file accepts 'rU+' as a mode

[brettcannon]

BPO	2091
Nosy	@brettcannon, @amauryfa, @pitrou, @vstinner, @larryhastings, @rbtcollins, @benjaminp, @bitdancer, @berkerpeksag, @serhiy-storchaka
PRs	bpo-2091: Fix typo in exception message #12987 [3.7] bpo-2091: Fix typo in exception message (GH-12987) #12988
Files	issue2091.diff issue2091-2.patch: Updated patch for new io issue2091-3.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2015-07-25.18:45:53.834>
created_at = <Date 2008-02-12.22:55:09.244>
labels = ['interpreter-core', 'type-bug', 'expert-IO']
title = "file accepts 'rU+' as a mode"
updated_at = <Date 2019-04-27.20:39:08.758>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2019-04-27.20:39:08.758>
actor = 'berker.peksag'
assignee = 'none'
closed = True
closed_date = <Date 2015-07-25.18:45:53.834>
closer = 'rbcollins'
components = ['Interpreter Core', 'IO']
creation = <Date 2008-02-12.22:55:09.244>
creator = 'brett.cannon'
dependencies = []
files = ['9697', '22217', '39980']
hgrepos = []
issue_num = 2091
keywords = ['patch', 'needs review']
message_count = 19.0
messages = ['62343', '63741', '104582', '116954', '116955', '136425', '137409', '223368', '223810', '247133', '247193', '247212', '247215', '247334', '247365', '247366', '247370', '341010', '341013']
nosy_count = 14.0
nosy_names = ['brett.cannon', 'amaury.forgeotdarc', 'pitrou', 'vstinner', 'larry', 'rbcollins', 'benjamin.peterson', 'jeff.balogh', 'r.david.murray', 'python-dev', 'jcon', 'francismb', 'berker.peksag', 'serhiy.storchaka']
pr_nums = ['12987', '12988']
priority = 'high'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2091'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5']

[brettcannon]

The docs on file's constructor says that the 'U' mode should not work
with '+', and yet 'rU+' does not throw an error.

[jeffbalogh]

Attaching a patch that checks for '+' in the mode string, updates the
docs, and tests bad mode strings.

[tseaver]

I can confirm that:

• the patch applies cleanly to the release26-maint branch, with the
  exception of the Misc/NEWS portion::

  $ hg branch
  release26-maint
  $ ./python -E -tt Lib/test/regrtest.py test_file
  test_file
  1 test OK.
  $ patch -p0 < /tmp/issue2091.diff 
  patching file Objects/fileobject.c
  Hunk #1 succeeded at 209 (offset 54 lines).
  Hunk #2 succeeded at 2269 (offset 194 lines).
  patching file Misc/NEWS
  Hunk #1 FAILED at 12.
  1 out of 1 hunk FAILED -- saving rejects to file Misc/NEWS.rej
  patching file Lib/test/test_file.py
  Hunk #1 succeeded at 176 (offset 55 lines).

• the added test fails before rebuilding Objects/fileobject.c::

  $ ./python -E -tt Lib/test/regrtest.py test_file
  test_file
  test test_file failed -- Traceback (most recent call last):
    File "/home/tseaver/projects/hg-repo/py-2.6/Lib/test/test_file.py", line 181, in testModeStrings
      f = open(TESTFN, mode)
  IOError: [Errno 2] No such file or directory: '@test'
  
  1 test failed:
      test_file

• the added test passes after rebuilding Objects/fielobject.c::

  $ make
  gcc -pthread -c -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Objects/fileobject.o Objects/fileobject.c
  ...
  $ ./python -E -tt Lib/test/regrtest.py test_file
  test_file
  1 test OK.

[BreamoreBoy]

The patch is now way out of date to the extent that I can't find the code in fileobject.c, perhaps I'm just blind Can someone please provide a new patch, thanks.

[amauryfa]

Of course, the implementation is now in the io module:
Modules/_io/_iomodule.c *and* Lib/_pyio.py

[elibendersky]

In Python 3, the documentation no longer mentions that 'U' should not work with '+' (or 'w' or 'a', for that matter), and the code throws ValueError if 'U' is used with 'w' or 'a', but not '+'.

On the other hand, the documentation *does* mention that 'U' is for backwards compatibility and shouldn't be used with new code.

In light of this, how do you suggest to proceed with this issue?

[jcon]

It seems to me that adding the proper check is a good idea. It also may not be obvious to some that 'U' is now more or less an alias for 'r'. I have updated the patch so that it at least applies.

I also removed a redundant reading = 1

[francismb]

On the other hand, the documentation *does* mention that
'U' is for backwards compatibility and shouldn't be used
with new code.

Shouldn't be some deprecation warning somewhere?

[serhiy-storchaka]

In Python 3.4+ 'U' already emits deprecation warning.

[rbtcollins]

Updated patch. I'm not going to apply right now - giving it a little time to let folk chime on whether this should be applied all the way back to 3.4, or not.

My inclination is to only apply it to 3.6.

[bitdancer]

I agree, a change that emits an error where none was emitted before should only go in the next release. With a what's new entry.

It is possible there should be a deprecation or -3 warning in 2.7, but I'm not sure it is worth the effort.

[rbtcollins]

@larry - should this go in 3.5, or would you rather it not?