decompressobj: return bytes from flush()

This makes behavior more inline with similar types in Python's
standard library.

Closes #78.

NEWS.rst

@@ -136,6 +136,9 @@ Backwards Compatibility Notes
   and ``ZstdDecompressor.copy_stream()`` now raise the original exception
   on error calling the source stream's ``read()`` instead of raising
   ``ZstdError``. This only affects the C backend.
+* ``ZstdDecompressionObj.flush()`` now returns ``bytes`` instead of
+  ``None``. This makes it behave more similarly to ``flush()`` methods
+  for similar types in the Python standard library. (#78)
 
 Bug Fixes
 ---------

c-ext/decompressobj.c

@@ -130,7 +130,7 @@ static PyObject *DecompressionObj_flush(ZstdDecompressionObj *self,
         return NULL;
     }
 
-    Py_RETURN_NONE;
+    return PyBytes_FromString("");
 }
 
 static PyMethodDef DecompressionObj_methods[] = {

tests/test_decompressor.py

@@ -877,9 +877,9 @@ def test_simple(self):
         dctx = zstd.ZstdDecompressor()
         dobj = dctx.decompressobj()
         self.assertEqual(dobj.decompress(data), b"foobar")
-        self.assertIsNone(dobj.flush())
-        self.assertIsNone(dobj.flush(10))
-        self.assertIsNone(dobj.flush(length=100))
+        self.assertEqual(dobj.flush(), b"")
+        self.assertEqual(dobj.flush(10), b"")
+        self.assertEqual(dobj.flush(length=100), b"")
 
     def test_input_types(self):
         compressed = zstd.ZstdCompressor(level=1).compress(b"foo")
@@ -897,11 +897,11 @@ def test_input_types(self):
 
         for source in sources:
             dobj = dctx.decompressobj()
-            self.assertIsNone(dobj.flush())
-            self.assertIsNone(dobj.flush(10))
-            self.assertIsNone(dobj.flush(length=100))
+            self.assertEqual(dobj.flush(), b"")
+            self.assertEqual(dobj.flush(10), b"")
+            self.assertEqual(dobj.flush(length=100), b"")
             self.assertEqual(dobj.decompress(source), b"foo")
-            self.assertIsNone(dobj.flush())
+            self.assertEqual(dobj.flush(), b"")
 
     def test_reuse(self):
         data = zstd.ZstdCompressor(level=1).compress(b"foobar")
@@ -914,7 +914,7 @@ def test_reuse(self):
             zstd.ZstdError, "cannot use a decompressobj"
         ):
             dobj.decompress(data)
-            self.assertIsNone(dobj.flush())
+            self.assertEqual(dobj.flush(), b"")
 
     def test_bad_write_size(self):
         dctx = zstd.ZstdDecompressor()

zstandard/__init__.pyi

@@ -316,7 +316,7 @@ class ZstdCompressor(object):
 
 class ZstdDecompressionObj(object):
     def decompress(self, data: ByteString) -> bytes: ...
-    def flush(self, length: int = 0): ...
+    def flush(self, length: int = 0) -> bytes: ...
 
 class ZstdDecompressionReader(BinaryIO):
     def __enter__(self) -> "ZstdDecompressionReader": ...

zstandard/backend_cffi.py

@@ -1901,7 +1901,7 @@ def decompress(self, data):
         return b"".join(chunks)
 
     def flush(self, length=0):
-        pass
+        return b""
 
 
 class ZstdDecompressionReader(object):