bpo-38971: Open file in codecs.open() closes if exception raised. (GH-17666)

Open issue in the BPO indicated a desire to make the implementation of
codecs.open() at parity with io.open(), which implements a try/except to
assure file stream gets closed before an exception is raised.

Lib/codecs.py

@@ -905,11 +905,16 @@ def open(filename, mode='r', encoding=None, errors='strict', buffering=-1):
     file = builtins.open(filename, mode, buffering)
     if encoding is None:
         return file
-    info = lookup(encoding)
-    srw = StreamReaderWriter(file, info.streamreader, info.streamwriter, errors)
-    # Add attributes to simplify introspection
-    srw.encoding = encoding
-    return srw
+
+    try:
+        info = lookup(encoding)
+        srw = StreamReaderWriter(file, info.streamreader, info.streamwriter, errors)
+        # Add attributes to simplify introspection
+        srw.encoding = encoding
+        return srw
+    except:
+        file.close()
+        raise
 
 def EncodedFile(file, data_encoding, file_encoding=None, errors='strict'):
 

Lib/test/test_codecs.py

@@ -1154,6 +1154,7 @@ class UTF8SigTest(UTF8Test, unittest.TestCase):
             got = ostream.getvalue()
             self.assertEqual(got, unistring)
 
+
 class EscapeDecodeTest(unittest.TestCase):
     def test_empty(self):
         self.assertEqual(codecs.escape_decode(b""), (b"", 0))
@@ -1725,6 +1726,14 @@ class CodecsModuleTest(unittest.TestCase):
             self.assertRaises(UnicodeError,
                 codecs.decode, b'abc', 'undefined', errors)
 
+    def test_file_closes_if_lookup_error_raised(self):
+        mock_open = mock.mock_open()
+        with mock.patch('builtins.open', mock_open) as file:
+            with self.assertRaises(LookupError):
+                codecs.open(support.TESTFN, 'wt', 'invalid-encoding')
+
+            file().close.assert_called()
+
 
 class StreamReaderTest(unittest.TestCase):
 

Misc/NEWS.d/next/Library/2019-12-20-16-06-28.bpo-38971.fKRYlF.rst

@@ -0,0 +1,3 @@
+Open issue in the BPO indicated a desire to make the implementation of
+codecs.open() at parity with io.open(), which implements a try/except to
+assure file stream gets closed before an exception is raised.