[issue25341] File mode wb+ appears as rb+

[Irit Katriel]

Irit Katriel  added the comment:

Reproduced on 3.11.

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nosy: +iritkatriel
versions: +Python 3.11 -Python 2.7, Python 3.5, Python 3.6

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[issue25341] File mode wb+ appears as rb+

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> 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 issue4362 and Barry asked the same question about "w+" 
(msg76134).

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

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[issue25341] File mode wb+ appears as rb+

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis :


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[issue25341] File mode wb+ appears as rb+

[Xiang Zhang]

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.

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[issue25341] File mode wb+ appears as rb+

[Mark Williams]

Mark Williams added the comment:

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  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 
> 
> ___
>

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[issue25341] File mode wb+ appears as rb+

[Xiang Zhang]

Changes by Xiang Zhang <18518281...@126.com>:


--
keywords: +patch
Added file: http://bugs.python.org/file40739/file_mode.patch

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[issue25341] File mode wb+ appears as rb+

[Xiang Zhang]

Xiang Zhang added the comment:

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.

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[issue25341] File mode wb+ appears as rb+

[Terry J. Reedy]

Changes by Terry J. Reedy :


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[issue25341] File mode wb+ appears as rb+

[Mahmoud Hashemi]

Changes by Mahmoud Hashemi :


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[issue25341] File mode wb+ appears as rb+

[Mark Williams]

New submission from Mark Williams:

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 "", line 1, in 
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 "", line 2, in 
  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.

--
components: IO, Interpreter Core, Library (Lib)
messages: 252518
nosy: Mark.Williams
priority: normal
severity: normal
status: open
title: File mode wb+ appears as rb+
type: behavior
versions: Python 2.7, Python 3.5, Python 3.6

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