wxPython - expanding controls

wx_sizer_ex.py

import wx
import os
 
class MainWindow(wx.Frame):
    def __init__(self,parent,title):
        wx.Frame.__init__(self, parent, title=title)
        self.workbookDir = r'C:\Users\Nick\Desktop'
        self.workbookName = 'rota.xlsx'
        self.panel = wx.Panel(self)
        self.label = wx.StaticText(self.panel, label = 'Workbook path:  ')
        self.filePathBox = wx.TextCtrl(self.panel)
        self.filePathBox.SetValue(self.workbookPath())
        self.filePathBoxSizer = wx.BoxSizer(wx.HORIZONTAL)
        self.filePathBoxSizer.Add(self.label, 0, wx.EXPAND)
        self.filePathBoxSizer.Add(self.filePathBox, 1, wx.EXPAND)
        self.openButton = wx.Button(self.panel, wx.ID_OPEN, 'Open')
        #self.Bind(wx.EVT_BUTTON, self.OnOpen, self.openButton)
        
        self.panelSizer = wx.BoxSizer(wx.VERTICAL)
        self.panelSizer.Add((0, 0), 1, wx.EXPAND)
        self.panelSizer.Add(self.filePathBoxSizer, 0, wx.CENTER)
        self.panelSizer.Add((0, 0), 1, wx.EXPAND)
        self.panelSizer.Add(self.openButton, 0, wx.CENTER)
        self.panelSizer.Add((0, 0), 1, wx.EXPAND)
        self.panel.SetSizer(self.panelSizer)
        self.panelSizer.Fit(self.panel)
        self.panel.SetAutoLayout(True)
        self.Fit()
        self.Show()
        
    def workbookPath(self):
        return os.path.join(self.workbookDir, self.workbookName)
 
app = wx.App(False)
frame = MainWindow(None, "Invoice Generator")
frame.Show(True)
app.MainLoop()