[Python-ideas] a in x or in y

[MRAB]

On 2014-02-13 11:27, Chris Angelico wrote:
> On Thu, Feb 13, 2014 at 9:57 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>> - given the restrictions on the parser, is this even possible? and
>
> Generalizing the syntax, I'd see this as:
>
> operand1 binary-op1 operand2 {and|or} binary-op2 operand3
>
> which implicitly places the value (not the code) of operand1 between
> and/or and binary-op2. Since all binary operators have higher
> precedence than either and or or (all that's lower is lambda and
> if/else), this notation is currently guaranteed to fail... except in
> two cases, namely + and -, which exist in unary form as well. So if +
> and - are excluded, it should be unambiguous. Whether or not the
> parser can actually handle it is a question for someone who knows what
> he's talking about, though :)
>
> The way I see it, there should ideally be no syntactic rule against
> using different operators on the two sides:
>
> input("> ") in legalcommands and not in forbiddencommands
> value > 20 or in {3,5,7,11}
>
> even though it would allow insanity:
>
> if 5 < int(input("Enter a number: ")) or < int(input("Greater than
> five please: ")) or < int(input("Come on now! ")) or == print("Bah, I
> give up."):
>      print("Thank you.")
>
> In this way, it's like chained comparisons:
>
> 1 < x <= 10
>
> which don't mind mixing and matching.
>
I think it would require 3-token lookahead (there would be new binary
operators such "and in" and "and not in"). It's certainly achievable.