title: how to get filename from TkInter filedialog.askopenfile from a button command
body: <p>I'm quite new to python, and even more new to Tkinter. 
  Sorry in advance for any obvious mistake I might be doing here ...</p>
  
  <pre><code>class application():
      def __init__(self):
          self.root = Tk()
          frameCSV = LabelFrame(self.root)
  
          Button(frameCSV, text="browse csv", command= self.browseCSV)
  
          Label(frameCSV,text=csvFilename ,bg='white').grid(row =1,column=1)
  
      def browseCSV(self):
          global csvFilename
          csvFilename = filedialog.askopenfilename(   initialdir="/Volumes/", title="select the file", filetypes=[("CSV files", ".csv"),("all files", "*.*")] )
  </code></pre>
  
  <p>The <code>frameCSV</code> is a frame within my root window. I'd like to add inside this frame a <code>Label</code> with the returned path of the selected file.</p>
  
  <p>But it doesn't work!</p>
  
  <pre><code>Traceback (most recent call last):
    File "/Users/guillaume/Downloads/uploader_v1_0_200312.py", line 106, in &lt;module&gt;
      f=application()
    File "/Users/guillaume/Downloads/uploader_v1_0_200312.py", line 70, in __init__
      Label(frameCSV,text=csvFilename ,bg='white').grid(row =1,column=1)
  NameError: name 'csvFilename' is not defined
  </code></pre>
  
  <p>What am I doing wrong? 
  I don't get why the function doesn't pass the PATH to the <code>Label</code> to display it.</p>
  
  <p>Thanks a lot for your help.</p>