Larry Hastings added the comment:
As an example, consider pattern_match() in _sre.c. This implements the match
method on a pattern object; in other words, re.compile().match(). The third
parameter, endpos, defaults to PY_SSIZE_T_MAX in C. What should
inspect.Signature() report as the default value for endpos? We can't look up
the proper value for PY_SSIZE_T_MAX, because the default value
inspect.Signature() uses was written out as text when Argument Clinic processed
the function.
As a hint, the documentation for match() dodges this problem by outright lying.
It claims that the prototype for the function is:
match(string[, pos[, endpos]])
which is a lie. pattern_match() parses its arguments by calling
PyArg_ParseTupleAndKeywords() with a format string of "O|nn". which means, for
example, you could call:
match("abc", endpos=5)
The documentation claims this is invalid but it works fine.
In other words, it's a sticky problem, and the people who had it before us
didn't try to solve it properly.
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<http://bugs.python.org/issue20144>
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