I am trying to get the output from an external program into python using `subprocess.Popen` and `select.select`.   For some reason though select.select is at times telling me that stdout is not ready to read, even when it is (reading from it works).   

This problem exists in python 3.1 & 3.2 but not in python 2.7.



For my particular application I am connecting to external program via ssh.  I would not expect that the ssh connection would be an issue.

The program that I am calling produces some output and eventually asks for user input.  In the example below I only get a portion of the output, then at some point select.select tells me that read is not available.  If I try to read anyway I can keep getting more output.  As soon as I press a key (passing something to stdin),  select.select tells me that I can read again and I get the rest of the output.   

Any ideas?
    

    def wrapExternal(host=None):

       command = ["ssh", "-t", host, "some_program"]

       child = subprocess.Popen(command
                                ,bufsize=0
                                ,stdout=subprocess.PIPE
                                ,stderr=subprocess.STDOUT)
        
       out = ''
       while True:
          r, w, x = select.select([child.stdout], [], [], 1.0)
    
          if child.poll() is not None:
             break
                        
          if r:
             out = child.stdout.read(1)
             print(out.decode(), end='')
             sys.stdout.flush()
    
       return child.returncode

I forgot to mention, I am running on OS X 10.6.6.

Could you try with the attached patch ?
The problem is that subprocess silently replaces bufsize=0, so child.stdout is actually buffered, and when you read just one byte, everything that's available for reading is read into the python's object buffer. Then, select/poll doesn't see the pipe as ready for reading since everything as already been read.
Mixing buffered I/O and select leads to trouble, you're right to pass bufsize=0, but I don't know why subprocess goes out of its way and buffers it anyway:
        if bufsize == 0:
            bufsize = 1  # Nearly unbuffered (XXX for now)

Applying the patch appears to fix this problem. Thanks!


I am definitely confused about why the buffer was changed to line buffered in the first place, especially since the default is bufsize=0 and the comment ("# Nearly unbuffered (XXX for now)") does not appear to match the actual behavior.

The Charles' patch fixes the problem but breaks [test_os test_poll test_popen test_select test_uuid] when running make test.

Those two lines were introduced by Guido in [1f7891d84d93] but that was back in 2007 when subprocess used os.fdopen and the new IO hadn't been invented (I think). They should probably be removed now.

I think it breaks those tests because despite the fact that the docs claim subprocess defaults to unbuffered, it has been set at bufsize=1 which actually causes line buffering mode. os.popen wraps the returned stream with a TextIOWrapper which doesn't work with an unbuffered stream.

The attached patch causes os.popen (and platform.popen) to default to line buffered mode and to throw an error if buffering=0 is passed (unbuffered).

I don't think this should be backported since there is some behavior change.

ross's patch looks good to me.

Isn't the behavior just plain broken in 3.1 and 3.2?  The docs say that the default bufsize=0 is unbuffered in Popen but the implementation has that nasty XXX to make it line buffered instead of unbuffered in it.

This appears to have hid the problems with the textiowrapper being used in os.popen.

I think fixing it in 3.1 and 3.2 is the right thing to do.

It is desirable for os.popen()'s default buffering to match that of open() itself which is that they're buffered a buffer size of io.DEFAULT_BUFFER_SIZE.

to get that behavior, change the =1 default to =io.DEFAULT_BUFFER_SIZE in ross's patch.

> to get that behavior, change the =1 default to =io.DEFAULT_BUFFER_SIZE in ross's patch.

The problem is, it doesn't seem like you can import and use io.DEFAULT_BUFFER_SIZE from inside os.py. Python fails to start.

You can import _io.DEFAULT_BUFFER_SIZE though.

I agree with Gregory that fixing this in 3.1 and 3.2 is the way to go. 

Otherwise I would suggest changing the documentation to match the behavior.

yeah i figured importing io from os at the top level might be a problem. it is not important for the default to be that exact value, even something safely on the small side like 512 will work.  but we could just have the default be set in the code by doing the 'import io' at os.popen() runtime.

Attached is a patch which uses -1 for the buffer size of popen(). This gets translated by the io.open() to the default io buffer size.

Here is the adapted patch for 3.1, with a test case.

New changeset 1dc52ecb8949 by Antoine Pitrou in branch '3.1':
Issue #11459: A `bufsize` value of 0 in subprocess.Popen() really creates
http://hg.python.org/cpython/rev/1dc52ecb8949

New changeset 7451da272111 by Antoine Pitrou in branch '3.2':
Issue #11459: A `bufsize` value of 0 in subprocess.Popen() really creates
http://hg.python.org/cpython/rev/7451da272111

New changeset cb148da52c47 by Antoine Pitrou in branch 'default':
Issue #11459: A `bufsize` value of 0 in subprocess.Popen() really creates
http://hg.python.org/cpython/rev/cb148da52c47

Fixed now, thank you!