[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Terry J. Reedy]

Terry J. Reedy added the comment:

In 3.6, the code in question is

def _showwarnmsg_impl(msg):
file = msg.file
if file is None:
file = sys.stderr
if file is None:
# sys.stderr is None when run with pythonw.exe:
# warnings get lost
return
text = _formatwarnmsg(msg)
try:
file.write(text)
except OSError:
# the file (probably stderr) is invalid - this warning gets lost.
pass

When the write is attempted, *file* is not None.  It can either be sys.stderr 
or something else, successfully opened.  If writing to sys.stderr fails, we 
properly should give up.

If writing to something else fails, I now think we should try to write an 
exception to stderr.  So I thing the bug here, if any, is unconditionally 
passing.  Perhaps the exception clause should be replaced (in a new issue) with 
something like

 except (OSError, ValueError):
if msg.file is not sys.stderr:
raise

--
resolution:  -> not a bug
stage: test needed -> resolved
status: open -> closed

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Julius Lehmann-Richter]

Julius Lehmann-Richter added the comment:

Hey Wolfgang,

thank you for the clarification.

I would never write such code of course, I stumbled across this with a 
third-party program.

I understand your point of view and I actually agree. I would not have even 
opened this report if it had not been for that comment.

If the error is still reproducable in the software I will open a bug with them 
instead and tell them to please fix it :P

As far as I am concerned this is not a bug (any more)

Cheers Julius

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Wolfgang Maier]

Wolfgang Maier added the comment:

Ah, I just found Issue607668, which discusses the changeset 2e7fe55c0e11 that 
introduced IOError suppression. The actual use case at the time was different 
and I have no idea whether it would still be a problem today.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Wolfgang Maier]

Wolfgang Maier added the comment:

Hi Julius,

I guess it's a question of control and responsibilities.
If you write a program that may issue warnings under some conditions, that 
program may, for example, be used with stderr redirected to a file. Now it is 
possible that file gets removed while your program is running, then, upon the 
next warning, the program would crash with an IOError. In effect, this would 
let external cirumstances that are not under control by the programmer escalate 
a non-critical warning message to a true exception. It's debatable whether that 
should be prevented or not, but that seems to be the decision that Mark Hammond 
made long ago.
If, OTOH, your program chooses to close sys.stderr, that is a deliberate 
decision and it's the programmers responsibility then not to try to write to it 
anymore. Suppressing the resulting ValueError would only hide the fact that due 
to a programming error no warnings can be issued anymore.
Not sure if this makes any sense to you or others, but that's how I came to 
think about this.

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Julius Lehmann-Richter]

Julius Lehmann-Richter added the comment:

Hey Wolfgang,

thank you for looking into this old one again ;)

The argument you are making does not answer the original bug report though as 
far as I can see.

My initial problem was not the AttributeError about the missing write but a 
ValueError (I/O operation on closed file).

Also this seems to have started a discussion about good programming practice 
between Terry and Antoine, my initial argument though was that, when you are 
catching an IOError for an invalid file, why not catch the ValueError for the 
closed file (since a closed file is surely not a valid target for a write 
operation).

I would really like to be educated on this if I am missing something, why does 
the argument for not silencing errors and expecting good programming practice 
apply to the ValueError of a closed standard error which has not been set to 
None but not to an IOError of a passed in file object?

Cheers Julius

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Wolfgang Maier]

Wolfgang Maier added the comment:

Issue23016 fixed the "AttributeError: 'NoneType' object has no attribute 
'write'" problem when sys.stderr is None.
With that it's now possible and good practice to set sys.stderr = None when 
closing it, just as Antoine suggested.
Should this issue be closed then?

--

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Wolfgang Maier]

Changes by Wolfgang Maier :


--
nosy: +wolma

___
Python tracker 

___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Antoine Pitrou]

Antoine Pitrou added the comment:

This is not about stderr though, this is about the `file` argument that is 
passed to showwarning(). That stderr may be an invalid file is a rather rare 
condition, for good reason (even if you want to silence any program output, it 
is generally better to redirect stderr to /dev/null rather than make it an 
invalid or closed file).

It would be better to research why the original except clause was added than 
try to blindly extend it, IMO.

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue22298
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Antoine Pitrou]

Antoine Pitrou added the comment:

And so the original silencing came without a bug reference or an explanation:

changeset:   25204:2e7fe55c0e11
branch:  legacy-trunk
user:Mark Hammond mhamm...@skippinet.com.au
date:Wed Sep 11 13:22:35 2002 +
files:   Lib/warnings.py
description:
Ignore IOError exceptions when writing the message.


