Custom Formatting The Output Of subprocess.Popen

[Steve Holden]

thedsadude at gmail.com wrote:
> On Nov 21, 8:50 pm, Scott David Daniels <Scott.Dani... at Acm.Org> wrote:
>> thedsad... at gmail.com wrote:
>>>   Hello,
>>>   I'm launching a script as follows:
>>> <code>
>>> p = subprocess.Popen(['./p.py', 'aa'])
>>> p.wait()
>>> </code>
>>>   If p.py writes to sys.stdout, then it is shown on the console....
>> You seem to be missing the fact that ./py is run in a different process.
>> The "sys.stdout" that p.py uses is different from that in the program
>> calling Popen.  In fact, it could be using a different Python.  The
>> situation is really similar to
>>      p = subprocess.Popen([<basic program>, 'aa'])
>> in that you have no way to "muck with the guts" of the subprocess, you
>> can only post-process its output.
>>
>> --Scott David Daniels
>> Scott.Dani... at Acm.Org
> 
> 
>   Hello,
> 
>   Thanks Diez & Scott for your replies.
> 
>   The subprocess.Popen documentation states, concerning the stding,
> stdout, stderr, arguments, that:
> <quote>
> 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. PIPE indicates that a new
> pipe to the child should be created. With None, no redirection will
> occur; the child's file handles will be inherited from the parent.
> Additionally, stderr can be STDOUT, which indicates that the stderr
> data from the applications should be captured into the same file
> handle as for stdout.
> </quote>
>   so, it seems to me that if I would know how to write a file object,
> then I could write one that prefixes each line, and that would be
> fine, no? I don't see how this would necessitate waiting for p.py's
> termination, or matter that it is a different process. I just don't
> know how to create a file object.
> 
It was an astute observation that you could provide your own "file-like
object" as the output for the subprocess. However, you may find that due
to buffering effects the subprocess's output doesn't get completely
intermingled with the calling process's output.

Although you can specify "unbuffered", which is the default for
subprocess.popen, I don't believe this guarantees that the subprocess
itself won't perform buffering. Others may know better.

regards
 Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC              http://www.holdenweb.com/