Question

when reading a python ndb api of google app engine, I cam across this line.

Guestbook.query().order(-Guestbook.post_date)

Guestbook is an entity class and post_date is a property class.

But how order function can take that as a input ? its not value its name, more specifically
its definition of class.

How to define function to take such an input ?

Answer 1

Guestbook.post_date is an instance of a class that implements the __neg__ method to return an object that will tell the database API to order by that column descending.

Here’s a small example on how it works (obviously the real class is much more complex):

class SqlOrder(object):
    def __init__(self, column, order='ASC'):
        self.column = column
        self.order = order
    def __neg__(self):
        return SqlOrder(self.column, 'DESC')
    def __str__(self):
        return '%s %s' % (self.column, self.order)

>>> foo = SqlOrder('foo')
>>> str(foo)
'foo ASC'
>>> str(-foo)
'foo DESC'