Mockup of quick expression validator syntax

Here's a quick mockup of the idea.

The idea is to provide a special token VAL that constructs validators using magic methods so you can write:

@attr.s
class Test:
    positive = attr.ib(validator=[VAL.isa(int, float), VAL > 0.0])
    nonempty = attr.ib(validator=[VAL.len() >= 1])

And, of course, it can report the expression in the exception it raises.

Things to support in a real version:

• __getitem__ to check keys
• __getattr__ to allow property and method checks
• the other operators, especially in and such
• __call__ validates in the mockup, but attr.ib should tell the object to convert itself to a validator

I'd like to see what the community thinks before putting together a proper PR.

Start by reviewing the linked mockup and the existing attr.ib validator behavior. No implementation files or tests are named, and the requested scope includes multiple magic methods, operators, and conversion behavior. Done would require community agreement on the syntax and a properly scoped implementation with tests.