Mine

import wx

class MainFrame( wx.Frame ):

    def __init__(self):
        wx.Frame.__init__(self,None,-1,"Verifica Chaves")

        painel = wx.Panel(self)
        mainSizer = wx.BoxSizer( wx.VERTICAL )
        


        toplbl = wx.StaticText(painel,-1,"Insira a Sua Chave:")
        num1 = wx.TextCtrl(painel,-1,"",size=(30,25))
        num2 = wx.TextCtrl(painel,-1,"",size=(30,25))
        num3 = wx.TextCtrl(painel,-1,"",size=(30,25))
        num4 = wx.TextCtrl(painel,-1,"",size=(30,25))
        num5 = wx.TextCtrl(painel,-1,"",size=(30,25))
        plus = wx.StaticText(painel,-1,"+",size=(30,25))
        str1 = wx.TextCtrl(painel,-1,"",size=(30,25))
        str2 = wx.TextCtrl(painel,-1,"",size=(30,25))

        _2line = wx.FlexGridSizer( cols=10, hgap=5, vgap=5)
        _2line.Add(num1)
        _2line.Add(num2)
        _2line.Add(num3)
        _2line.Add(num4)
        _2line.Add(num5)
        _2line.Add(plus)
        _2line.Add(str1)
        _2line.Add(str2)
        _2line.Add((10,10))

        openfilelbl = wx.StaticText(painel,-1,"Ficheiro: ",size=(50,25))
## Change This field
        openfiledata = wx.TextCtrl(painel,-1,"",size=(100,25))
#########
        openfilebutton = wx.Button ( painel, 100, 'Open' )
        wx.EVT_BUTTON ( painel, 100, self.Open )

        _3line = wx.FlexGridSizer( cols=3,vgap=5 ,hgap=5)

        _3line.Add( openfilelbl )
        _3line.Add( openfiledata )
        _3line.Add( openfilebutton )

        mainSizer.Add( toplbl )
        mainSizer.Add( _2line )
        mainSizer.Add( _3line )

#        openfiledata.SetValue( "lol" )
        
        painel.SetSizer( mainSizer )

        mainSizer.Fit(self)
        self.SetSizeHints(*self.GetSize())

    def Open(self,event):
        openfile = wx.FileDialog(None,"Escolha o Ficheiro:")
        if openfile.ShowModal()== wx.ID_OK:
            directory = openfile.GetDirectory()
            filename  = openfile.GetFilename()
#### FROM HERE
            self.openfiledata.SetValue("lol" )
        return 0

app = wx.PySimpleApp()
frame = MainFrame()
frame.Show()
app.MainLoop()