gh-96526: Clarify format and __format__ docstrings (gh-96648)

Objects/clinic/longobject.c.h

@@ -88,7 +88,8 @@ int___getnewargs__(PyObject *self, PyObject *Py_UNUSED(ignored))
 PyDoc_STRVAR(int___format____doc__,
 "__format__($self, format_spec, /)\n"
 "--\n"
-"\n");
+"\n"
+"Convert to a string according to format_spec.");
 
 #define INT___FORMAT___METHODDEF    \
     {"__format__", (PyCFunction)int___format__, METH_O, int___format____doc__},
@@ -466,4 +467,4 @@ skip_optional_kwonly:
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=b29b4afc65e3290e input=a9049054013a1b77]*/
+/*[clinic end generated code: output=bf6074ecf2f32cf4 input=a9049054013a1b77]*/

Objects/clinic/typeobject.c.h

@@ -204,7 +204,9 @@ PyDoc_STRVAR(object___format____doc__,
 "__format__($self, format_spec, /)\n"
 "--\n"
 "\n"
-"Default object formatter.");
+"Default object formatter.\n"
+"\n"
+"Return str(self) if format_spec is empty. Raise TypeError otherwise.");
 
 #define OBJECT___FORMAT___METHODDEF    \
     {"__format__", (PyCFunction)object___format__, METH_O, object___format____doc__},
@@ -267,4 +269,4 @@ object___dir__(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
     return object___dir___impl(self);
 }
-/*[clinic end generated code: output=3312f873c970bfd1 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=d2fc52440a89f2fa input=a9049054013a1b77]*/

Objects/longobject.c

@@ -5528,11 +5528,13 @@ int.__format__
 
     format_spec: unicode
     /
+
+Convert to a string according to format_spec.
 [clinic start generated code]*/
 
 static PyObject *
 int___format___impl(PyObject *self, PyObject *format_spec)
-/*[clinic end generated code: output=b4929dee9ae18689 input=e31944a9b3e428b7]*/
+/*[clinic end generated code: output=b4929dee9ae18689 input=d5e1254a47e8d1dc]*/
 {
     _PyUnicodeWriter writer;
     int ret;

Objects/typeobject.c

@@ -5824,11 +5824,13 @@ object.__format__
   /
 
 Default object formatter.
+
+Return str(self) if format_spec is empty. Raise TypeError otherwise.
 [clinic start generated code]*/
 
 static PyObject *
 object___format___impl(PyObject *self, PyObject *format_spec)
-/*[clinic end generated code: output=34897efb543a974b input=7c3b3bc53a6fb7fa]*/
+/*[clinic end generated code: output=34897efb543a974b input=b94d8feb006689ea]*/
 {
     /* Issue 7994: If we're converting to a string, we
        should reject format specifications */

Python/bltinmodule.c

@@ -677,16 +677,19 @@ format as builtin_format
     format_spec: unicode(c_default="NULL") = ''
     /
 
-Return value.__format__(format_spec)
+Return type(value).__format__(value, format_spec)
 
-format_spec defaults to the empty string.
-See the Format Specification Mini-Language section of help('FORMATTING') for
-details.
+Many built-in types implement format_spec according to the
+Format Specification Mini-language. See help('FORMATTING').
+
+If type(value) does not supply a method named __format__
+and format_spec is empty, then str(value) is returned.
+See also help('SPECIALMETHODS').
 [clinic start generated code]*/
 
 static PyObject *
 builtin_format_impl(PyObject *module, PyObject *value, PyObject *format_spec)
-/*[clinic end generated code: output=2f40bdfa4954b077 input=88339c93ea522b33]*/
+/*[clinic end generated code: output=2f40bdfa4954b077 input=45ef3934b86d5624]*/
 {
     return PyObject_Format(value, format_spec);
 }

Python/clinic/bltinmodule.c.h

@@ -183,11 +183,14 @@ PyDoc_STRVAR(builtin_format__doc__,
 "format($module, value, format_spec=\'\', /)\n"
 "--\n"
 "\n"
-"Return value.__format__(format_spec)\n"
+"Return type(value).__format__(value, format_spec)\n"
 "\n"
-"format_spec defaults to the empty string.\n"
-"See the Format Specification Mini-Language section of help(\'FORMATTING\') for\n"
-"details.");
+"Many built-in types implement format_spec according to the\n"
+"Format Specification Mini-language. See help(\'FORMATTING\').\n"
+"\n"
+"If type(value) does not supply a method named __format__\n"
+"and format_spec is empty, then str(value) is returned.\n"
+"See also help(\'SPECIALMETHODS\').");
 
 #define BUILTIN_FORMAT_METHODDEF    \
     {"format", _PyCFunction_CAST(builtin_format), METH_FASTCALL, builtin_format__doc__},
@@ -1212,4 +1215,4 @@ builtin_issubclass(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=f3da5510745785af input=a9049054013a1b77]*/
+/*[clinic end generated code: output=3c9497e0ffeb8a30 input=a9049054013a1b77]*/