Category Archives: Python

High-polish use of subprocess.Popen

Python has a pretty decent facility to launch and operate a child process, subprocess.popen. However, like many “scripting systems”, it’s easy to do something that mostly works but is rough around the edges and not all that robust, and this is because sub-processes don’t all run in 100 milliseconds without errors.

First off, avoid the use of It waits for the process to terminate before returning, which means that if your subprocess hangs, your Python program will hang.

Second, if you’re using Python 2.7 on POSIX, use subprocess32, which is a backport of subprocess from Python 3.

Third, stop using os.popen in favor of subprocess.Popen. It’s a little more complicated, but worth it.

Fourth, keep in mind that Popen.communicate() also blocks until the process terminates, so don’t use it either. Also, communicate() doesn’t seem to handle large amounts of output on some systems (reports of “no more than 65535 bytes of output due to Linux pipe implementation”).

Reading stdout

Now, on to actual details. Let’s call dir on Windows and number each line in the output
from __future__ import print_function

import subprocess
import sys

proc = subprocess.Popen(args=['dir'] + sys.argv[1:], stdin=subprocess.PIPE,
             stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True)
linenum = 1
while True:
  line = proc.stdout.readline()
  if len(line) == 0:
  print("%d: %s" % (linenum, line), end='')
  linenum += 1

We are merging stderr and stdout together in this example (stderr=subprocess.STDOUT). If we run this on C:\Windows\System32 like so /s C:\Windows\System32

we’ll start seeing output like this

1:  Volume in drive C is OSDisk
2:  Volume Serial Number is 062F-8F58
4:  Directory of c:\Windows\System32
6: 04/23/2014  06:09 PM    <DIR>          .
7: 04/23/2014  06:09 PM    <DIR>          ..
8: 04/12/2011  12:38 AM    <DIR>          0409
9: 01/14/2014  11:21 AM    <DIR>          1033
10: 06/10/2009  02:16 PM             2,151 12520437.cpx
11: 06/10/2009  02:16 PM             2,233 12520850.cpx
12: 02/14/2013  09:34 PM           131,584 aaclient.dll
13: 11/20/2010  08:24 PM         3,727,872 accessibilitycpl.dll

And since this is under our control, we can pipe to more, we can control-C to stop it, and so on.

There are still complications, mostly around buffering. The default for Popen is to not buffer data, but that only affects the reader – the source process can still buffer. You can trick programs into thinking they are writing into a console, which usually means that output will be unbuffered. You can use the low-level pty module directly (on Unix) or something higher-level like pexpect

  • Unix:
  • Windows:

Of course, not all processes write lines. You can use a more generalized approach by reading bytes from the stdout pipe. The previous program modifed to read 128 bytes at a time looks like this

while True:
  line =
  if len(line) == 0:
  print("<%d>: %s" % (linenum, line), end='')
  linenum += 1

and produces this output (with numbers changed to to stand out more)

<1>:  Volume in drive C is OSDisk
 Volume Serial Number is 062F-8F58

 Directory of c:\Windows\System32

04/23/2014  06:09 PM    <DIR<2>: >          .
04/23/2014  06:09 PM    <DIR>          ..
04/12/2011  12:38 AM    <DIR>          0409
01/14/2014  11:21 AM    <DIR><3>:           1033
06/10/2009  02:16 PM             2,151 12520437.cpx
06/10/2009  02:16 PM             2,233 12520850.cpx
02/14/201<4>: 3  09:34 PM           131,584 aaclient.dll
11/20/2010  08:24 PM         3,727,872 accessibilitycpl.dll

And of course this would work for programs that are reading and writing octet streams, not just text.

Reading stdout and stderr

Sometimes you want to read from stderr and stdout independently, because you need to react to output on stderr. You can’t just call read or readline, because it could block waiting for input on a handle.

On Unix systems, you can call select on the stdin and stdout handles, because select works on file-like objects, including pipes. On Windows, select only works on sockets, so you need to use some threads and a queue to have a blocking read per handle. Since this works on Unix as well, we can do it for both.

import Queue
io_q = Queue.Queue(5) # somewhat arbitrary, readers block when queue is full
def read_from_stream(identifier, stream):
  for line in stream:
    io_q.put((identifier, line))
  if not stream.closed:

import threading
threading.Thread(target=read_from_stream, name='stdout-stream', args=('STDOUT', proc.stdout)).start()
threading.Thread(target=read_from_stream, name='stderr-stream', args=('STDERR', proc.stderr)).start()

while True:
    item = io_q.get(False)
  except Queue.Empty:
    if proc.poll() is not None:
    identifier, line = item
    print(identifier + ':',  line, end='')

This works well, but has a flaw – it is basically busy-waiting, burning CPU while waiting for input to come in. We’re doing this because we don’t want to block at the reader level – consider that in a more complex situation, we might want to do processing while waiting for input to come in. There’s also a race condition here, in that we could check the queue, it could be empty, then a reader could put something in the queue while we are checking proc.poll(), and then we could miss that item.

