There is at least one mode in which a file can be opened that cannot be represented in its mode attribute: wb+. This mode instead appears as 'rb+' in the mode attribute:

Python 3.5.0 (default, Oct  3 2015, 10:40:38)
[GCC 4.2.1 Compatible FreeBSD Clang 3.4.1 (tags/RELEASE_34/dot1-final 208032)] on freebsd10
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> if os.path.exists('some_file'): os.unlink('some_file')
...
>>> with open('some_file', 'r+b') as f: print(f.mode)
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'some_file'
>>> with open('some_file', 'w+b') as f: print(f.mode)
...
rb+
>>> with open('some_file', 'r+b') as f: print(f.mode)
rb+

This means code that interacts with file objects cannot trust the mode of binary files. For example, you can't use tempfile.TemporaryFile (the mode argument of which defaults to 'wb+') and GzipFile:

>>> import gzip
>>> from tempfile import TemporaryFile
>>> with TemporaryFile() as f:
...     gzip.GzipFile(fileobj=f).write(b'test')
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/local/lib/python3.5/gzip.py", line 249, in write
    raise OSError(errno.EBADF, "write() on read-only GzipFile object")
OSError: [Errno 9] write() on read-only GzipFile object

This occurs because without a mode argument passed to its initializer, GzipFile checks that the fp object's mode starts with 'w', 'a', or 'x'.

For the sake of completeness/searchability: w+ and r+ are different modes, so rb+ and wb+ must be different modes. Per https://docs.python.org/3/library/functions.html#open :

"""
For binary read-write access, the mode 'w+b' opens and truncates the file to 0 bytes. 'r+b' opens the file without truncation.
"""

I haven't been able to test this on Windows, but I expect precisely the same behavior given my understanding of the relevant source.

_io_FileIO___init___impl in _io/fileio.c does the right thing and includes O_CREAT and O_TRUNC in the open(2) flags upon seeing 'w' in the mode:

https://hg.python.org/cpython/file/3.5/Modules/_io/fileio.c#l324

this ensures correct interaction with the file system. But it also sets self->readable and self->writable upon seeing '+' in the mode:

https://hg.python.org/cpython/file/3.5/Modules/_io/fileio.c#l341

The open flags are not retained. Consequently, when the mode attribute is accessed and the get_mode calls the mode_string function, the instance has insufficient information to differentiate between 'rb+' and 'wb+':

https://hg.python.org/cpython/file/3.5/Modules/_io/fileio.c#l1043

If the FileIO instance did retain the 'flags' variable that's declared and set in its initializer, then mode_string could use it to determine the difference between wb+ and rb+.

I would be happy to write a patch for this.

I think Mark is right. Since wb+ and rb+ have different behaviours they
should be treat separately.

But this behaviour treating wb+ and rb+ as the same is well tested and
seems to intended to do so.

Python's test suite may test the current behavior but that does not lessen
the problem.

I gave an example of apparently correct code that fails (that was actually
encountered by a Python user) in my original description. Another such
example: you cannot duplicate a file object -- same path, same mode --- and
be sure that the duplicate is a true duplicate. Data corruption could
occur in application code if the duplicated file were opened "rb+" instead
of "wb+", as the duplicate would not truncate existing data.

Another way to think about the problem is accuracy of intent. The mode
attribute on file objects can be incorrect, and by "incorrect" I mean "not
describe the mode under which the file was opened." Why have a mode
attribute at all, then? I, for one, would prefer *no* mode attribute to
one that's sometimes incorrect. But a correct one is even better!

On Sat, Oct 10, 2015 at 1:27 AM, Xiang Zhang <report@bugs.python.org> wrote:

Xiang Zhang added the comment:

I think Mark is right. Since wb+ and rb+ have different behaviours they
should be treat separately.

But this behaviour treating wb+ and rb+ as the same is well tested and
seems to intended to do so.

----------
nosy: +xiang.zhang

---

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue25341\>

---

I make a patch which now identifies the difference between wb+ and rb+,
and modifies the corresponding tests. Though I don't know whether this
need to be fixed.

But this behaviour treating wb+ and rb+ as the same is well tested and
seems to intended to do so.

I think this is not intended behavior. Tests just test that the current behavior is not changed accidentally. If I'm right, the patch LGTM. But since third-party code can depend on this behavior, I would fix it only in 3.6.

Tests were added in bpo-4362 and Barry asked the same question about "w+" (msg76134).

Barry, Benjamin, what are you think about this now?

Reproduced on 3.11.

>>> import gzip
>>> from tempfile import TemporaryFile
>>> with TemporaryFile() as f:
...     gzip.GzipFile(fileobj=f).write(b'test')
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/local/lib/python3.5/gzip.py", line 249, in write
    raise OSError(errno.EBADF, "write() on read-only GzipFile object")
OSError: [Errno 9] write() on read-only GzipFile object

This occurs because without a mode argument passed to its initializer, GzipFile checks that the fp object's mode starts with 'w', 'a', or 'x'.

I think this is a problem in gzip. It has only READ and WRITE modes, and it ignores the '+'.