I find it's a valid expression: `1 in [1, 2, 3] in [4, 5, 6]`

`a in b in c` is equivalent to `a in b and b in c` 

but this expression seems useless, and easy to confused with (a in b) in c .
in this program, what I originally want is `if a in b and a in c` , But it was mistakenly written as `a in b in c`

This expression is similar to `a<b<c` 。
But it seems very unreasonable here, is this a bug ?

This is the expected behavior, documented here: https://docs.python.org/3/reference/expressions.html#comparisons


That page says:

    * The comparison operators are  "<" | ">" | "==" | ">=" | "<=" | "!=" | "is" ["not"] | ["not"] "in"

    * "Comparisons can be chained arbitrarily"

    * "Note that a op1 b op2 c doesn’t imply any kind of comparison between a and c, so that, e.g., x < y > z is perfectly legal (though perhaps not pretty)."


So I'll close this for now.

I think it would be hard to change this behavior without introducing a needless backwards-incompatibility, but if you have a proposal, you could bring it up on the Python-Ideas mailing list.