DO-NOT-MERGE: bpo-34595: Add %t format to PyUnicode_FromFormatV()

[vstinner]

• The %T format of PyUnicode_FromFormatV() now returns the fully
  qualified name of an object type (ex: "module.namespace.typename").
• Add %t format to PyUnicode_FromFormatV(), and so to
  PyUnicode_FromFormat() and PyErr_Format(), to format the "short
  name" of an object type: equivalent to "%s" with
  _PyType_Name(Py_TYPE(obj)).
• Replace %T format with %t format in unicodeobject.c.

https://bugs.python.org/issue34595

[serhiy-storchaka]

In case of non-heap types you can just return PyUnicode_FromString(type->tp_name).

In case of heap types the code can be a tiny bit simpler if inline _PyType_Module() and _PyType_QualName() because you can get rid of increfs/decrefs and NULL checks.

[vstinner]

In case of non-heap types you can just return PyUnicode_FromString(type->tp_name).

Done. This change removes "builtins." when formatting builtins type. So %T of a string becomes "str" (instead of "builtins.str"). IMHO it's the expected behaviour.

In case of heap types the code can be a tiny bit simpler if inline _PyType_Module() and _PyType_QualName() because you can get rid of increfs/decrefs and NULL checks.

I don't think that performance matters here. I prefer to reuse the same code, to make sure that type.qualname, type.module and %T behave the same for heap types.

[serhiy-storchaka]

Why type_qualname() and type_module() have been renamed to _PyType_QualName() and _PyType_Module()?

[vstinner]

Hum. I moved code in a weird way. I fixed that.

[serhiy-storchaka]

Except that the module name is omitted for types in the builtins module (and for non-heap extension types that don't specify the module, but this can be considered as a bug).

[serhiy-storchaka]

What was used instead of %t/%T before?

[vstinner]

Python 3.7 code:

        PyErr_Format(PyExc_TypeError,
                     "must be str, not %.100s",
                     Py_TYPE(obj)->tp_name);

[serhiy-storchaka]

Thus it was closer to %T. In error messages it is better to use fully qualified names.

[vstinner]

I rebased my changed and made requested changes.

[vstinner]

I propose to replace Py_TYPE(obj)->tp_name with %t in C to mimick Python code which uses type(obj).name or obj.class.name.

If we want to use %T in C code, I suggest to also update the related Python code, especially for "C accelerators" modules like _asyncio/_pickle. I suggest to only start to use %T on a case by base basis.

[vstinner]

#Linux-PR_20180911.05 failed: the CI is broken, it failed on apt-get install.

[vstinner]

I changed the status of this PR to "DO-NOT-MERGE", since Petr Viktorin asked me to open a discussion on python-dev:

• https://mail.python.org/pipermail/python-dev/2018-September/155150.html
• https://bugs.python.org/issue34595#msg325079