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mozprocess provides python process management via an operating system and platform transparent interface to Mozilla platforms of interest. Mozprocess aims to provide the ability to robustly terminate a process (by timeout or otherwise), along with any child processes, on Windows, OS X, and Linux. Mozprocess utilizes and extends subprocess.Popen to these ends.


mozprocess.processhandler:ProcessHandler is the central exposed API for mozprocess. ProcessHandler utilizes a contained subclass of subprocess.Popen, Process, which does the brunt of the process management.

Basic usage

process = ProcessHandler(['command', '-line', 'arguments'],
                         cwd=None, # working directory for cmd; defaults to None
                         env={},   # environment to use for the process; defaults to os.environ
                         ) # seconds

ProcessHandler offers several other properties and methods as part of its API:

def __init__(self,
             ignore_children = False,
    cmd = Command to run
    args = array of arguments (defaults to None)
    cwd = working directory for cmd (defaults to None)
    env = environment to use for the process (defaults to os.environ)
    ignore_children = when True, causes system to ignore child processes,
    defaults to False (which tracks child processes)
    processOutputLine = handlers to process the output line
    onTimeout = handlers for timeout event
    kwargs = keyword args to pass directly into Popen

    NOTE: Child processes will be tracked by default. If for any reason
    we are unable to track child processes and ignore_children is set to False,
    then we will fall back to only tracking the root process. The fallback
    will be logged.

def timedOut(self):
    """True if the process has timed out."""

def run(self, timeout=None, outputTimeout=None):
    Starts the process.

    If timeout is not None, the process will be allowed to continue for
    that number of seconds before being killed.

    If outputTimeout is not None, the process will be allowed to continue
    for that number of seconds without producing any output before
    being killed.

def kill(self):
    Kills the managed process and if you created the process with
    'ignore_children=False' (the default) then it will also
    also kill all child processes spawned by it.
    If you specified 'ignore_children=True' when creating the process,
    only the root process will be killed.

    Note that this does not manage any state, save any output etc,
    it immediately kills the process.

def readWithTimeout(self, f, timeout):
    Try to read a line of output from the file object |f|.
    |f| must be a pipe, like the |stdout| member of a subprocess.Popen
    object created with stdout=PIPE. If no output
    is received within |timeout| seconds, return a blank line.
    Returns a tuple (line, did_timeout), where |did_timeout| is True
    if the read timed out, and False otherwise.

    Calls a private member because this is a different function based on
    the OS

def processOutputLine(self, line):
    """Called for each line of output that a process sends to stdout/stderr."""
    for handler in self.processOutputLineHandlers:

def onTimeout(self):
    """Called when a process times out."""
    for handler in self.onTimeoutHandlers:

def onFinish(self):
    """Called when a process finishes without a timeout."""
    for handler in self.onFinishHandlers:

def wait(self, timeout=None):
    Waits until all output has been read and the process is 

    If timeout is not None, will return after timeout seconds.
    This timeout only causes the wait function to return and
    does not kill the process.

See for the python implementation.

ProcessHandler extends ProcessHandlerMixin which by default prints the output, logs to a file (if specified), and stores the output (if specified, by default True). ProcessHandlerMixin, by default, does none of these things and has no handlers for onTimeout, processOutput, or onFinish.

ProcessHandler may be subclassed to handle process timeouts (by overriding the onTimeout() method), process completion (by overriding onFinish()), and to process the command output (by overriding processOutputLine()).


In the most common case, a process_handler is created, then run followed by wait are called:

proc_handler = ProcessHandler([cmd, args]) # will time out after 60 seconds without output

Often, the main thread will do other things:

proc_handler = ProcessHandler([cmd, args]) # will time out after 60 seconds regardless of output

if proc_handler.proc.poll() is None:

By default output is printed to stdout, but anything is possible:

# this example writes output to both stderr and a file called 'output.log'
def some_func(line):
    print >> sys.stderr, line

    with open('output.log', 'a') as log:
        log.write('%s\n' % line)

proc_handler = ProcessHandler([cmd, args], processOutputLine=some_func)


  • Document improvements over subprocess.Popen.kill
  • Introduce test the show improvements over subprocess.Popen.kill