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What is Sarge for?

If you want to interact with external programs from your Python applications, Sarge is a library which is intended to make your life easier than using the subprocess module in Python’s standard library.

Sarge is, of course, short for sergeant – and like any good non-commissioned officer, sarge works to issue commands on your behalf and to inform you about the results of running those commands.

The acronym lovers among you might be amused to learn that sarge can also stand for “Subprocess Allegedly Rewards Good Encapsulation” :-)

Here’s a taster (example suggested by Kenneth Reitz’s Envoy documentation):

>>> from sarge import capture_stdout
>>> p = capture_stdout('fortune|cowthink')
>>> p.returncode
>>> p.commands
[Command('fortune'), Command('cowthink')]
>>> p.returncodes
[0, 0]
>>> print(p.stdout.text)
( The last thing one knows in        )
( constructing a work is what to put )
( first.                             )
(                                    )
( -- Blaise Pascal                   )
        o   ^__^
         o  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

The capture_stdout() function is a convenient form of an underlying function, run(). You can also use conditionals:

>>> from sarge import run
>>> p = run('false && echo foo')
>>> p.commands
>>> p.returncodes
>>> p.returncode
>>> p = run('false || echo foo')
>>> p.commands
[Command('false'), Command('echo foo')]
>>> p.returncodes
[1, 0]
>>> p.returncode

The conditional logic is being done by sarge and not the shell – which means you can use the identical code on Windows. Here’s an example of some more involved use of pipes, which also works identically on Posix and Windows:

>>> cmd = 'echo foo | tee stdout.log 3>&1 1>&2 2>&3 | tee stderr.log > %s' % os.devnull
>>> p = run(cmd)
>>> p.commands
[Command('echo foo'), Command('tee stdout.log'), Command('tee stderr.log')]
>>> p.returncodes
[0, 0, 0]
vinay@eta-oneiric64:~/projects/sarge$ cat stdout.log
vinay@eta-oneiric64:~/projects/sarge$ cat stderr.log

In the above example, the first tee invocation swaps its stderr and stdout – see this post for a longer explanation of this somewhat esoteric usage.

Why not just use subprocess?

The subprocess module in the standard library contains some very powerful functionality. It encapsulates the nitty-gritty details of subprocess creation and communication on Posix and Windows platforms, and presents the application programmer with a uniform interface to the OS-level facilities. However, subprocess does not do much more than this, and is difficult to use in some scenarios. For example:

  • You want to use command pipelines, but using subprocess out of the box often leads to deadlocks because pipe buffers get filled up.
  • You want to use bash-style pipe syntax on Windows, but Windows shells don’t support some of the syntax you want to use, like &&, ||, |& and so on.
  • You want to process output from commands in a flexible way, and communicate() is not flexible enough for your needs – for example, you need to process output a line at a time.
  • You want to avoid shell injection problems by having the ability to quote your command arguments safely.
  • subprocess allows you to let stderr be the same as stdout, but not the other way around – and you need to do that.

Main features

Sarge offers the following features:

  • A simple run command which allows a rich subset of Bash-style shell command syntax, but parsed and run by sarge so that you can run on Windows without cygwin.

  • The ability to format shell commands with placeholders, such that variables are quoted to prevent shell injection attacks:

    >>> from sarge import shell_format
    >>> shell_format('ls {0}', '*.py')
    "ls '*.py'"
    >>> shell_format('cat {0}', 'a file name with spaces')
    "cat 'a file name with spaces'"
  • The ability to capture output streams without requiring you to program your own threads. You just use a Capture object and then you can read from it as and when you want:

    >>> from sarge import Capture, run
    >>> with Capture() as out:
    ...     run('echo foobarbaz', stdout=out)
    <sarge.Pipeline object at 0x175ed10>
    >>> out.read(3)
    >>> out.read(3)
    >>> out.read(3)
    >>> out.read(3)
    >>> out.read(3)

    A Capture object can capture the output from multiple commands:

    >>> from sarge import run, Capture
    >>> p = run('echo foo; echo bar; echo baz', stdout=Capture())
    >>> p.stdout.readline()
    >>> p.stdout.readline()
    >>> p.stdout.readline()
    >>> p.stdout.readline()

    Delays in commands are honoured in asynchronous calls:

    >>> from sarge import run, Capture
    >>> cmd = 'echo foo & (sleep 2; echo bar) & (sleep 1; echo baz)'
    >>> p = run(cmd, stdout=Capture(), async=True) # returns immediately
    >>> p.close() # wait for completion
    >>> p.stdout.readline()
    >>> p.stdout.readline()
    >>> p.stdout.readline()

Here, the sleep commands ensure that the asynchronous echo calls occur in the order foo (no delay), baz (after a delay of one second) and bar (after a delay of two seconds); the capturing works as expected.

