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I'm not comfortable adding a dependency on inspect to the contextlib module, and I think it would be undesirable to have too many cases where the early validation is silently skipped.
While I like the idea of adding early signature validation to these APIs in principle, given the above concerns and the fact this probably crosses the line into "feature" territory, I think we want to wait until 3.4 and the introduction of a mechanism that adds introspection support to callables implemented in C.
That will likely involve moving the signature support somewhere lower in the module stack (perhaps to a private "_signature" module, with inspect remaining the official API), and will reduce the number of cases where pre-validation is skipped.