How to sort a list and handle None values properly?

Question

I am trying to sort a list of objects using the date attribute with

list_of_objects.sort(key=lambda x: x.date, reverse=True)

but some dates are just None, which means that I get the error

TypeError: can't compare datetime.datetime to NoneType

is there a way to account for this? e.g. Have objects with date == None at the top or bottom of the sorted list—or do I need to do this manually?

Answer 1

You want to sort based on two properties:

1. Primarily, whether a date is present
2. Secondarily, the date (if present)

You can express your intent in a straightforward manner by sorting on a tuple, where

1. the first element of the tuple specifies whether the date is None, and
2. the second element of the tuple is the date itself.

list_of_objects.sort(key=lambda x: (x.date is None, x.date), reverse=True)

This approach circumvents the type error that you are getting, because comparison between tuples is performed left-to-right lazily. The second tuple elements don't get compared unless the first elements are equal.

Here are some examples demonstrating the concept:

>>> xs = [None, 1, 3, None, 2]

>>> sorted(xs, key=lambda x: (x is None, x))
[1, 2, 3, None, None]

>>> sorted(xs, key=lambda x: (x is not None, x))
[None, None, 1, 2, 3]

>>> sorted(xs, key=lambda x: (x is None, x), reverse=True)
[None, None, 3, 2, 1]

>>> sorted(xs, key=lambda x: (x is not None, x), reverse=True)
[3, 2, 1, None, None]

Answer 2

You can modify your lambda slightly:

from datetime import datetime

list_of_objects.sort(key=lambda x: x.date or datetime.min, reverse=True)

If they're appearing at the wrong end of the sort, use datetime.max instead.