Elegant way to filter by none or not none

Question

The following SQLAlchemy code works, but looks un-pythonic:

if has_died: # has_died: True or False
  query = query.filter(User.died_at != None)
else:
  query = query.filter(User.died_at == None)

What is a more elegant way to add the filter?

I think it's mainly ugly that I have to rewrite the whole line just for a simple change from != to == ... — Thorben Croisé, Aug 18 '15 at 11:56
I don't know SQLAlchemy, but I assume there's some magic going on there that's returning something other than `True` or `False` for the comparisons. — Cyphase, Aug 18 '15 at 11:58
Yes, it is. This [magic](http://stackoverflow.com/a/21784915/1377864). I don't think it's possible to do it any better. — Yaroslav Admin, Aug 18 '15 at 12:00
Repeated code is a _sign_ that you should _make sure_ that you can't do it better. But sometimes it would take more/uglier code just to not have any repetition. — Cyphase, Aug 18 '15 at 12:04

score 5 · Answer 1 · answered Aug 18 '15 at 11:53

5

Well, you could do this:

query = query.filter((User.died_at != None) if has_died else (User.died_at == None))

But it's a bit hard to read. I think how you're doing it is fine.

answered Aug 18 '15 at 11:53

Cyphase

@Tobo, I agree, which is why I think you should stick with what you have :). – Cyphase Aug 18 '15 at 12:15

SuperBiasedMan · Answer 2 · 2015-08-18T11:55:07.787

3

You could rewrite it as one line with a ternary operator

query = query.filter((User.died_at != None) if has_died else (User.died_at == None))

edited Aug 18 '15 at 11:55

answered Aug 18 '15 at 11:53

SuperBiasedMan

score 0 · Answer 3 · answered Aug 18 '15 at 12:04

0

What about:

query = query.filter(bool(User.died_at) == has_died)

It returns:

Which is the intended behavior...

But then again, not sure if it is easier to understand than your piece of code!

answered Aug 18 '15 at 12:04

Challensois

1

`User.died_at != None` and `User.died_at == None` don't return a `bool`; they return a SQLAlchemy object. As linked by [Yaroslav Admin](https://stackoverflow.com/users/1377864/yaroslav-admin) in the question comments: https://stackoverflow.com/questions/21784851/sqlalchemy-is-not-null-select/21784915#21784915 – Cyphase Aug 18 '15 at 12:05
Oh I am not familiar with SQLAlchemy syntax. Stricly speaking in python, a `None` is considered as `False`. Is there a way to make this call valid ? Because then I think it is the most elegant way to do it. Otherwise sorry for my faulty contribution. – Challensois Aug 18 '15 at 12:06
No problem. Even if it behaved closer to how you expected, while `bool(None) == False`, `None != False`, so they're not the same thing. You could have a value that could be `True`, `False`, or `None`, with each one having it's own meaning. For example, if it's the answer to a yes/no question, they might mean `Yes`, `No`, and `Unanswered`. – Cyphase Aug 18 '15 at 12:10
Indeed, my comment could be misleading, it is `bool(None)` that is considered as `False`. It is true that `None`, `False` and `True` all have different meanings. – Challensois Aug 18 '15 at 12:14

3 Answers3