[This only applies to Win32, obviously!]
Perhaps you want to be able to close an app cleanly by sending it a WM_CLOSE? Or maybe you want to minimize it on demand? To do that you have to get the hWnd of its top-level window. You’d have thought there’d be some kind of get-window-from-process-id API call, wouldn’t you? Well, according to all the available literature, there isn’t. (Maybe there is in .NET? I wouldn’t know). The code below pretty much illustrates the canonical approach: loop over available windows, find their process id, and compare against the process id you first thought of. Since a process could have multiple visible top-level windows, I’ve allowed for a list of HWNDs but this is probably overkill.

import subprocess
import time

import win32con
import win32gui
from win32gui import IsWindowVisible, IsWindowEnabled
import win32process

def get_hwnds_for_pid (pid):

  def callback (hwnd, hwnds):
    if IsWindowVisible (hwnd) and IsWindowEnabled (hwnd):
      _, found_pid = win32process.GetWindowThreadProcessId (hwnd)
      if found_pid == pid:
        hwnds.append (hwnd)
    return True

  hwnds = []
  win32gui.EnumWindows (callback, hwnds)
  return hwnds

if __name__ == '__main__':
  notepad = subprocess.Popen ([r"notepad.exe"])
  #
  # sleep to give the window time to appear
  #
  time.sleep (2.0)

  for hwnd in get_hwnds_for_pid (notepad.pid):
    print hwnd, "=>", win32gui.GetWindowText (hwnd)
    win32gui.SendMessage (hwnd, win32con.WM_CLOSE, 0, 0)