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Right, Larry and I had a fairly long discussion about this idea at the sprints, and I was satisfied that all the cases where he's proposing to use this are safe: in order to exploit them you need to be able to set __text_signature__ on arbitrary objects, and if an attacker can do that, you've already lost control of the process.
However, a natural future extension is to expose this as a public alternative constructor for Signature objects, and for that, the fact that it ultimately calls eval() under the hood presents more of a security risk. The "trusted=False" default on _signature_fromstr allows the function to be used safely on untrusted data, while allowing additional flexibility when you *do* trust the data you're evaluating.