Update reference implementation

For `print(..., pretty=object)` simplify so that it's not an object that has a .pformat() method, but it just
needs to be a callable taking a single arg, returning the pretty formatted string.

Author: warsaw

Lib/test/test_print.py

@@ -205,6 +205,9 @@ class PPrintable:
     def __pprint__(self):
         yield 'I feel pretty'
 
+    def __str__(self):
+        return 'Pretty, pretty, pretty good'
+
 
 class PrettySmart(PrettyPrinter):
     def pformat(self, obj):
@@ -232,11 +235,15 @@ def test_pprint_magic(self):
         self.assertEqual(self.file.getvalue(), "'one' PPrintable('I feel pretty') 2\n")
 
     def test_custom_pprinter(self):
-        print('one', PPrintable(), 2, file=self.file, pretty=PrettySmart())
+        print('one', PPrintable(), 2, file=self.file, pretty=PrettySmart().pformat)
         self.assertEqual(self.file.getvalue(), "one PPrintable('I feel pretty') 2\n")
 
+    def test_callable_pprinter(self):
+        print('one', PPrintable(), 2, file=self.file, pretty=str)
+        self.assertEqual(self.file.getvalue(), "one Pretty, pretty, pretty good 2\n")
+
     def test_bad_pprinter(self):
-        with self.assertRaises(AttributeError):
+        with self.assertRaises(TypeError):
             print('one', PPrintable(), 2, file=self.file, pretty=object())
 
     def test_fstring(self):

Python/bltinmodule.c

@@ -2343,14 +2343,24 @@ builtin_print_impl(PyObject *module, PyObject * const *objects,
     if (pretty == Py_True) {
         /* Use default `pprint.PrettyPrinter` */
         PyObject *printer_factory = PyImport_ImportModuleAttrString("pprint", "PrettyPrinter");
+        PyObject *printer_instance = NULL;
 
         if (!printer_factory) {
             Py_DECREF(file);
             return NULL;
         }
-        printer = PyObject_CallNoArgs(printer_factory);
+
+        printer_instance = PyObject_CallNoArgs(printer_factory);
         Py_DECREF(printer_factory);
 
+        if (!printer_instance) {
+            Py_DECREF(file);
+            return NULL;
+        }
+
+        printer = PyObject_GetAttrString(printer_instance, "pformat");
+        Py_DECREF(printer_instance);
+
         if (!printer) {
             Py_DECREF(file);
             return NULL;
@@ -2381,7 +2391,7 @@ builtin_print_impl(PyObject *module, PyObject * const *objects,
         }
 
         if (printer) {
-            PyObject *prettified = PyObject_CallMethod(printer, "pformat", "O", objects[i]);
+            PyObject *prettified = PyObject_CallOneArg(printer, objects[i]);
 
             if (!prettified) {
                 Py_DECREF(file);