It's common to format a type name by accessing PyTypeObject.tp_name member. Example:

PyErr_Format(PyExc_TypeError,
             "__format__ must return a str, not %.200s",
             Py_TYPE(result)->tp_name);

Problems:

• PyTypeObject.tp_name (type.__name__) is less helpful than PyHeapTypeObject.ht_qualname (type.__qualname__). I would prefer to display the qualified type name.
• PyTypeObject.tp_name is a UTF-8 encoded string, it requires to decode the UTF-8 string at each call.
• I would like to remove PyTypeObject members (tp_name) from the public C API: issue C API: Investigate how the PyTypeObject members can be removed from the public C API #105970.
• By the way, in the early days of Python, PyString_FromFormat() used a buffer with a fixed size, so the output string should be truncated to avoid overflow. But nowadays, PyUnicode_FromFormat() allocates a buffer on the heap and is no longer limited to 200 characters. Truncated a type name can miss important information in the error message. I would prefer to not truncate the type name.

I propose adding a %T format to PyUnicode_FromUnicode() to format the qualified name of an object type. For example, the example would become:

PyErr_Format(PyExc_TypeError,
             "__format__ must return a str, not %T", result);

In 2018, I already added %T format to PyUnicode_FromFormat(): issue GH-78776. See related python-dev discussion. The change was reverted.

Linked PRs

• PEP 737: gh-111696: Add %T format to PyUnicode_FromFormat() #111703
• gh-111696: type(str) returns the fully qualified name #112129
• PEP 737: gh-111696: Add type.__fully_qualified_name__ attribute #112133
• gh-111696, PEP 737: Add PyType_GetFullyQualifiedName() function #116815
• gh-111696, PEP 737: Add PyType_GetModuleName() function #116824
• gh-111696, PEP 737: Add %T and %N to PyUnicode_FromFormat() #116839