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just use "_P", a fairly-widely-recognized substitute from the Lisp world.

As a python programmer I would have no idea what `valid_P` is supposed to mean. Valid Pascals?

P is shaped like a question mark, though I think it was retronymed to stand for "Predicate"

I think "predicate" is the original meaning of that postfix.

Emacs Lisp, at least, can use question marks in names without batting an eyelash.

I see no reason that would not have been true for Lisps generally since their birth.

The suffix is just "p" in Common Lisp[1], so I think the "predicate" explanation is more likely to be correct than the visual analogy to the question mark.

[1] http://www.lispworks.com/documentation/HyperSpec/Body/f_list...

Or even better, just `valid?` Oh, wait .... that's Ruby.

Ruby to the rescue!