I'm nosying Mark Hammond in case he remembers what his changeset addressed, but 
it's 12 years old and may not be relevant anymore.

--
nosy: +mhammond

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue22298
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Julius Lehmann-Richter]

New submission from Julius Lehmann-Richter:

In Lib/warnings.py the _show_warning function catches IOError with the 
commented intention to ward against an invalid file:

def _show_warning(message, category, filename, lineno, file=None, line=None):
Hook to write a warning to a file; replace if you like.
if file is None:
file = sys.stderr
try:
file.write(formatwarning(message, category, filename, lineno, line))
except IOError:
pass # the file (probably stderr) is invalid - this warning gets lost.

If for some reason the file like object, and in the default case stderr, is 
closed, a calling program is faced with a ValueError, which is not being caught.

It seems to me, and correct me if I am wrong, that a file object which has been 
closed is a case of an invalid file and that the warning subsystem should in 
that case behave in the same manner as in the case of the IOError.

This behavior is the same for python 3.2 with the function renamed to 
showwarning and can be reproduced with for example

from sys import stderr  
from warnings import warn
 
stderr.close()
try:
warn(foo)
except ValueError as e:
print(e)

--
components: Library (Lib)
messages: 226058
nosy: Julius.Lehmann-Richter
priority: normal
severity: normal
status: open
title: Lib/warnings.py _show_warning does not protect against being called with 
a file like object which is closed
type: behavior
versions: Python 2.7, Python 3.2

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue22298
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Terry J. Reedy]

Terry J. Reedy added the comment:

3.1-3.3 only get security fixes. 3.4 outputs write to closed file, so it 
appears to have the same issue. The proposed change seems reasonable to me, as 
a bugfix.

--
nosy: +terry.reedy
stage:  - test needed
versions: +Python 3.4, Python 3.5 -Python 3.2

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue22298
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Antoine Pitrou]

Antoine Pitrou added the comment:

I frankly don't think silencing more exceptions is a good idea. If someone 
decided to close sys.stderr without setting sys.stderr to None I'd say it's a 
programming error and deserves to be notified...

--
nosy: +brett.cannon, pitrou -Julius.Lehmann-Richter, terry.reedy

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue22298
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Antoine Pitrou]

Antoine Pitrou added the comment:

Oops, sorry for the bogus un-nosying.

--
nosy: +Julius.Lehmann-Richter, terry.reedy

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue22298
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue22298] Lib/warnings.py _show_warning does not protect against being called with a file like object which is closed

[Terry J. Reedy]

Terry J. Reedy added the comment:

If sys.stderr is set to None, then if file is None: file = sys.stderr will 
have no effect and 'file.write' raises AttributeError: 'NoneType' object has 
no attribute 'write' (actual quote).  Do you propose to catch that?  If not, 
it is not much of an improvement. Multiple people have reported on python-list 
that they are mystified by messages like this when they never used a None 
object as an argument.

I am sympathetic to not overlooking bugs.  However, a traceback is normally 
lost in this situation anyway.  If sys.stderr is closed, trying to write a 
ValueError traceback (or any other traceback) to stderr generates another 
ValueError.  Python apparently tries to write a double traceback to 
sys.__stderr__ as a backup (see below).  If that is also unusable (perhaps 
because it *is* sys.stderr), then the traceback disappears anyway.  This is 
what happens (on Windows, at least) with python started from either an icon or 
in Command Prompt.

 import sys; sys.stderr.close()
 from warnings import warn; warn('foo')
 sys.stderr is sys.__stderr__
True

When Idle starts an icon, the result is the same except that the user process 
is restarted.  Also, two streams are not the same; sys.__stderr__ is None.

If Idle is run with python from a console, there is still no message in the 
Idle shell but since sys.__stderr__ points back to the console, the double 
traceback appears there: first the warn ValueError, then 'while 
proccessing...', the traceback ValueError.  Idle replaces showwarning.  I 
believe it now only changes the formatting, but I could have it use 
sys.__stderr__ when not None to print the warning instead of or in addition to 
the tracebacks.

If the code above is placed in a file and run in batch mode, python quits with 
no output.

C:\Programs\Python34python temop.py

C:\Programs\Python34

When run from the idle editor running under pythonw, the user process does the 
same (exit without output).

--

___
Python tracker rep...@bugs.python.org
http://bugs.python.org/issue22298
___
___
Python-bugs-list mailing list
Unsubscribe: 
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com