gh-77566: Make setdefault methods require explicit default argument for dbm backends

Lib/dbm/dumb.py

@@ -287,6 +287,21 @@ def __enter__(self):
     def __exit__(self, *args):
         self.close()
 
+    def setdefault(self, key, default):
+        if isinstance(key, str):
+            key = key.encode('utf-8')
+        try:
+            if key in self._index:
+                return self[key]
+            else:
+                self[key] = default
+                return default
+        except TypeError:
+            if self._index is None:
+                raise error('DBM object has already been closed') from None
+            else:
+                raise
+
     def reorganize(self):
         if self._readonly:
             raise error('The database is opened for reading only')

Lib/dbm/sqlite3.py

@@ -130,6 +130,12 @@ def __exit__(self, *args):
     def reorganize(self):
         self._execute(REORGANIZE)
 
+    def setdefault(self, key, default):
+        if key in self:
+            return self[key]
+        else:
+            self[key] = default
+            return default
 
 def open(filename, /, flag="r", mode=0o666):
     """Open a dbm.sqlite3 database and return the dbm object.

Lib/test/test_dbm.py

@@ -93,6 +93,22 @@ def test_anydbm_modification(self):
         # setdefault() works as in the dict interface
         self.assertEqual(f.setdefault(b'xxx', b'foo'), b'foo')
         self.assertEqual(f[b'xxx'], b'foo')
+        self.assertEqual(f.setdefault(b'g', b'bar'), b"indented")
+        self.assertEqual(f[b'g'], b"indented")
+        self.assertNotEqual(f[b'g'], b'bar')
+        with self.assertRaises(TypeError):
+            f.setdefault(b'key_without_default')
+        with self.assertRaises(TypeError):
+            f.setdefault(b'g', b'value', b'extra')
+        if self.module.__name__ in ('dbm.dumb', 'dbm.gnu', 'dbm.ndbm'):
+            with self.assertRaises(TypeError):
+                f.setdefault(b'new_key', 123)
+                f.setdefault(b'g', 123)
+        if self.module.__name__ == "dbm.sqlite3":
+            f.setdefault(b'new_key', 123)
+            self.assertEqual(f[b'new_key'], b'123')
+            f.setdefault(b'g', 123)
+            self.assertNotEqual(f[b'g'], b'123')
         f.close()
 
     def test_anydbm_read(self):
@@ -105,6 +121,11 @@ def test_anydbm_read(self):
         self.assertIsNone(f.get(b'xxx'))
         with self.assertRaises(KeyError):
             f[b'xxx']
+        with self.assertRaises(dbm.error):
+            f.setdefault(b'readonly_key', b'readonly_value')
+        f.setdefault(b'a', b'foo')
+        self.assertEqual(f[b'a'], b'Python:')
+        self.assertNotEqual(f[b'a'], b'foo')
         f.close()
 
     def test_anydbm_keys(self):
@@ -124,7 +145,7 @@ def test_empty_value(self):
         self.assertIn(b'empty', f)
         self.assertEqual(f[b'empty'], b'')
         self.assertEqual(f.get(b'empty'), b'')
-        self.assertEqual(f.setdefault(b'empty'), b'')
+        self.assertEqual(f.setdefault(b'empty', b'x'), b'')
         f.close()
 
     def test_anydbm_access(self):

Lib/test/test_dbm_ndbm.py

@@ -48,7 +48,7 @@ def test_empty_value(self):
         self.assertIn(b'empty', self.d)
         self.assertEqual(self.d[b'empty'], b'')
         self.assertEqual(self.d.get(b'empty'), b'')
-        self.assertEqual(self.d.setdefault(b'empty'), b'')
+        self.assertEqual(self.d.setdefault(b'empty', b'x'), b'')
         self.d.close()
 
     def test_modes(self):

Modules/_dbmmodule.c

@@ -425,7 +425,7 @@ _dbm_dbm_get_impl(dbmobject *self, PyTypeObject *cls, const char *key,
 _dbm.dbm.setdefault
     cls: defining_class
     key: str(accept={str, robuffer}, zeroes=True)
-    default: object(c_default="NULL") = b''
+    default: object
     /
 
 Return the value for key if present, otherwise default.
@@ -436,7 +436,7 @@ If key is not in the database, it is inserted with default as the value.
 static PyObject *
 _dbm_dbm_setdefault_impl(dbmobject *self, PyTypeObject *cls, const char *key,
                          Py_ssize_t key_length, PyObject *default_value)
-/*[clinic end generated code: output=9c2f6ea6d0fb576c input=c01510ef7571e13b]*/
+/*[clinic end generated code: output=9c2f6ea6d0fb576c input=12069156f2ddea7e]*/
 {
     datum dbm_key, val;
     Py_ssize_t tmp_size;
@@ -449,20 +449,13 @@ _dbm_dbm_setdefault_impl(dbmobject *self, PyTypeObject *cls, const char *key,
     if (val.dptr != NULL) {
         return PyBytes_FromStringAndSize(val.dptr, val.dsize);
     }
-    if (default_value == NULL) {
-        default_value = Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
-        val.dptr = NULL;
-        val.dsize = 0;
-    }
-    else {
-        if ( !PyArg_Parse(default_value, "s#", &val.dptr, &tmp_size) ) {
-            PyErr_SetString(PyExc_TypeError,
-                "dbm mappings have bytes or string elements only");
-            return NULL;
-        }
-        val.dsize = tmp_size;
-        Py_INCREF(default_value);
+    if ( !PyArg_Parse(default_value, "s#", &val.dptr, &tmp_size) ) {
+        PyErr_SetString(PyExc_TypeError,
+                        "dbm mappings have bytes or string elements only");
+        return NULL;
     }
+    val.dsize = tmp_size;
+    Py_INCREF(default_value);
     if (dbm_store(self->di_dbm, dbm_key, val, DBM_INSERT) < 0) {
         dbm_clearerr(self->di_dbm);
         PyErr_SetString(state->dbm_error, "cannot add item to database");

Modules/_gdbmmodule.c

@@ -375,7 +375,7 @@ gdbm_ass_sub(PyObject *op, PyObject *v, PyObject *w)
 _gdbm.gdbm.setdefault
 
     key: object
-    default: object = None
+    default: object
     /
 
 Get value for key, or set it to default and return default if not present.
@@ -384,7 +384,7 @@ Get value for key, or set it to default and return default if not present.
 static PyObject *
 _gdbm_gdbm_setdefault_impl(gdbmobject *self, PyObject *key,
                            PyObject *default_value)
-/*[clinic end generated code: output=f3246e880509f142 input=f4008b358165bbb8]*/
+/*[clinic end generated code: output=f3246e880509f142 input=6e9519905bc18910]*/
 {
     PyObject *res;