Jesh Kundem Jesh Kundem - 2 months ago 26
Python Question

Python - Cannot join thread - No multiprocessing

I have this piece of code in my program. Where OnDone function is an event in a wxPython GUI. When I click the button DONE, the OnDone event fires up, which then does some functionality and starts the thread self.tstart - with target function StartEnable. This thread I want to join back using self.tStart.join(). However I am getting an error as follows:

Exception in thread StartEnablingThread:
Traceback (most recent call last):
File "C:\Python27\lib\threading.py", line 801, in __bootstrap_inner
self.run()
File "C:\Python27\lib\threading.py", line 754, in run
self.__target(*self.__args, **self.__kwargs)
File "//wagnernt.wagnerspraytech.com/users$/kundemj/windows/my documents/Production GUI/Trial python Codes/GUI_withClass.py", line 638, in StartEnable
self.tStart.join()
File "C:\Python27\lib\threading.py", line 931, in join
raise RuntimeError("cannot join current thread")
RuntimeError: cannot join current thread


I have not got this type of error before. Could any one of you guys tell me what I am missing here.

def OnDone(self, event):
self.WriteToController([0x04],'GuiMsgIn')
self.status_text.SetLabel('PRESSURE CALIBRATION DONE \n DUMP PRESSURE')
self.led1.SetBackgroundColour('GREY')
self.add_pressure.Disable()
self.tStart = threading.Thread(target=self.StartEnable, name = "StartEnablingThread", args=())
self.tStart.start()

def StartEnable(self):
while True:
time.sleep(0.5)
if int(self.pressure_text_control.GetValue()) < 50:
print "HELLO"
self.start.Enable()
self.tStart.join()
print "hello2"
break


I want to join the thread after the "if" condition has executed. Until them I want the thread to run.

Answer

When the StartEnable method is executing, it is running on the StartEnablingThread you created in the __init__ method. You cannot join the current thread. This is clearly stated in the documentation for the join call.

join() raises a RuntimeError if an attempt is made to join the current thread as that would cause a deadlock. It is also an error to join() a thread before it has been started and attempts to do so raises the same exception.