[issue17159] Remove explicit type check from inspect.Signature.from_function()

[Nick Coghlan]

Nick Coghlan added the comment:

Stefan is suggesting that rather than emulating the Python signature line
(which is a concise human readable notation that with the PEP 457 tweaks
will handle even the most obscure custom argument parsing, whether that
parsing is implemented in C or in Python), it would make more sense to
instead emulate the confusing jumble of CPython implementation details
exposed by Python level function and code objects. That ducktyping has been
the traditional way to support signature introspection for anyone that
didn't have the ability to change how the inspect module works.

However, I'm not sure *how* generating and storing an assortment of hard to
interpret lists and strings and mappings would qualify as being simpler
than generating and storing a single string that is only converted into
real signature data if requested. Once __text_signature__ becomes a public
API in 3.5 (as part of PEP 457), inspect shouldn't need to special case any
type ever again: it will be up to the type to set __text_signature__
properly instead.

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<http://bugs.python.org/issue17159>
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