实现思路：

1)用os.getpid()获取当前程序运行PID，将PID存入文件中

2)用psutil模块获取当前系统所有正在运行的pid

3)读取之前存入的PID，判断该PID是否在系统PID中

4)如果文件中的PID在系统PID中，则退出程序，否则存入新的PID，运行程序。

# coding: utf-8
import os
import psutil
import time

def write_pid():
    pid = os.getpid()
    fp = open("pid.log",'w')
    fp.write(str(pid))
    fp.close()

def read_pid():
    if os.path.exists("pid.log"):
        fp = open("pid.log",'r')
        pid = fp.read()
        fp.close()
        return pid
    else:
        return False

def write_log(log_content):
    time_now = time.strftime("%Y-%m-%d %H:%M:%S", time.localtime())
    log_content = time_now+"---->"+log_content+os.linesep
    fp = open('recognition.log','a+')
    fp.write(log_content)
    fp.close()

def run():
    pid = read_pid()
    #print pid
    pid = int(pid)
    if pid:
        running_pid = psutil.pids()
        if pid in running_pid:
            log_content =  "process is running..."
            write_log(log_content)
        else:
            write_pid()
            time.sleep(20)
    else:
        write_pid()
        time.sleep(20)

if __name__ == "__main__":
    run()