bpo-28286: Deprecate opening GzipFile for writing implicitly. (GH-16417)

Always specify the mode argument for writing.

2 files changed, 11 insertions, 1 deletions

diff --git a/Lib/gzip.py b/Lib/gzip.py
index 2968f47..e60d8ad 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -177,6 +177,7 @@ class GzipFile(_compression.BaseStream):
                 filename = ''
         else:
             filename = os.fspath(filename)
+        origmode = mode
         if mode is None:
             mode = getattr(fileobj, 'mode', 'rb')
 
@@ -187,6 +188,13 @@ class GzipFile(_compression.BaseStream):
             self.name = filename
 
         elif mode.startswith(('w', 'a', 'x')):
+            if origmode is None:
+                import warnings
+                warnings.warn(
+                    "GzipFile was opened for writing, but this will "
+                    "change in future Python releases.  "
+                    "Specify the mode argument for opening it for writing.",
+                    FutureWarning, 2)
             self.mode = WRITE
             self._init_write(filename)
             self.compress = zlib.compressobj(compresslevel,
diff --git a/Lib/test/test_gzip.py b/Lib/test/test_gzip.py
index 48a36a3..57d851c 100644
--- a/Lib/test/test_gzip.py
+++ b/Lib/test/test_gzip.py
@@ -469,7 +469,9 @@ class TestGzip(BaseTest):
             if "x" in mode:
                 support.unlink(self.filename)
             with open(self.filename, mode) as f:
-                with gzip.GzipFile(fileobj=f) as g:
+                with self.assertWarns(FutureWarning):
+                    g = gzip.GzipFile(fileobj=f)
+                with g:
                     self.assertEqual(g.mode, gzip.WRITE)
 
     def test_bytes_filename(self):