wxPython: Object has no attribute error

teaching myself Python and wxPython. Not sure what I am doing wrong here, could use some insight from folks much smarter than myself...

import wx
from wxPython.wx import *

class myGUI(wx.Frame):

    def __init__(self, parent, title):    
        super(myGUI, self).__init__(parent, title=title,
            size=(450, 350))
        panel = wx.Panel(self)
        sizer = wx.GridBagSizer(5, 5)

    # Main Database Text, Entry and Browse Button ------------------------------
        label_MainDatabase = wx.StaticText(panel, label="Main Database:")
        sizer.Add(label_MainDatabase, pos=(0, 0), flag=wx.LEFT|
            wx.ALIGN_CENTER_VERTICAL, border=20)

        tc_MainDatabase = wx.TextCtrl(panel)
        sizer.Add(tc_MainDatabase, pos=(0, 1), span=(1, 3), flag=wx.TOP|
            wx.EXPAND|wx.ALIGN_CENTER_VERTICAL)
        tc_MainDatabase.Value = "DBG: I am properly initialized."

        bt_MainDatabase = wx.Button(panel, label="Browse...")
        sizer.Add(bt_MainDatabase, pos=(0, 4), flag=wx.LEFT|wx.RIGHT|
        wx.ALIGN_CENTER_VERTICAL, border=0)
        bt_MainDatabase.Bind(wx.EVT_BUTTON, self.bt_MainDatabaseClick,
            bt_MainDatabase)
    # --------------------------------------------------------------------------
        sizer.AddGrowableCol(2)
        panel.SetSizer(sizer)

        self.Centre()
        self.Show()

    def bt_MainDatabaseClick(self, event):
        # Create a list of filters
        self.tc_MainDatabase.SetValue = "A"


if __name__ == '__main__':
    app = wx.App()
    myGUI(None, title="myGUI")

    app.MainLoop()

I get the following error when I click the "Browse" button: AttributeError: 'myGUI' object has no attribute 'tc_MainDatabase'

What am I doing wrong? I am trying to capture information from the Browse button and then update a text control field (tc_MainDatabase). I've tried rearranging the order of the def statements, etc.

And yes, I always jump in with both feet. It's the only way I know how to learn... :)

Thanks.

-Chow