> Please recall that chained comparisons such are

> >>> A < B < C

> is to produce simple code when B is not an identifier, but an expression. For example

> >>> low < len(x) < high

> or

> >>> inner ** 2 < x**2 + y**2 + z**2 < outer**2

> which determines if (x, y, z) is in a spherical shell.

Even in those cases, each individual expression could be stored in a variable before doing the comparison:

>>> a < b and b < c

But then again I don't find the rich boolean comparison operators (<, <=, >, =>, ==, !=) to be an issue when they're chained together on standard numeric types such as the above example, particularly when it's the same operator. IMO, it only becomes difficult to read when the operators start to become "mixed up"; more so when the objects are not the builtin types and have overridden operators:

>>> a is b < c in d

Not only do you have to sort out what is happening logically with the operators, you also have to figure out what those operators actually do for that particular object. Whether or not that is "readable" might be somewhat subjective though.

But I think we're probably veering a bit off topic here, the main focus of this thread is on whether or not "A in B < C" should be permissible syntax. Although I'm not a fan of the syntax, I don't think it would be worthwhile for it to raise a SyntaxError for the reasons I mentioned previously. This seems to be something to either enforce in a linter or manually revise in a code review process, rather than through the language itself.