[3.6] bpo-30605: Fix compiling binary regexs with BytesWarnings enabled. (GH-2016) (#2214)

Running our unit tests with `-bb` enabled triggered this failure..
(cherry picked from commit 171b9a354e816eebc6d4c3a8553303942e9c5025)

1 files changed, 14 insertions, 2 deletions

diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index e88d0b3..a6cbbd0 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -1346,7 +1346,7 @@ class ReTests(unittest.TestCase):
             self.assertTrue(re.match(p, lower_char))
         self.assertEqual(
             str(warns.warnings[0].message),
-            'Flags not at the start of the expression %s' % p
+            'Flags not at the start of the expression %r' % p
         )
         self.assertEqual(warns.warnings[0].filename, __file__)
 
@@ -1355,10 +1355,22 @@ class ReTests(unittest.TestCase):
             self.assertTrue(re.match(p, lower_char))
         self.assertEqual(
             str(warns.warnings[0].message),
-            'Flags not at the start of the expression %s (truncated)' % p[:20]
+            'Flags not at the start of the expression %r (truncated)' % p[:20]
         )
         self.assertEqual(warns.warnings[0].filename, __file__)
 
+        # bpo-30605: Compiling a bytes instance regex was throwing a BytesWarning
+        with warnings.catch_warnings():
+            warnings.simplefilter('error', BytesWarning)
+            p = b'A(?i)'
+            with self.assertWarns(DeprecationWarning) as warns:
+                self.assertTrue(re.match(p, b'a'))
+            self.assertEqual(
+                str(warns.warnings[0].message),
+                'Flags not at the start of the expression %r' % p
+            )
+            self.assertEqual(warns.warnings[0].filename, __file__)
+
         with self.assertWarns(DeprecationWarning):
             self.assertTrue(re.match('(?s).(?i)' + upper_char, '\n' + lower_char))
         with self.assertWarns(DeprecationWarning):