gh-138912: Improve MATCH_CLASS opcode performance

Lib/test/test_syntax.py

@@ -2345,6 +2345,12 @@
  Traceback (most recent call last):
  SyntaxError: positional patterns follow keyword patterns
 
+ >>> match ...:
+ ... case Foo(y=1, x=2, y=3):
+ ... ...
+ Traceback (most recent call last):
+ SyntaxError: attribute name repeated in class pattern: y
+
  >>> match ...:
  ... case C(a=b, c, d=e, f, g=h, i, j=k, ...):
  ... ...

Misc/NEWS.d/next/Core_and_Builtins/2025-09-15-13-28-48.gh-issue-138912.61EYbn.rst

@@ -0,0 +1 @@
+Improve :opcode:`MATCH_CLASS` performance by up to 52% in certain cases. Patch by Marc Mueller.

Python/ceval.c

@@ -509,15 +509,18 @@ match_class_attr(PyThreadState *tstate, PyObject *subject, PyObject *type,
  PyObject *name, PyObject *seen)
 {
  assert(PyUnicode_CheckExact(name));
- assert(PySet_CheckExact(seen));
- if (PySet_Contains(seen, name) || PySet_Add(seen, name)) {
- if (!_PyErr_Occurred(tstate)) {
- // Seen it before!
- _PyErr_Format(tstate, PyExc_TypeError,
- "%s() got multiple sub-patterns for attribute %R",
- ((PyTypeObject*)type)->tp_name, name);
+ // Only check for duplicates if seen is not NULL.
+ if (seen != NULL) {
+ assert(PySet_CheckExact(seen));
+ if (PySet_Contains(seen, name) || PySet_Add(seen, name)) {
+ if (!_PyErr_Occurred(tstate)) {
+ // Seen it before!
+ _PyErr_Format(tstate, PyExc_TypeError,
+ "%s() got multiple sub-patterns for attribute %R",
+ ((PyTypeObject*)type)->tp_name, name);
+ }
+ return NULL;
  }
- return NULL;
  }
  PyObject *attr;
  (void)PyObject_GetOptionalAttr(subject, name, &attr);
@@ -540,14 +543,26 @@ _PyEval_MatchClass(PyThreadState *tstate, PyObject *subject, PyObject *type,
  if (PyObject_IsInstance(subject, type) <= 0) {
  return NULL;
  }
+ // Short circuit if there aren't any arguments:
+ Py_ssize_t nkwargs = PyTuple_GET_SIZE(kwargs);
+ Py_ssize_t nattrs = nargs + nkwargs;
+ if (!nattrs) {
+ return PyTuple_New(0);
+ }
  // So far so good:
- PyObject *seen = PySet_New(NULL);
- if (seen == NULL) {
- return NULL;
+ PyObject *seen = NULL;
+ // Only check for duplicates if there is at least one positional attribute
+ // and two or more attributes in total. Duplicate keyword attributes are
+ // detected during the compile stage and raise a SyntaxError.
+ if (nargs > 0 && nattrs > 1) {
+ seen = PySet_New(NULL);
+ if (seen == NULL) {
+ return NULL;
+ }
  }
- PyObject *attrs = PyList_New(0);
+ PyObject *attrs = PyTuple_New(nattrs);
  if (attrs == NULL) {
-Py_DECREF(seen);
+Py_XDECREF(seen);
  return NULL;
  }
  // NOTE: From this point on, goto fail on failure:
@@ -588,9 +603,8 @@ _PyEval_MatchClass(PyThreadState *tstate, PyObject *subject, PyObject *type,
  }
  if (match_self) {
  // Easy. Copy the subject itself, and move on to kwargs.
- if (PyList_Append(attrs, subject) < 0) {
- goto fail;
- }
+ assert(PyTuple_GET_ITEM(attrs, 0) == NULL);
+ PyTuple_SET_ITEM(attrs, 0, Py_NewRef(subject));
  }
  else {
  for (Py_ssize_t i = 0; i < nargs; i++) {
@@ -606,36 +620,29 @@ _PyEval_MatchClass(PyThreadState *tstate, PyObject *subject, PyObject *type,
  if (attr == NULL) {
  goto fail;
  }
- if (PyList_Append(attrs, attr) < 0) {
- Py_DECREF(attr);
- goto fail;
- }
- Py_DECREF(attr);
+ assert(PyTuple_GET_ITEM(attrs, i) == NULL);
+ PyTuple_SET_ITEM(attrs, i, attr);
  }
  }
  Py_CLEAR(match_args);
  }
  // Finally, the keyword subpatterns:
- for (Py_ssize_t i = 0; i < PyTuple_GET_SIZE(kwargs); i++) {
+ for (Py_ssize_t i = 0; i < nkwargs; i++) {
  PyObject *name = PyTuple_GET_ITEM(kwargs, i);
  PyObject *attr = match_class_attr(tstate, subject, type, name, seen);
  if (attr == NULL) {
  goto fail;
  }
- if (PyList_Append(attrs, attr) < 0) {
- Py_DECREF(attr);
- goto fail;
- }
- Py_DECREF(attr);
+ assert(PyTuple_GET_ITEM(attrs, nargs + i) == NULL);
+ PyTuple_SET_ITEM(attrs, nargs + i, attr);
  }
- Py_SETREF(attrs, PyList_AsTuple(attrs));
- Py_DECREF(seen);
+ Py_XDECREF(seen);
  return attrs;
 fail:
  // We really don't care whether an error was raised or not... that's our
  // caller's problem. All we know is that the match failed.
  Py_XDECREF(match_args);
-Py_DECREF(seen);
+Py_XDECREF(seen);
  Py_DECREF(attrs);
  return NULL;
 }