fix os.fspath call on file-like objects

CHANGES.rst

@@ -14,6 +14,8 @@ Unreleased
     :issue:`1739`
 -   Fix missing local variables in interactive debugger console.
     :issue:`1746`
+-   Fix passing file-like objects like ``io.BytesIO`` to
+    ``FileStorage.save``. :issue:`1733`
 
 
 Version 1.0.0

src/werkzeug/datastructures.py

@@ -3058,7 +3058,9 @@ def save(self, dst, buffer_size=16384):
         from shutil import copyfileobj
 
         close_dst = False
-        dst = fspath(dst)
+
+        if hasattr(dst, "__fspath__"):
+            dst = fspath(dst)
 
         if isinstance(dst, string_types):
             dst = open(dst, "wb")

tests/test_datastructures.py

@@ -1243,6 +1243,20 @@ def test_save_to_pathlib_dst(self, tmp_path):
         storage.save(dst)
         assert dst.read_text() == "test"
 
+    def test_save_to_bytes_io(self):
+        storage = self.storage_class(io.BytesIO(b"one\ntwo"))
+        dst = io.BytesIO()
+        storage.save(dst)
+        assert dst.getvalue() == b"one\ntwo"
+
+    def test_save_to_file(self, tmp_path):
+        path = tmp_path / "file.data"
+        storage = self.storage_class(io.BytesIO(b"one\ntwo"))
+        with path.open("wb") as dst:
+            storage.save(dst)
+        with path.open("rb") as src:
+            assert src.read() == b"one\ntwo"
+
 
 @pytest.mark.parametrize("ranges", ([(0, 1), (-5, None)], [(5, None)]))
 def test_range_to_header(ranges):