bpo-41520: codeop no longer ignores SyntaxWarning (GH-21838)

(cherry picked from commit 369a1cbdee14d9f27356fb3a8bb21e4fde289d25)

Co-authored-by: Victor Stinner <vstinner@python.org>

3 files changed, 12 insertions, 2 deletions

diff --git a/Lib/codeop.py b/Lib/codeop.py
index 7e192ea..5476292 100644
--- a/Lib/codeop.py
+++ b/Lib/codeop.py
@@ -84,9 +84,11 @@ def _maybe_compile(compiler, source, filename, symbol):
     except SyntaxError:
         pass
 
-    # Suppress warnings after the first compile to avoid duplication.
+    # Catch syntax warnings after the first compile
+    # to emit SyntaxWarning at most once.
     with warnings.catch_warnings():
-        warnings.simplefilter("ignore")
+        warnings.simplefilter("error", SyntaxWarning)
+
         try:
             code1 = compiler(source + "\n", filename, symbol)
         except SyntaxError as e:
diff --git a/Lib/test/test_codeop.py b/Lib/test/test_codeop.py
index 45cb1a7..72840ca 100644
--- a/Lib/test/test_codeop.py
+++ b/Lib/test/test_codeop.py
@@ -4,6 +4,7 @@
 """
 import sys
 import unittest
+import warnings
 from test import support
 
 from codeop import compile_command, PyCF_DONT_IMPLY_DEDENT
@@ -309,5 +310,11 @@ class CodeopTests(unittest.TestCase):
             compile_command("0 is 0")
             self.assertEqual(len(w.warnings), 1)
 
+        # bpo-41520: check SyntaxWarning treated as an SyntaxError
+        with self.assertRaises(SyntaxError):
+            warnings.simplefilter('error', SyntaxWarning)
+            compile_command('1 is 1\n', symbol='exec')
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst b/Misc/NEWS.d/next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst
new file mode 100644
index 0000000..ca5501c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst
@@ -0,0 +1 @@
+Fix :mod:`codeop` regression: it no longer ignores :exc:`SyntaxWarning`.