Pydoc: fall back to __text_signature__ if inspect.signature() fails

[serhiy-storchaka]

Pydoc uses inspect.signature() to get a function signature. inspect.signature() supports Python functions, and also extension functions defined with Argument Clinic (by parsing the __text_signature__ attribute). Unfortunately Argument Clinic is used with functions whose signature cannot be expressed in Python, e.g. getattr() or dict.pop(). It produces a human readable __text_signature__, but it can not be represented as a Signature object, so inspect.signature() fails. Pydoc display generic (...) for such functions.

Since pydoc only needs a string representation of signature, not a Signature object, I propose to use __text_signature__ if inspect.signature() fails. It needs only some trivial processing.

Before:

getattr(...)
    Get a named attribute from an object.

    getattr(x, 'y') is equivalent to x.y
    When a default argument is given, it is returned when the attribute doesn't
    exist; without it, an exception is raised in that case.

After:

getattr(object, name, default=<unrepresentable>, /)
    Get a named attribute from an object.

    getattr(x, 'y') is equivalent to x.y
    When a default argument is given, it is returned when the attribute doesn't
    exist; without it, an exception is raised in that case.

Linked PRs

• gh-107782: Pydoc: fall back to __text_signature__ if inspect.signature() fails #107786
• gh-107782: Use _testcapi to test non-representable signatures #109325