title: Thread safety in wxpython
body: <p>I'm using wxpython to build the front-end GUI for a command-line tool that analyzes and processes audio files. The files are loaded into the GUI; then threads are launched that perform the analysis and adjustment steps; and finally the results of those processes are displayed in the main window. </p>
  
  <p>I have endeavored to write thread safe code; however, some threads still arbitrarily fail to complete (it should be noted that when I manually launch them a second time, they typically run to completion). Below I have included an abridged version of my program, which contains classes for the AnalysisThread, the AdjustThread, and the MainWindow. Buttons in the main window are bound to the functions "OnAnalyze" and "OnAdjust," which create instances of the appropriate thread classes. The threads themselves communicate with the GUI via wx.CallAfter and Publisher. To my understanding, this should allow data to be passed safely back and forth between the main process and the threads. If someone could kindly point out where I went wrong with the code below, I'd be very grateful.</p>
  
  <p>If I cannot fix the thread safety issue, my back-up plan is to somehow detect the death of a thread and try to "resuscitate" it under the hood, without the user knowing there was a glitch. Does this seem reasonable? If so, advice on how to accomplish this would be most welcome.</p>
  
  <p>Thanks very much.</p>
  
  <pre><code>#!/usr/bin/python
  
  import wx
  import time
  from threading import Thread
  import os, sys, re, subprocess, shutil
  from wx.lib.pubsub import setuparg1
  from wx.lib.pubsub import pub as Publisher
  
  
  #Start a thread that analyzes audio files.
  class AnalysisThread(Thread):
      def __init__(self,args):
          Thread.__init__(self)
          self.file = args[0]
          self.index = args[1]
          self.setDaemon(True)
          self.start()
  
      def run(self):
         proc = subprocess.Popen(['ffmpeg', '-nostats', '-i', self.file, '-filter_complex', 'ebur128=peak=true+sample', '-f', 'null', '-'], bufsize=1, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
         flag = 0
         summary = ""
         while proc.poll() is None:
            line = proc.stdout.readline()
            if line:
               endProcess = re.search(r'Summary', line)
               if endProcess is not None:
                  flag = 1
               if flag:
                  summary += line
  
         wx.CallAfter(Publisher.sendMessage, "update", (self.file, summary, self.index))
  
  #Start a thread that adjusts audio files so that they conform to EBU loudness standards.
  class AdjustThread(Thread):
      def __init__(self,args):
          Thread.__init__(self)
          self.file = args[0]
          self.index = args[1]
          self.IL = args[2]
          self.TP = args[3]
          self.SP = args[4]
          self.setDaemon(True)
          self.start()
  
      def run(self):
  
         proc = subprocess.Popen(['ffmpeg', '-nostats', '-i', adjusted_file, '-filter_complex', 'ebur128=peak=true+sample', '-f', 'null', '-'], bufsize=1, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
         flag = 0
         summary = ""
         while proc.poll() is None:
            line = proc.stdout.readline()
            if line:
               endProcess = re.search(r'Summary', line)
               if endProcess is not None:
                  flag = 1
               if flag:
                  summary += line
  
         wx.CallAfter(Publisher.sendMessage, "update", (self.file, summary, self.index))
  
  
  class MainWindow(wx.Frame):
  
      fileList = collections.OrderedDict()
  
      def __init__(self, parent, id, title):
          wx.Frame.__init__(self, parent, id, title, size=(900, 400))
  
          Publisher.subscribe(self.UpdateDisplay, "update")
  
          #Add "analyze" and "Adjust" buttons to the main frame.
          panel = wx.Panel(self, -1)
          vbox = wx.BoxSizer(wx.VERTICAL)
          self.ana = wx.Button(panel, -1, 'Analyze', size=(100, -1))
          self.adj = wx.Button(panel, -1, 'Adjust', size=(100, -1))
          self.Bind(wx.EVT_BUTTON, self.OnAnalyze, id=self.ana.GetId())
          self.Bind(wx.EVT_BUTTON, self.OnAdjust, id=self.adj.GetId())
          vbox.Add(self.ana, 0, wx.ALL, 10)
          vbox.Add(self.adj, 0, wx.ALL, 10)
          vbox.Add(self.list, 1, wx.EXPAND | wx.TOP, 3)
          vbox.Add((-1, 10))
          panel.SetSizer(hbox)
  
          self.Centre()
          self.Show(True)
  
      #This function gets called when "Analyze" is pressed.
      def OnAnalyze(self, event):
  
          for (file,index) in toAnalyze:
  
             #Add a progess bar
             item = self.list.GetItem(index,2)
             gauge = item.GetWindow()
             gauge.Pulse()
  
             #Launch the analysis thread
             AnalysisThread(args=(file,index,))
  
      #This function gets called when "Adjust" is pressed.
      def OnAdjust(self, event):
  
          for (file,index) in toAdjust:
             gauge = wx.Gauge(self.list,-1,range=50,size=(width,15),style=wx.GA_HORIZONTAL | wx.GA_SMOOTH)
             gauge.Pulse() #shouldn't start this right away...
             item.SetWindow(gauge, wx.ALIGN_CENTRE)
             self.list.SetItem(item) 
  
             #Launch the adjust thread
             AdjustThread(args=(file,index,intloud,truepeak,samplepeak))
  
      #This function is invoked by the Publisher.
      def UpdateDisplay(self, msg):
         t = msg.data 
         file = t[0]
         summary = t[1]
         i = t[2]
         self.ProcessSummary(file, summary, i)  
         item = self.list.GetItem(i,2)
         gauge = item.GetWindow()
         gauge.SetValue(50)
         self.fileList[file][1] = True
  
      #Display information from the threads in the main frame.
      def ProcessSummary(self, file, summary, i):
        loudnessRange = re.search(r'LRA:\s(.+?) LU', summary)
        if loudnessRange is not None:
           LRA = loudnessRange.group(1)
        else:
           LRA = "n/a"
        self.list.SetStringItem(i,7,LRA)         
        self.fileList[file][6] = LRA
  
        intloud = re.search(r'I:\s(.+?) LUFS', summary)
        if intloud is not None:
           IL = intloud.group(1)
        else:
           IL = "n/a"
        self.list.SetStringItem(i,4,IL)
        self.fileList[file][3] = IL
  
        truePeak = re.search(r'True peak:\s+Peak:\s(.+?) dBFS', summary)
        if truePeak is not None:
            TP = truePeak.group(1)
        else:
            TP = "n/a"
        self.list.SetStringItem(i,5,TP)
        self.fileList[file][4] = TP
  
        samplePeak = re.search(r'Sample peak:\s+Peak:\s(.+?) dBFS', summary)
        if samplePeak is not None:
            SP = samplePeak.group(1)
        else:
            SP = "n/a"
        self.list.SetStringItem(i,6,SP)
        self.fileList[file][5] = SP
  
  
  app = wx.App()
  MainWindow(None, -1, 'Leveler')
  app.MainLoop()
  </code></pre>