title: StringVar bound to an Entry doesn't update Entry value
body: <p>I'm starting with GUI in Python and Tkinter and want to make a small window that load an image from a file and show the path of the file and also de image. Until now, I've got my window and the button to pick the image (<em>tkinter.filedialog.askopenfilename</em>) but I'm trying to update the <em>Entry</em> that is supposed to show the path using a <em>tkinter.StringVar</em>. The value of the StringVar is changing, but not the value shown in the Entry.</p>
  
  <p><strong>Frame Code:</strong> </p>
  
  <pre><code>import tkinter as tk
  import tkinter.filedialog as tkf
  
  class ImageViewer(tk.Frame):
  
      def __init__(self, master=None):
          tk.Frame.__init__(self, master)
          self.master.title("Image Viewer")
          self.drawButtons()
          self.grid()
  
      def drawButtons(self):
          self.lblfilename = tk.Label(self, text="Image: ")
          self.lblfilename.grid(padx=1, column=0, row=0, sticky=tk.N+tk.E+tk.S+tk.W)
          self.eText = tk.StringVar()
          self.eText.set("")
          self.filenametext = tk.Entry(self, background='#fff', width=65, textvariable=self.eText)
          self.filenametext.grid(padx=10, column=1, row=0, columnspan=2, sticky=tk.N+tk.E+tk.S+tk.W)
          self.pickerbut = tk.Button(self, text="Load", command=self.picker)
          self.pickerbut.grid(column=3, row=0, sticky=tk.N + tk.S + tk.W)
          self.image = tk.Canvas(self, height=480, width=640)
          self.image.grid(padx=10, pady=5, column=0, row=1, columnspan=4, rowspan=3)
          self.cancelbut = tk.Button(self, text="Exit", command=self.cancel)
          self.cancelbut.grid(column=3, row=4, sticky=tk.N + tk.W + tk.S)
  
      def picker(self):
          self.imgpicker = tkf.askopenfilename(parent=self)
          self.eText.set(self.imgpicker)
          print(self.eText.get())
  
      def cancel(self):
          self.master.destroy()
  </code></pre>
  
  <p><strong>main:</strong></p>
  
  <pre><code>if __name__ == "__main__":
      imageviewerbutton = tk.Tk()
      imageviewerbutton.geometry('660x550+100+90')
      ImageViewer(imageviewerbutton)
      imageviewerbutton.mainloop()
  </code></pre>
  
  <p>When I print the value of <em>self.eText</em> with <em>self.eText.get()</em>, it show the correct image path, but my Entry remain empty. </p>
  
  <p>In my opinion, it is any kind of problem when bindind the StringVar and the Entry, although I've already search on internet and try everything that came to my mind, I didn't found a solution. </p>
  
  <p>Can someone give me a hint?</p>