Unexpected result with `in` operator chaining

Question

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

Answer 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)).