Alex Hall

Python metaprogrammer

Oct 27, 2020

Since 0 is considered a "Falsy" value, then the comparison returns 'True`.

No, that's the kind of thing JavaScript does. Most Falsy values (None, empty string/list/dict/etc) are not equal to False.

False == 0 and True == 1 because bool is a subclass of int. You can treat booleans just like those numbers, for example True + True == 2. This can be very useful.

Miguel Brito

Machine Learning Software Engineer - Hobbyst Technical Writer - Interested in Software Testing, Best Practices, Scalability, and Python.

Oct 27, 2020

Interesting, thanks for the correction. I'll update the post.

Regardless, I wish it boolean wasn't subclass of ints.

IMHO, True + True + True shouldn't be equal to 3, or False == 0. This would avoid bugs such as the ones when using chained comparisons.

Alex Hall

Python metaprogrammer

Oct 27, 2020

To me it seems obvious that the chained comparisons wouldn't work. I've never seen someone make that mistake before. Doing that breaks extremely easily without the fact about booleans. If a == b == c is True, then (a == b) == c is guaranteed to be False unless they are all equal to True (or 1).

I often use this property of booleans to write something like sum(predicate(x) for x in lst) to count the number of items in a list satisfying a condition, or conveniently making something plural with f"thing{(len(things) > 1) * 's'}". It's hacky but it's fun.

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