Python version and platform compatibility

Sarge is intended to be used on any Python version >= 2.6 and is tested on Python versions 2.6, 2.7, 3.1, 3.2 and 3.3 on Linux, Windows, and Mac OS X (not all versions are tested on all platforms, but are expected to work correctly).

Project status

The project has reached alpha status in its development: there is a test suite and it has been exercised on Windows, Ubuntu and Mac OS X. However, because of the timing sensitivity of the functionality, testing needs to be performed on as wide a range of hardware and platforms as possible.

The source repository for the project is on BitBucket:


You can leave feedback by raising a new issue on the issue tracker (BitBucket registration not necessary, but recommended).


For testing under Windows, you need to install the GnuWin32 coreutils package, and copy the relevant executables (currently libiconv2.dll, libintl3.dll, cat.exe, echo.exe, tee.exe, false.exe, true.exe, sleep.exe and touch.exe) to the directory from which you run the test harness (test_sarge.py).

API stability

Although every attempt will be made to keep API changes to the absolute minimum, it should be borne in mind that the software is in its very early stages. For example, the asynchronous feature (where commands are run in separate threads when you specify & in a command pipeline) can be considered experimental, and there may be changes in this area. However, you aren’t forced to use this feature, and sarge should be useful without it.

Change log

0.1.5 (future)

Released: Not yet.


Released: 2015-01-24

  • Fixed issue #21: Don’t parse if shell=True.
  • Fixed issue #20: Run pipeline in separate thread if async.
  • Fixed issue #23: Return the correct return code when shell=True.
  • Improved logging.
  • Minor documentation updates.
  • Minor additions to tests.


Released: 2014-01-17

  • Fixed issue #15: Handled subprocess internal changes in Python 2.7.6.
  • Improved logging support.
  • Minor documentation updates.


Released: 2013-12-17

  • Fixed issue #13: Removed module globals to improve thread safety.
  • Fixed issue #12: Fixed a hang which occurred when a redirection failed.
  • Fixed issue #11: Added + to the characters allowed in parameters.
  • Fixed issue #10: Removed a spurious debugger breakpoint.
  • Fixed issue #9: Relative pathnames in redirections are now relative to the current working directory for the redirected process.
  • Added the ability to pass objects with fileno() methods as values to the input argument of run(), and a Feeder class which facilitates passing data to child processes dynamically over time (rather than just an initial string, byte-string or file).
  • Added functionality under Windows to use PATH, PATHEXT and the registry to find appropriate commands. This can e.g. convert a command 'foo bar', if 'foo.py' is a Python script in the c:\Tools directory which is on the path, to the equivalent 'c:\Python26\Python.exe c:\Tools\foo.py bar'. This is done internally when a command is parsed, before it is passed to subprocess.
  • Fixed issue #7: Corrected handling of whitespace and redirections.
  • Fixed issue #8: Added a missing import.
  • Added Travis integration.
  • Added encoding parameter to the Capture initializer.
  • Fixed issue #6: addressed bugs in Capture logic so that iterating over captures is closer to subprocess behaviour.
  • Tests added to cover added functionality and reported issues.
  • Numerous documentation updates.


Released: 2013-06-04

  • expect method added to Capture class, to allow searching for specific patterns in subprocess output streams.
  • added terminate, kill and poll methods to Command class to operate on the wrapped subprocess.
  • Command.run now propagates exceptions which occur while spawning subprocesses.
  • Fixed issue #4: shell_shlex does not split on @.
  • Fixed issue #3: run et al now accept commands as lists, just as subprocess.Popen does.
  • Fixed issue #2: shell_quote implementation improved.
  • Improved shell_shlex resilience by handling Unicode on 2.x (where shlex breaks if passed Unicode).
  • Added get_stdout, get_stderr and get_both for when subprocess output is not expected to be voluminous.
  • Added an internal lock to serialise access to shared data.
  • Tests added to cover added functionality and reported issues.
  • Numerous documentation updates.


Released: 2012-02-10

  • Initial release.

Next steps

You might find it helpful to look at the Tutorial, or the API Reference.

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