GeezerVison

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.

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.

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

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

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).

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

Python 3.7 code:

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

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