gh-111696: Add %T format to PyUnicode_FromFormat()

Author: vstinner

Doc/c-api/unicode.rst

@@ -518,6 +518,11 @@ APIs:
         - :c:expr:`PyObject*`
         - The result of calling :c:func:`PyObject_Repr`.
 
+      * - ``T``
+        - :c:expr:`PyObject*`
+        - Get the name of a type (``type.__name__``): the result of calling
+          ``PyType_GetName(type)``.
+
    .. note::
       The width formatter unit is number of characters rather than bytes.
       The precision formatter unit is number of bytes or :c:type:`wchar_t`
@@ -553,6 +558,9 @@ APIs:
       In previous versions it caused all the rest of the format string to be
       copied as-is to the result string, and any extra arguments discarded.
 
+   .. versionchanged:: 3.13
+      Support for ``"%T"`` added.
+
 
 .. c:function:: PyObject* PyUnicode_FromFormatV(const char *format, va_list vargs)
 

Doc/whatsnew/3.13.rst

@@ -1164,6 +1164,10 @@ New Features
   :c:func:`PyErr_WriteUnraisable`, but allow to customize the warning mesage.
   (Contributed by Serhiy Storchaka in :gh:`108082`.)
 
+* Add support for ``"%T"`` format to :c:func:`PyUnicode_FromFormat`: format
+  the name of a type (``type.__name__``).
+  (Contributed by Victor Stinner in :gh:`111696`.)
+
 
 Porting to Python 3.13
 ----------------------

Lib/test/test_capi/test_unicode.py

@@ -609,6 +609,23 @@ def check_format(expected, format, *args):
         check_format('xyz',
                      b'%V', None, b'xyz')
 
+        # test %T and %#T
+        obj = 'abc'
+        obj_type = type(obj)
+        check_format('type: str',
+                     b'type: %T', py_object(obj_type))
+        class LocalType:
+            pass
+        obj = LocalType()
+        obj_type = type(obj)
+        name = 'LocalType'
+        check_format(f'type: {name}',
+                     b'type: %T', py_object(obj_type))
+        check_format(f'type: {name[:3]}',
+                     b'type: %.3T', py_object(obj_type))
+        check_format(f'type: {name.rjust(20)}',
+                     b'type: %20T', py_object(obj_type))
+
         # test %ls
         check_format('abc', b'%ls', c_wchar_p('abc'))
         check_format('\u4eba\u6c11', b'%ls', c_wchar_p('\u4eba\u6c11'))

Misc/NEWS.d/next/C API/2023-11-03-15-53-11.gh-issue-111696.LObNGG.rst

@@ -0,0 +1,2 @@
+Add support for ``"%T"`` format to :c:func:`PyUnicode_FromFormat`: format the
+name of a type (``type.__name__``).  Patch by Victor Stinner.

Objects/unicodeobject.c

@@ -2464,6 +2464,7 @@ unicode_fromformat_arg(_PyUnicodeWriter *writer,
         switch (*f++) {
         case '-': flags |= F_LJUST; continue;
         case '0': flags |= F_ZERO; continue;
+        case '#': flags |= F_ALT; continue;
         }
         f--;
         break;
@@ -2787,6 +2788,37 @@ unicode_fromformat_arg(_PyUnicodeWriter *writer,
         break;
     }
 
+    case 'T':
+    {
+        PyObject *type_raw = va_arg(*vargs, PyObject *);
+        assert(type_raw != NULL);
+
+        if (!PyType_Check(type_raw)) {
+            PyErr_SetString(PyExc_TypeError,
+                            "%T argument must be a type; did you forget Py_TYPE()?");
+            return NULL;
+        }
+        PyTypeObject *type = (PyTypeObject*)type_raw;
+
+        PyObject *type_name;
+        if (flags & F_ALT) {
+            type_name = _PyType_GetFullyQualName(type, 0);
+        }
+        else {
+            type_name = PyType_GetName(type);
+        }
+        if (!type_name) {
+            return NULL;
+        }
+        if (unicode_fromformat_write_str(writer, type_name,
+                                         width, precision, flags) == -1) {
+            Py_DECREF(type_name);
+            return NULL;
+        }
+        Py_DECREF(type_name);
+        break;
+    }
+
     default:
     invalid_format:
         PyErr_Format(PyExc_SystemError, "invalid format string: %s", p);