GH-46412: More efficient bool() for ndbm/_gdbmmodule (#96692)

Lib/test/test_dbm_gnu.py

@@ -118,6 +118,20 @@ class TestGdbm(unittest.TestCase):
         self.assertEqual(str(cm.exception),
                          "GDBM object has already been closed")
 
+    def test_bool_empty(self):
+        with gdbm.open(filename, 'c') as db:
+            self.assertFalse(bool(db))
+
+    def test_bool_not_empty(self):
+        with gdbm.open(filename, 'c') as db:
+            db['a'] = 'b'
+            self.assertTrue(bool(db))
+
+    def test_bool_on_closed_db_raises(self):
+        with gdbm.open(filename, 'c') as db:
+            db['a'] = 'b'
+        self.assertRaises(gdbm.error, bool, db)
+
     def test_bytes(self):
         with gdbm.open(filename, 'c') as db:
             db[b'bytes key \xbd'] = b'bytes value \xbd'

Lib/test/test_dbm_ndbm.py

@@ -133,6 +133,20 @@ class DbmTestCase(unittest.TestCase):
     def test_open_with_pathlib_bytes_path(self):
         dbm.ndbm.open(os_helper.FakePath(os.fsencode(self.filename)), "c").close()
 
+    def test_bool_empty(self):
+        with dbm.ndbm.open(self.filename, 'c') as db:
+            self.assertFalse(bool(db))
+
+    def test_bool_not_empty(self):
+        with dbm.ndbm.open(self.filename, 'c') as db:
+            db['a'] = 'b'
+            self.assertTrue(bool(db))
+
+    def test_bool_on_closed_db_raises(self):
+        with dbm.ndbm.open(self.filename, 'c') as db:
+            db['a'] = 'b'
+        self.assertRaises(dbm.ndbm.error, bool, db)
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2022-09-08-20-12-48.gh-issue-46412.r_cfTh.rst

@@ -0,0 +1 @@
+Improve performance of ``bool(db)`` for large ndb/gdb databases. Previously this would call ``len(db)`` which would iterate over all keys -- the answer (empty or not) is known after the first key.

Modules/_dbmmodule.c

@@ -130,6 +130,37 @@ dbm_length(dbmobject *dp)
     return dp->di_size;
 }
 
+static int
+dbm_bool(dbmobject *dp)
+{
+    _dbm_state *state = PyType_GetModuleState(Py_TYPE(dp));
+    assert(state != NULL);
+
+    if (dp->di_dbm == NULL) {
+        PyErr_SetString(state->dbm_error, "DBM object has already been closed");
+        return -1;
+    }
+
+    if (dp->di_size > 0) {
+        /* Known non-zero size. */
+        return 1;
+    }
+    if (dp->di_size == 0) {
+        /* Known zero size. */
+        return 0;
+    }
+
+    /* Unknown size.  Ensure DBM object has an entry. */
+    datum key = dbm_firstkey(dp->di_dbm);
+    if (key.dptr == NULL) {
+        /* Empty. Cache this fact. */
+        dp->di_size = 0;
+        return 0;
+    }
+    /* Non-empty. Don't cache the length since we don't know. */
+    return 1;
+}
+
 static PyObject *
 dbm_subscript(dbmobject *dp, PyObject *key)
 {
@@ -416,6 +447,7 @@ static PyType_Slot dbmtype_spec_slots[] = {
     {Py_mp_length, dbm_length},
     {Py_mp_subscript, dbm_subscript},
     {Py_mp_ass_subscript, dbm_ass_sub},
+    {Py_nb_bool, dbm_bool},
     {0, 0}
 };
 

Modules/_gdbmmodule.c

@@ -162,6 +162,35 @@ gdbm_length(gdbmobject *dp)
     return dp->di_size;
 }
 
+static int
+gdbm_bool(gdbmobject *dp)
+{
+    _gdbm_state *state = PyType_GetModuleState(Py_TYPE(dp));
+    if (dp->di_dbm == NULL) {
+        PyErr_SetString(state->gdbm_error, "GDBM object has already been closed");
+        return -1;
+    }
+    if (dp->di_size > 0) {
+        /* Known non-zero size. */
+        return 1;
+    }
+    if (dp->di_size == 0) {
+        /* Known zero size. */
+        return 0;
+    }
+    /* Unknown size.  Ensure DBM object has an entry. */
+    datum key = gdbm_firstkey(dp->di_dbm);
+    if (key.dptr == NULL) {
+        /* Empty. Cache this fact. */
+        dp->di_size = 0;
+        return 0;
+    }
+
+    /* Non-empty. Don't cache the length since we don't know. */
+    free(key.dptr);
+    return 1;
+}
+
 // Wrapper function for PyArg_Parse(o, "s#", &d.dptr, &d.size).
 // This function is needed to support PY_SSIZE_T_CLEAN.
 // Return 1 on success, same to PyArg_Parse().
@@ -569,6 +598,7 @@ static PyType_Slot gdbmtype_spec_slots[] = {
     {Py_mp_length, gdbm_length},
     {Py_mp_subscript, gdbm_subscript},
     {Py_mp_ass_subscript, gdbm_ass_sub},
+    {Py_nb_bool, gdbm_bool},
     {Py_tp_doc, (char*)gdbm_object__doc__},
     {0, 0}
 };