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

Always specify the mode argument for writing.

Doc/library/gzip.rst

@@ -88,7 +88,8 @@ The module defines the following items:
    The *mode* argument can be any of ``'r'``, ``'rb'``, ``'a'``, ``'ab'``, ``'w'``,
    ``'wb'``, ``'x'``, or ``'xb'``, depending on whether the file will be read or
    written.  The default is the mode of *fileobj* if discernible; otherwise, the
-   default is ``'rb'``.
+   default is ``'rb'``.  In future Python releases the mode of *fileobj* will
+   not be used.  It is better to always specify *mode* for writing.
 
    Note that the file is always opened in binary mode. To open a compressed file
    in text mode, use :func:`.open` (or wrap your :class:`GzipFile` with an
@@ -164,6 +165,10 @@ The module defines the following items:
    .. versionchanged:: 3.6
       Accepts a :term:`path-like object`.
 
+   .. deprecated:: 3.9
+      Opening :class:`GzipFile` for writing without specifying the *mode*
+      argument is deprecated.
+
 
 .. function:: compress(data, compresslevel=9, *, mtime=None)
 

Doc/whatsnew/3.9.rst

@@ -239,6 +239,12 @@ Deprecated
   the module will restrict its seeds to :const:`None`, :class:`int`,
   :class:`float`, :class:`str`, :class:`bytes`, and :class:`bytearray`.
 
+* Opening the :class:`~gzip.GzipFile` file for writing without specifying
+  the *mode* argument is deprecated.  In future Python versions it will always
+  be opened for reading by default.  Specify the *mode* argument for opening
+  it for writing and silencing a warning.
+  (Contributed by Serhiy Storchaka in :issue:`28286`.)
+
 * Deprecated the ``split()`` method of :class:`_tkinter.TkappType` in
   favour of the ``splitlist()`` method which has more consistent and
   predicable behavior.

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,

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):

Misc/NEWS.d/next/Library/2019-09-26-12-16-30.bpo-28286.LdSsrN.rst

@@ -0,0 +1,2 @@
+Deprecate opening :class:`~gzip.GzipFile` for writing implicitly.  Always
+specify the *mode* argument for writing.