Raising exception on STDIN read

Ian Clark iclark at mail.ewu.edu
Thu Feb 28 18:22:21 CET 2008


On 2008-02-28, Gabriel Genellina <gagsl-py2 at yahoo.com.ar> wrote:
> En Thu, 28 Feb 2008 14:29:04 -0200, Ian Clark <iclark at mail.ewu.edu>  
> escribió:
>
>> On 2008-02-27, Gabriel Genellina <gagsl-py2 at yahoo.com.ar> wrote:
>>> En Wed, 27 Feb 2008 15:06:36 -0200, Ian Clark <iclark at mail.ewu.edu>
>>> escribi�:
>>>
>>>> On 2008-02-27, Michael Goerz <newsgroup898sfie at 8439.e4ward.com> wrote:
>>>>> Hi,
>>>>>
>>>>> I would like to raise an exception any time a subprocess tries to read
>>>>> from STDIN:
>>>>>
>>>>> latexprocess = subprocess.Popen( \
>>>>>          'pdflatex' + " " \
>>>>>           + 'test' + " 2>&1", \
>>>>>          shell=True, \
>>>>>          cwd=os.getcwd(), \
>>>>>          env=os.environ, \
>>>>>          stdin=StdinCatcher() # any ideas here?
>>>>>      )
>>>>>
>>>>> An exception should be raised whenever the pdflatex process
>>>>> reads from STDIN... and I have no idea how to do it. Any suggestions?
>>>
>>>> How about with a file-like object? I haven't tested this with  
>>>> subprocess
>>>> so you might want to read the manual on files if it doesn't work[1].
>>>
>>> Won't work for an external process, as pdflatex (and the OS) knows  
>>> nothing
>>> about Python objects. The arguments to subprocess.Popen must be actual
>>> files having real OS file descriptors.
>>
>> Taken from the subprocess documentation (emphasis mine). [1]
>>
>> 	stdin, stdout and stderr specify the executed programs' standard
>> 	input, standard output and standard error file handles,
>> 	respectively. Valid values are PIPE, an existing file descriptor (a
>> 	positive integer), *an existing file object*, and None.
>>
>> The following peice of code works fine for me with the subprocess
>> module. NOTE: the only difference from this and the last I posted is
>> that I set fileno() to _error().
>>
>>     import sys
>>     import subprocess
>>
>>     class ErrorFile(object):
>>         def _error(self, *args, **kwargs):
>>             raise AssertionError("Illegal Access")
>>
>>         def _noop(self, *args, **kwargs):
>>             pass
>>
>>         close = flush = seek = tell = _noop
>>         next = read = readline = readlines = xreadlines = tuncate =  
>> _error
>>         truncate = write = writelines = fileno = _error
>>         #                               ^^^^^^
>>
>>     proc = subprocess.Popen("cat -", shell=True, stdin=ErrorFile())
>>     ret = proc.wait()
>>     print "return", ret
>
> I don't see how this could ever work. The shell knows nothing about your  
> ErrorFile objects. If subprocess.Popen doesn't reject that ErrorFile  
> instance, it's a bug. An ErrorFile instance is not "an existing file  
> object".

Could you please explain why this won't work. ErrorFile exposes most all
attributes of file objects, so I don't understand why it wouldn't be
considered a file object.

Did you try running the code by any chance? It works fine for me on
2.5.1 on Linux.

Ian




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