Yancey (or a C extension, which you cannot use on Google App Engine): 

import re
from datetime import datetime

def parseDateTime(s):
  """Create datetime object representing date/time
     expressed in a string

  Takes a string in the format produced by calling str()
  on a python datetime object and returns a datetime
  instance that would produce that string.

  Acceptable formats are: "YYYY-MM-DD HH:MM:SS.ssssss+HH:MM",
              "YYYY-MM-DD HH:MM:SS.ssssss",
              "YYYY-MM-DD HH:MM:SS+HH:MM",
              "YYYY-MM-DD HH:MM:SS"
  Where ssssss represents fractional seconds.   The timezone
  is optional and may be either positive or negative
  hours/minutes east of UTC.
  """
  if s is None:
    return None
  # Split string in the form 2007-06-18 19:39:25.3300-07:00
  # into its constituent date/time, microseconds, and
  # timezone fields where microseconds and timezone are
  # optional.
  m = re.match(r'(.*?)(?:\.(\d+))?(([-+]\d{1,2}):(\d{2}))?$',
         str(s))
  datestr, fractional, tzname, tzhour, tzmin = m.groups()

  # Create tzinfo object representing the timezone
  # expressed in the input string.  The names we give
  # for the timezones are lame: they are just the offset
  # from UTC (as it appeared in the input string).  We
  # handle UTC specially since it is a very common case
  # and we know its name.
  if tzname is None:
    tz = None
  else:
    tzhour, tzmin = int(tzhour), int(tzmin)
    if tzhour == tzmin == 0:
      tzname = 'UTC'
    tz = FixedOffset(timedelta(hours=tzhour,
                   minutes=tzmin), tzname)

  # Convert the date/time field into a python datetime
  # object.
  x = datetime.strptime(datestr, "%Y-%m-%d %H:%M:%S")

  # Convert the fractional second portion into a count
  # of microseconds.
  if fractional is None:
    fractional = '0'
  fracpower = 6 - len(fractional)
  fractional = float(fractional) * (10 ** fracpower)

  # Return updated datetime object with microseconds and
  # timezone information.
  return x.replace(microsecond=int(fractional), tzinfo=tz)

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