Check if a process is running or not on Windows?

You can not rely on lock files in Linux or Windows. I would just bite the bullet and iterate through all the running programs. I really do not believe it will be as "expensive" as you think. psutil is an excellent cross-platform python module cable of enumerating all the running programs on a system.

import psutil    
"someProgram" in (p.name() for p in psutil.process_iter())

Although @zeller said it already here is an example how to use tasklist. As I was just looking for vanilla python alternatives...

import subprocess

def process_exists(process_name):
    call = 'TASKLIST', '/FI', 'imagename eq %s' % process_name
    # use buildin check_output right away
    output = subprocess.check_output(call).decode()
    # check in last line for process name
    last_line = output.strip().split('\r\n')[-1]
    # because Fail message could be translated
    return last_line.lower().startswith(process_name.lower())

and now you can do:

>>> process_exists('eclipse.exe')
True

>>> process_exists('AJKGVSJGSCSeclipse.exe')
False

To avoid calling this multiple times and have an overview of all the processes this way you could do something like:

# get info dict about all running processes
import subprocess
output = subprocess.check_output(('TASKLIST', '/FO', 'CSV')).decode()
# get rid of extra " and split into lines
output = output.replace('"', '').split('\r\n')
keys = output[0].split(',')
proc_list = [i.split(',') for i in output[1:] if i]
# make dict with proc names as keys and dicts with the extra nfo as values
proc_dict = dict((i[0], dict(zip(keys[1:], i[1:]))) for i in proc_list)
for name, values in sorted(proc_dict.items(), key=lambda x: x[0].lower()):
    print('%s: %s' % (name, values))

win32ui.FindWindow(classname, None) returns a window handle if any window with the given class name is found. It raises window32ui.error otherwise.

import win32ui

def WindowExists(classname):
    try:
        win32ui.FindWindow(classname, None)
    except win32ui.error:
        return False
    else:
        return True

if WindowExists("DropboxTrayIcon"):
    print "Dropbox is running, sir."
else:
    print "Dropbox is running..... not."

I found that the window class name for the Dropbox tray icon was DropboxTrayIcon using Autohotkey Window Spy.

See also

MSDN FindWindow

Lock files are generally not used on Windows (and rarely on Unix). Typically when a Windows program wants to see if another instance of itself is already running, it will call FindWindow with a known title or class name.

def iTunesRunning():
    import win32ui
    # may need FindWindow("iTunes", None) or FindWindow(None, "iTunes")
    # or something similar
    if FindWindow("iTunes", "iTunes"):
        print "Found an iTunes window"
        return True