We could do something like this, which is not clean, but works

import Queue
io_q = Queue.Queue(5)
def read_from_stream(identifier, stream):
  if not stream:
    print('%s does not exist' % identifier)
    io_q.put(('EXIT', identifier))
  for line in stream:
    io_q.put((identifier, line))
  if not stream.closed:
  print('%s is done' % identifier)
  io_q.put(('EXIT', identifier))

import threading
active = 2
threading.Thread(target=read_from_stream, name='stdout-stream', args=('STDOUT', proc.stdout)).start()
threading.Thread(target=read_from_stream, name='stderr-stream', args=('STDERR', proc.stderr)).start()

while True:
    item = io_q.get(True, 1)
  except Queue.Empty:
    if proc.poll() is not None:
    identifier, line = item
    if identifier == 'EXIT':
      active -= 1
      if active == 0:
      print(identifier + ':',  line, end='')


Now there’s no busy-waiting, and we exit instantly. This is also a lot of scaffolding to write for each time we use subprocess.Popen(). One answer would be to wrap this up into a helper class, or rather a set of helper classes.

stdin and stdout and stderr

There are two cases here

  1. Feeding a pipe that takes input and returns output.
  2. Running an interactive process

For the former, you could just have a file or psuedo-file feed the Popen process instead of subprocess.PIPE. For the latter, you definitely need to trick your Popen process into thinking that it’s writing to a TTY, otherwise the buffering will kill you.




Python 2.7 end-of-life extended to 2020

Guido Van Rossum evidently announced at PyCon 2014 that Python 2.7 would be supported through 2020 (the previous cut-off date was 2015).

A HackerNews thread started by intimating that this was partly in release to RedHat needing long-term support for the version of Python in RHEL 7, and that version of Python will almost certainly be Python 2.7.

I doubt that it was anything more than a very minor contributing factor.

Python argparse

Command-line processing

As of Python 2.7, the preferred command-line parsing library is argparse. The pattern is straightforward – create an ArgumentParser, add all the command-line options, then call it and extract arguments.

import argparse
def commandline():
  parser = argparse.ArgumentParser(
    description='Your command-line description',

  parser.add_argument('path', nargs='*', help='help string')
  parser.add_argument('--user', action='append', help='help string')
  parser.add_argument('--verbose', action='count', default=0, help='help string')
  parser.add_argument('--quiet', action='store_true', help='help string')

  args = parser.parser_args()

  print '  path   =    %s' % ',\n              '.join(args.path)
  print '  user   =    %s' % ',\n              '.join(args.user)
  print '  verbose= %d' % args.verbose
  print '  quiet = %s' % 'True' if args.quiet else 'False'


sub/one sub/three --user=sub/two --verbose --quiet --verbose

this produces

path    = sub/one
user    = sub/two
verbose = 2
quiet   = True

Arguments are positional or optional. In the example above, all arguments not attached to one of the listed options is gathered up into the ‘path’ positional argument.

More reading



Gitstats, source at

I think this could be turned into something pretty cool with a little bit of work. As-is, it uses gnuplot to do the visualization, so that makes it a little hard to use on Windows. It would be nicer if it used matplotlib or the Python ggplot2, or the approach that IPython Notebook took.

So, speaking of “the future of visualization in Python”, here are some links.

And for fun, to emulate XKCD

Cog – interesting Python-based code generator

I like the approach, which is that you can (1) do generated code for any language and (2) edit the generated code with some freedom, because there are markers to separate generated code from non-generated code.

Cog uses Python – it runs any Python code inside the file, replacing the whole file with the output from the generation. I’ll have to try it out in conjunction with SCons.

Extending Python import

I want Python’s import to look something like this

import google.protobuf.message from ('Python Google Protocol Buffers', '2.5.0',

The extra information is hints – hints to the import system to make sure that it found the correct google package, hints to a search/download system on where to look for it if it’s not already present locally. This would basically replace pip. I don’t want to need to install packages. I don’t mind installing packages, you can prebake a system, but it should be an option for efficiency’s sake, not something that everyone must do.

Go has taken a big step in this direction, but it’s a little inflexible. It will download a package from a remote repository, but you have to specify the path, and it’s to a specific repository. This has long-term implications, all your code will need updating if the repository moves. Also, this is not very distributed, and you can’t package things up easily for bundle distribution. Every first-time run of a Go program is going to hit the remote repositories – packages are cached in your local program, but not even system-wide.

What I want is very loose coupling. If I have modules preinstalled on my system and they are the right modules, they get used. If not, my import casts a net to find it, and this search process can be enhanced as desired. For example, a company might want to keep a local cache server of packages so that most searches can be satisfied locally, only going out to other networks for unknown packages. Or an organization might push a giant set of likely-defined modules to every system.

I also want as much flexibility as possible. I like the Google search approach – no schema that has to absolutely be followed, it’s all hints, hints to a search engine. But the hints are specific enough that you’ll find exactly what you are looking for.

Also, I want something that automatically handles architecture differences, platform differences, compiler/interpreter differences, and so on. If I run my code in Python 2.7, I want it to use a Python 2.7-compatible library (e.g. perhaps already precompiled for my exact version of Python), but if I run it in Python 3.3, I might expect a different Python 3.3-savvy library to be used. Or if I’m running on a Windows 7 64-bit machine, then I want any native code to be specific to my architecture and operating system.


surrogate is a library used to create stubs for non-existing modules in sys.modules. Its aim is for unit testing, but it’s an example of manipulating sys.modules.

PEP 302 — New Import Hooks is the original specification for import hooks, kept around because the current system uses it for documentation on how to make custom importers.

The current system is described in The import system and importlib in the Python 3 documentation; also see sys.meta_path and related in the sys module reference. I don’t yet know how much of this is in Python 2.7. However, this still doesn’t let me get extra hint information to my custom importer.

The sum of all knowledge of Python packaging is contained herein: Python Packaging User Guide. OK, maybe not quite, but this is from the Python Packaging Authority, a group working on packaging and installing distributions in Python.

Bento is another interesting Python package system.

Some related module-system PEPs