Improve error message of subprocess when cannot open

[mmokrejs]

BPO	4925
Nosy	@terryjreedy, @amauryfa, @benjaminp, @bitdancer, @briancurtin
Files	issue4925.diff: now with errno.ENOENT instead of 2

^{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 2010-11-20.18:08:24.044>
created_at = <Date 2009-01-12.16:26:57.020>
labels = ['extension-modules', 'easy', 'type-feature']
title = 'Improve error message of subprocess when cannot open'
updated_at = <Date 2010-11-20.18:29:36.043>
user = 'https://bugs.python.org/mmokrejs'

bugs.python.org fields:

activity = <Date 2010-11-20.18:29:36.043>
actor = 'brian.curtin'
assignee = 'none'
closed = True
closed_date = <Date 2010-11-20.18:08:24.044>
closer = 'benjamin.peterson'
components = ['Extension Modules']
creation = <Date 2009-01-12.16:26:57.020>
creator = 'mmokrejs'
dependencies = []
files = ['19688']
hgrepos = []
issue_num = 4925
keywords = ['patch', 'easy']
message_count = 8.0
messages = ['79684', '79685', '79697', '108644', '108895', '121678', '121700', '121701']
nosy_count = 8.0
nosy_names = ['terry.reedy', 'amaury.forgeotdarc', 'mmokrejs', 'benjamin.peterson', 'LambertDW', 'r.david.murray', 'brian.curtin', 'endian']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue4925'
versions = ['Python 3.2']

[mmokrejs]

I think the following error output unsatisfactory as it does not give me
any hint what file was not found:

$ fetch_quals.py blah.txt 
Traceback (most recent call last):
  File "/home/mmokrejs/bin/fetch_quals.py", line 15, in <module>
    _p1 = subprocess.Popen(['query_tracedb', 'retrieve', 'quality',
_id], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
  File "/usr/lib/python2.5/subprocess.py", line 594, in __init__
    errread, errwrite)
  File "/usr/lib/python2.5/subprocess.py", line 1091, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory
$

I know now that I had to specify full path tot he binary or include
shell=True argument but anyway, please improve the error message.

[lambertdw]

Related, but outside python realm, this error likewise confuses:

$ cat <<EOF > s.sh
#! invalid path
echo hi
EOF
$ ./s.sh 
zsh: no such file or directory: ./s.sh

[lambertdw]

(Actual command stream includes chmod +x ./s.sh)

[terryjreedy]

Improved error messages are feature requests because 1) there is no particular guarantee in the doc and 2) changes can break existing code, so should only happen in an new x.y version.

When reporting behavior, it is helpful to give *minimal* code that will reproduce, either in the message if small, or attached. For example, with 3.1.2 on WixXP

>>> import subprocess
>>> subprocess.Popen("xyz")
gives
WindowsError: [Error 2] The system cannot find the file specified

Although in this case the literal name is in the traceback, it is not always. So I am leaving this open. Given that

>>> open('xyx')
gives the more helpful
IOError: [Errno 2] No such file or directory: 'xyx'

it seems it might be possible by using the io code.

[amauryfa]

in PC/_subprocess.c, it should be enough to use PyErr_SetFromWindowsErrWithFilename() instead of PyErr_SetFromWindowsErr()

[endian]

I'm working this for platforms that use OSError. #pythonbugday2010 #myfirstpatch

[benjaminp]

r86593

[bitdancer]

Note that Benjamin's commit only addresses the posix side. Amaury, do you want to fix the windows side?