Why "a in b == c" is equal to "(a in b) and (b == c)"

Question

I found that the result of evaluating a in b == c is always equal to (a in b) and (b == c). Could anyone explain that?

Here is an example running on Python3.7:

>>>> "123" in "1234" == "1234"
True

>>>> "123" in "1234" != "1234"
False

>>>> "123" in "1234" != "12345"
True

>>>> "123" in "1234" == "12345"
False

>>>> "888" in "1234" == "1234"
False

@Selcuk the putative dupe is a laughably horrendously bad fit for this question. — hobbs, Mar 24 '21 at 04:12
@hobbs I disagree. The question confirms the findings of this one, and the answer explains how it works internally. I am happy to add another duplicate to the list of duplicates if you suggest one though. — Selcuk, Mar 24 '21 at 04:14
Nice question. I hadn't appreciated how weird chaining can get! — Greg Ball, Mar 24 '21 at 04:25
The 1st dupe was not perfect, I added another one, which isn't perfect either. But there is no clear canonical dupe. I would even consider reopening this in case someone can answer it well, that would help the community I think and we can refer back to it. — jamylak, Mar 24 '21 at 04:27
Why are those expressions equivalent? Because the Python language designers decided they would be. There's not much more to it than that. — kaya3, Mar 24 '21 at 23:15
The `and` isn't explicitly added, the `in`, `==` and `!=` are evaluated left to right https://docs.python.org/3/reference/expressions.html#operator-precedence — OneCricketeer, Mar 24 '21 at 23:17
The relevant section of the docs is this: https://docs.python.org/3/reference/expressions.html#comparisons — kaya3, Mar 24 '21 at 23:21
@jamylak: I added new dupes, pretty sure the first two are basically perfect (even combining the same two operators). — ShadowRanger, Mar 24 '21 at 23:27
@ShadowRanger Feel free to close it again if you see fit, I see your point, I just thought that there wasn't 1 big detailed answer that got enough attention, but yes the first two do answer it — jamylak, Mar 24 '21 at 23:35

score 1 · Answer 1 · answered Mar 24 '21 at 23:22

Comparisons

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false

Where in is also a comparison operator

Also refer section on operator precedence to see that in and equality checks occur at the same time, left to right

Why "a in b == c" is equal to "(a in b) and (b == c)"

1 Answers1