[issue39573] Make PyObject an opaque structure in the limited C API

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

Py_TYPE() is commonly used to render the type name in an error message. Example:

PyErr_Format(PyExc_TypeError,
             "cannot convert '%.200s' object to bytearray",
             Py_TYPE(arg)->tp_name);

This code has multiple issues:

* It truncates type name to 200 characters: there is no Python exception, not even a marker to indicate that the string has been truncated
* It's only the short name: the qualified name (tp_qualname) would be more helpful. The best would be to generate the fully qualified name: module + qualname.
* Py_TYPE() returns a borrowed reference which is causing multiple issues: https://pythoncapi.readthedocs.io/bad_api.html#borrowed-references

In September 2018, I created bpo-34595: "PyUnicode_FromFormat(): add %T format for an object type name". But there was disagreement, so I rejected my change.

I started "bpo-34595: How to format a type name?" thread on python-dev:
* https://mail.python.org/archives/list/python-dev@python.org/thread/HKYUMTVHNBVB5LJNRMZ7TPUQKGKAERCJ/#3UAMHYG6UF4MPLXBZORHO4JVKUBRUZ53

I didn't continue this work (until now), since it wasn't my priority.

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<https://bugs.python.org/issue39573>
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