What's the best way to do an existence check on a cookie with python/GAE?

Question

In my code I'm using

user_id = self.request.cookies.get( 'user_id', '' )

if user_id != '':
        me = User.get_by_id( int( user_id ) )

but to me this doesn't look right even though it technically works...it looks imprecise. Is there a better way I can check for the existence of a cookie?

Answer 1

I've never use AppEngine but I guess request.cookies is just a normal dictionary object like for example in Django. You can try the following:

if 'user_id' in self.request.cookies:
    # cookie exists

Answer 2

Try and Except clauses are very handy for situations like this where you need a clear and obvious workflow with a hair trigger invalidate everything catch all.

To be clear, this does not approach the various nuances involved in securely tracking/managing a user session by leaving data on the client.

try:
  user_id = self.request.cookies['user_id'] #will raise a 'KeyError' exception if not set.
  if isinstance(user_id, basestring):
    assert user_id # will raise a 'ValueError' exception if user_id == ''.
    try:
      user_id = int(user_id)
    except ValueError:
      logging.warn(u'user_id value in cookie was of type %s and could not be '
        u'coerced to an integer. Value: %s' % (type(user_id), user_id))
  if not isinstance(user_id, int):
    raise AssertionError(u'user_id value in cookie was INVALID! '
      u'TYPE:VALUE %s:%s' % (type(user_id), user_id))
except KeyError:
  # 'user_id' key did not exist in cookie object.
  logging.debug('No \'user_id\' value in cookie.')
except AssertionError:
  # The cookie value was invalid!
  clear_the_cookie_and_start_again_probably()
except Exception, e:
  #something else went wrong!
  logging.error(u'An exception you didn\'t count on. Exception: %s' % e)
  clear_the_cookie_and_start_again_probably()
  raise e
else:
  me = User.get_by_id(user_id)