How to check if there exists a process with a given pid in Python?

Sending signal 0 to a pid will raise an OSError exception if the pid is not running, and do nothing otherwise.

import os

def check_pid(pid):        
    """ Check For the existence of a unix pid. """
    try:
        os.kill(pid, 0)
    except OSError:
        return False
    else:
        return True

Have a look at the psutil module:

psutil (python system and process utilities) is a cross-platform library for retrieving information on running processes and system utilization (CPU, memory, disks, network) in Python. [...] It currently supports Linux, Windows, OSX, FreeBSD and Sun Solaris, both 32-bit and 64-bit architectures, with Python versions from 2.6 to 3.4 (users of Python 2.4 and 2.5 may use 2.1.3 version). PyPy is also known to work.

It has a function called pid_exists() that you can use to check whether a process with the given pid exists.

Here's an example:

import psutil
pid = 12345
if psutil.pid_exists(pid):
    print("a process with pid %d exists" % pid)
else:
    print("a process with pid %d does not exist" % pid)

For reference:

• https://pypi.python.org/pypi/psutil
• https://github.com/giampaolo/psutil
• http://pythonhosted.org/psutil/#psutil.pid_exists

mluebke code is not 100% correct; kill() can also raise EPERM (access denied) in which case that obviously means a process exists. This is supposed to work:

(edited as per Jason R. Coombs comments)

import errno
import os

def pid_exists(pid):
    """Check whether pid exists in the current process table.
    UNIX only.
    """
    if pid < 0:
        return False
    if pid == 0:
        # According to "man 2 kill" PID 0 refers to every process
        # in the process group of the calling process.
        # On certain systems 0 is a valid PID but we have no way
        # to know that in a portable fashion.
        raise ValueError('invalid PID 0')
    try:
        os.kill(pid, 0)
    except OSError as err:
        if err.errno == errno.ESRCH:
            # ESRCH == No such process
            return False
        elif err.errno == errno.EPERM:
            # EPERM clearly means there's a process to deny access to
            return True
        else:
            # According to "man 2 kill" possible error values are
            # (EINVAL, EPERM, ESRCH)
            raise
    else:
        return True

You can't do this on Windows unless you use pywin32, ctypes or a C extension module. If you're OK with depending from an external lib you can use psutil:

>>> import psutil
>>> psutil.pid_exists(2353)
True

The answers involving sending 'signal 0' to the process will work only if the process in question is owned by the user running the test. Otherwise you will get an OSError due to permissions, even if the pid exists in the system.

In order to bypass this limitation you can check if /proc/<pid> exists:

import os

def is_running(pid):
    if os.path.isdir('/proc/{}'.format(pid)):
        return True
    return False

This applies to linux based systems only, obviously.