How to stop a looping thread in Python

Question

What s the proper way to tell a looping thread to stop looping    I have a fairly simple program that pings a specified host in a separate threading Thread class  In this class it sleeps 60 seconds  the runs again until the application quits   I d like to implement a  Stop  button in my wx Frame to ask the looping thread to stop  It doesn t need to end the thread right away  it can just stop looping once it wakes up    Here is my threading class  note  I haven t implemented looping yet  but it would likely fall under the run method in PingAssets   class PingAssets threading Thread       def   init   self  threadNum  asset  window           threading Thread   init   self          self threadNum   threadNum         self window   window         self asset   asset      def run self           config   controller getConfig           fmt   config  timefmt           start time   datetime now   strftime fmt          try              if onlinecheck check status self asset                   status    online              else                  status    offline          except socket gaierror              status    an invalid asset tag           msg           is        n  format start time  self asset  status           wx CallAfter self window Logger  msg    And in my wxPyhton Frame I have this function called from a Start button   def CheckAsset self  asset           self count    1         thread   PingAssets self count  asset  self          self threads append thread          thread start

User · Answer

I had a different approach  I ve sub-classed a Thread class and in the constructor I ve created an Event object  Then I ve written custom join   method  which first sets this event and then calls a parent s version of itself   Here is my class  I m using for serial port communication in wxPython app   import wx  threading  serial  Events  Queue  class PumpThread threading Thread        def   init    self  port  queue  parent           super PumpThread  self    init             self port   port         self queue   queue         self parent   parent          self serial   serial Serial           self serial port   self port         self serial timeout   0 5         self serial baudrate   9600         self serial parity    N           self stopRequest   threading Event        def run  self           try              self serial open           except Exception  ex              print    ERROR  tUnable to open port     format self port               print    ERROR  t   n n    format ex message  ex traceback               self stopRequest set           else              print    INFO  tListening port     format self port               self serial write  FLOW  r            while not self stopRequest isSet                msg                  if not self queue empty                    try                      command   self queue get                       self serial write command                  except Queue Empty                      continue              while self serial inWaiting                    char   self serial read 1                  if   r  in char and len msg   gt  1                      char                             print   DATA  t    format msg                       event   Events PumpDataEvent Events SERIALRX  wx ID ANY  msg                      wx PostEvent self parent  event                      msg                          break                 msg    char         self serial close        def join  self  timeout None           self stopRequest set           super PumpThread  self  join timeout       def SetPort  self  serial           self serial   serial      def Write  self  msg           if self serial is open              self queue put msg          else              print   ERROR  tPort    is not open   format self port        def Stop self           if self isAlive                self join     The Queue is used for sending messages to the port and main loop takes responses back  I ve used no serial readline   method  because of different end-line char  and I have found the usage of io classes to be too much fuss

User · Answer

Threaded stoppable function  Instead of subclassing threading Thread  one can modify the function to allow stopping by a flag   We need an object  accessible to running function  to which we set the flag to stop running   We can use threading currentThread   object   import threading import time   def doit arg       t   threading currentThread       while getattr t   do run   True           print   working on  s    arg          time sleep 1      print  Stopping as you wish      def main        t   threading Thread target doit  args   task         t start       time sleep 5      t do run   False     t join    if   name         main         main     The trick is  that the running thread can have attached additional properties  The solution builds on assumptions    the thread has a property  do run  with default value True driving parent process can assign to started thread the property  do run  to False    Running the code  we get following output     python stopthread py                                                         working on task working on task working on task working on task working on task Stopping as you wish    Pill to kill - using Event  Other alternative is to use threading Event as function argument  It is by default False  but external process can  set it   to True  and function can learn about it using wait timeout  function   We can wait with zero timeout  but we can also use it as the sleeping timer  used below    def doit stop event  arg       while not stop event wait 1           print   working on  s    arg      print  Stopping as you wish      def main        pill2kill   threading Event       t   threading Thread target doit  args  pill2kill   task        t start       time sleep 5      pill2kill set       t join     Edit  I tried this in Python 3 6  stop event wait   blocks the event  and so the while loop  until release  It does not return a boolean value  Using stop event is set   works instead   Stopping multiple threads with one pill  Advantage of pill to kill is better seen  if we have to stop multiple threads at once  as one pill will work for all   The doit will not change at all  only the main handles the threads a bit differently   def main        pill2kill   threading Event       tasks     task ONE    task TWO    task THREE        def thread gen pill2kill  tasks           for task in tasks              t   threading Thread target doit  args  pill2kill  task               yield t      threads   list thread gen pill2kill  tasks       for thread in threads          thread start       time sleep 5      pill2kill set       for thread in threads          thread join

User · Answer

My solution is  import threading  time  def a        t   threading currentThread       while getattr t   quot do run quot   True       print  Do something       time sleep 1   def getThreadByName name       threads   threading enumerate    Threads list     for thread in threads          if thread name    name              return thread  threading Thread target a  name  228   start    Init thread t   getThreadByName  228    Get thread by name time sleep 5  t do run   False  Signal to stop thread t join

User · Answer

Depends on what you run in that thread  If that s your code  then you can implement a stop condition  see other answers   However  if what you want is to run someone else s code  then you should fork and start a process  Like this  import multiprocessing proc   multiprocessing Process target your proc function  args     proc start    now  whenever you want to stop that process  send it a SIGTERM like this  proc terminate   proc join    And it s not slow  fractions of a second  Enjoy

User · Answer

I read the other questions on Stack but I was still a little confused on communicating across classes  Here is how I approached it   I use a list to hold all my threads in the   init   method of my wxFrame class  self threads       As recommended in How to stop a looping thread in Python  I use a signal in my thread class which is set to True when initializing the threading class   class PingAssets threading Thread       def   init   self  threadNum  asset  window           threading Thread   init   self          self threadNum   threadNum         self window   window         self asset   asset         self signal   True      def run self           while self signal               do stuff                sleep     and I can stop these threads by iterating over my threads   def OnStop self  e           for t in self threads              t signal   False

User · Answer

This has been asked before on Stack  See the following links    Is there any way to kill a Thread in Python  Stopping a thread after a certain amount of time   Basically you just need to set up the thread with a stop function that sets a sentinel value that the thread will check  In your case  you ll have the something in your loop check the sentinel value to see if it s changed and if it has  the loop can break and the thread can die

User · Answer

I find it useful to have a class  derived from threading Thread  to encapsulate my thread functionality  You simply provide your own main loop in an overridden version of run   in this class  Calling start   arranges for the object   s run   method to be invoked in a separate thread  Inside the main loop  periodically check whether a threading Event has been set  Such an event is thread-safe  Inside this class  you have your own join   method that sets the stop event object before calling the join   method of the base class  It can optionally take a time value to pass to the base class s join   method to ensure your thread is terminated in a short amount of time  import threading import time  class MyThread threading Thread       def   init   self  sleep time 0 1           self  stop event   threading Event           self  sleep time   sleep time          quot  quot  quot call base class constructor quot  quot  quot          super     init          def run self            quot  quot  quot main control loop quot  quot  quot          while not self  stop event isSet                 do work             print  quot hi quot               self  stop event wait self  sleep time       def join self  timeout None            quot  quot  quot set stop event and join within a given time period quot  quot  quot          self  stop event set           super   join timeout    if   name       quot   main   quot       t   MyThread       t start        time sleep 5       t join 1   wait 1s max  Having a small sleep inside the main loop before checking the threading Event is less CPU intensive than looping continuously  You can have a default sleep time  e g  0 1s   but you can also pass the value in the constructor

[python] How to stop a looping thread in Python?

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