How to sort a list using a Boolean compare function using the Python sorted function

Question

I have a binary function which returns a Boolean indicating whether the first argument is strictly less than the second argument. Can I use the sorted(my_list, key=functools.cmp_to_key(boolean_cmp)) or some variant thereof to sort this list? Or do I need to refactor the boolean_cmp function so that it returns a positive, negative, or zero integer?

Do I really need to distinguish between the greater-than case and the equal-to case?

The boolean_cmp function is written as a several methods spread out over the program. Refactoring them would be possible, but seems like more work than is necessary.

I've done a bit of experimentation and it looks like converting False to 0 and True to -1 causes sorted to sort correctly. But my experiment is only anecdotal.

BTW, I was reading Sorting HOW TO and it took me a long time to figure out that the underlying compare function needed an integer return value rather than a Boolean. In my opinion, it would have been nice if that had been clearer. [grumble]

Rather than refactor it (since you use it elsewhere) wrap it in a lambda or proper function that returns what you want to return. — JonSG
– JonSG, Commented Jun 23, 2021 at 12:58
@JonSG, is it sufficient to convert False to 0 and True to -1, or do I need to distinguish between the == case and the > case? Because if I need to distinguish all three cases, I cannot do this with a simple wrapper function — Jim Newton
– Jim Newton, Commented Jun 23, 2021 at 13:00
How about lambda a, b : 1 if boolean_cmp(a, b) else -1 if boolean_cmp(b, a) else 0? — JonSG
– JonSG, Commented Jun 23, 2021 at 13:16
Though I suspect lambda a, b : 1 if boolean_cmp(a, b) else -1 might be sufficient — JonSG
– JonSG, Commented Jun 23, 2021 at 13:18
@JonSG I suggest posting that as an answer, since I don't think there will be any better answer than that to this question. — Stef
– Stef, Commented Jun 23, 2021 at 15:48

JonSG · Accepted Answer · 2021-06-24 13:43:40Z

1

Rather than refactor a method you already have in use, I recommend that you wrap a lambda around it that will try to return the -1, 0, 1 that functools.cmp_to_key() is hoping for.

Maybe something like:

lambda a, b : 1 if boolean_cmp(a, b) else -1 if boolean_cmp(b, a) else 0

edited Jun 24, 2021 at 13:43

answered Jun 23, 2021 at 16:41

JonSG

13.6k2 gold badges32 silver badges48 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How to sort a list using a Boolean compare function using the Python sorted function

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related