Unexpected result with `in` operator chaining

问题

As far as I know, the in operator in Python can't be chained or at least I couldn't find any info on it, here is my problem

Here is the code

arr = [1, True, 'a', 2]
print('a' in arr in arr)  # prints False
print(('a' in arr) in arr)  # prints True

What I don't understand is the first print, I know in the second the first in returns True and then it check if True is in arr, but what about the first one? Does it check if 'a' is in arr and then if arr is in arr?

回答1:

The premise is false; the in operator can be chained. See Comparisons in the docs:

comp_operator ::=  "<" | ">" | "==" | ">=" | "<=" | "!="
                   | "is" ["not"] | ["not"] "in"

So, just as with any other chained comparison, a in b in c is equivalent to (a in b) and (b in c) (except that b is only evaluated once.

The reason 'a' in arr in arr is false is that arr in arr is false. The only time x in x is true is if x is type that does substring comparisons for __contains__ (like str or bytes), or if it's a container that actually contains itself (like lst = []; lst.append(lst)).

来源：https://stackoverflow.com/questions/50866585/unexpected-result-with-in-operator-chaining