Success with subprocess communicate on Windows?

[Wolfgang Maier]

On 07/03/2014 06:09 AM, Terry Reedy wrote:
>
> Yes, but what puzzled me is that running
> subprocess.check_output(r'pyflakes c:\programs\python34\lib')
> in the regular interpreter *does* produce output instead of the error
> message. My guess is that it fills up the pipe, so that check_output
> starts reading the pipe long before pyflakes exits with status 1.
>
> Hmm. I tried it again, and I see some but not all of the output I got at
> the command line *and* I see the exit status message. So the subprocess
> must get some of the output but then stop when it sees the exit status.
>   Thanks.
>

For a partial explanation try this:

from the command line (again my path is slightly different):

pyflakes C:\Python34\lib > stdout.txt

this will still give you a few lines of output and these should be the 
same ones you're seeing from the python interpreter when you're doing:

subprocess.check_output(r'pyflakes C:\Python34\lib')

==> pyflakes sends these lines to stderr instead of stdout !!

confirmation:

subprocess.check_output(r'pyflakes C:\Python34\lib', stderr=subprocess.PIPE)

and the output is gone.

So the remaining questions are:
- why on earth is pyflakes sending these lines (and only these) to stderr ?

- what is happening to the stderr output when run in IDLE ? I guess it 
is caught and suppressed somewhere, but to add to your observations the 
check_output call doesn't hang on IDLE, but finishes eventually with no 
output other than the traceback.

Best wishes,
Wolfgang