title: How to make a ttk.Combobox callback
body: <p>I was wondering if there was a way to invoke a call back from a ttk.Combobox when the user selects an item from the drop-down list. I want to check to see what the value of the combobox is when an item is clicked so that I can display the associated dictionary value given the combobox key.</p>
  
  <pre><code>import Tkinter
  import ttk
  
  FriendMap = {}
  UI = Tkinter.Tk()
  UI.geometry("%dx%d+%d+%d" % (330, 80, 500, 450))
  UI.title("User Friend List Lookup")
  
  def TextBoxUpdate():
      if not  FriendListComboBox.get() == "":
          FriendList = FriendMap[FriendListComboBox.get()]
          FriendListBox.insert(0,FriendMap[FriendListComboBox.get()])`
  
  #Imports the data from the FriendList.txt file
  with open("C:\Users\me\Documents\PythonTest\FriendList.txt", "r+") as file:
  for line in file:
      items = line.rstrip().lower().split(":")
      FriendMap[items[0]] = items[1]
  
  #Creates a dropdown box with all of the keys in the FriendList file
  FriendListKeys = FriendMap.keys()
  FriendListKeys.sort()
  FriendListComboBox = ttk.Combobox(UI,values=FriendListKeys,command=TextBoxUpdate)`
  </code></pre>
  
  <p>The last line obviously doesn't work since there is no 'command' for Comboboxes but I am not really sure what I need to do here to get that to work. Any help would be appreciated.</p>