Fix zstd compression in ab mode (#833)

smart_open/tests/test_smart_open.py

@@ -607,6 +607,22 @@ def test_atplus(self):
         self.assertEqual(text, SAMPLE_TEXT * 2)
 
 
+class CompressionRealFileSystemTests(RealFileSystemTests):
+    """Same as RealFileSystemTests but with a compressed file."""
+
+    def setUp(self):
+        with named_temporary_file(prefix='test', suffix='.zst', delete=False) as fout:
+            self.temp_file = fout.name
+        with smart_open.open(self.temp_file, 'wb') as fout:
+            fout.write(SAMPLE_BYTES)
+
+    def test_aplus(self):
+        pass  # transparent (de)compression unsupported for mode 'ab+'
+
+    def test_atplus(self):
+        pass  # transparent (de)compression unsupported for mode 'ab+'
+
+
 #
 # What exactly to patch here differs on _how_ we're opening the file.
 # See the _shortcut_open function for details.

smart_open/utils.py

@@ -208,6 +208,8 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
 
 class FileLikeProxy(wrapt.ObjectProxy):
+    __inner = ...  # initialized before wrapt disallows __setattr__ on certain objects
+
     def __init__(self, outer, inner):
         super().__init__(outer)
         self.__inner = inner