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

(cherry picked from commit 369a1cbdee)

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

Lib/codeop.py

@@ -84,9 +84,11 @@ def _maybe_compile(compiler, source, filename, symbol):
     except SyntaxError as err:
         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:

Lib/test/test_codeop.py

@@ -3,6 +3,7 @@
    Nick Mathewson
 """
 import unittest
+import warnings
 from test import support
 
 from codeop import compile_command, PyCF_DONT_IMPLY_DEDENT
@@ -300,5 +301,11 @@ 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()

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`.