title: Python tkinter label won't change at beginning of function
body: <p>I'm using tkinter with Python to create a user interface for a program that converts Excel files to CSV.</p>
  
  <p>I created a label to act as a status bar, and set statusBarText as a StringVar() as the textvariable. inputFileEntry and outputFileEntry are textvariables that contain the input and output file paths.</p>
  
  <pre><code>def convertButtonClick():
      statusBarText.set('Converting...')
  
      if inputFileEntry.get() == '' or outputFileEntry.get() == '':
          statusBarText.set('Invalid Parameters.')
          return
  
      retcode = subprocess.('Program.exe' ,shell=true)
  
      if retcode == 0:
          statusBarText.set('Conversion Successful!')
      else:
          statusBarText.set('Conversion Failed!')
  </code></pre>
  
  <p>This function gets called when you click the convert button, and everything is working fine EXCEPT that the status bar never changes to say 'Converting...'.</p>
  
  <p>The status bar text will get changed to invalid parameters if either the input or output are empty, and it will change to success or failure depending on the return code. The problem is it never changes to 'Converting...' </p>
  
  <p>I've copied and pasted that exact line into the if statements and it works fine, but for some reason it just never changes before the subprocess runs when its at the top of the function. Any help would be greatly appreciated.</p>