From bcf4dccfa77df6fd75181e2a7cbfbf2d4b2c6255 Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@gmail.com>
Date: Tue, 22 Nov 2016 15:30:38 +0100
Subject: [PATCH] Issue #28727: Optimize pattern_richcompare() for a==a

A pattern is equal to itself.
---
 Lib/test/test_re.py | 4 ++++
 Modules/_sre.c      | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 4fcd2d463dc..84131d2b926 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -1781,6 +1781,10 @@ SUBPATTERN None 0 0
     def test_pattern_compare(self):
         pattern1 = re.compile('abc', re.IGNORECASE)
 
+        # equal to itself
+        self.assertEqual(pattern1, pattern1)
+        self.assertFalse(pattern1 != pattern1)
+
         # equal
         re.purge()
         pattern2 = re.compile('abc', re.IGNORECASE)
diff --git a/Modules/_sre.c b/Modules/_sre.c
index 1b7741696d0..979e61fb535 100644
--- a/Modules/_sre.c
+++ b/Modules/_sre.c
@@ -2683,6 +2683,12 @@ pattern_richcompare(PyObject *lefto, PyObject *righto, int op)
     if (Py_TYPE(lefto) != &Pattern_Type || Py_TYPE(righto) != &Pattern_Type) {
         Py_RETURN_NOTIMPLEMENTED;
     }
+
+    if (lefto == righto) {
+        /* a pattern is equal to itself */
+        return PyBool_FromLong(op == Py_EQ);
+    }
+
     left = (PatternObject *)lefto;
     right = (PatternObject *)righto;