title: Why Does .Hide()ing and .Show()ing Panels in wxPython Result in the Sizer Changing the Layout?
body: <p>As referenced in my <a href="https://stackoverflow.com/questions/2119067/in-wxpython-what-is-the-standard-process-of-making-an-application-slightly-more">previous question</a>, I am trying to make something slightly wizard-like in function.  I have settled on a single frame with a sizer added to it.  I build panels for each of the screens I would like users to see, add them to the frame's sizer, then switch between panels by <code>.Hide()</code>ing one panel, then calling a custom <code>.ShowYourself()</code> on the next panel.  Obviously, I would like the buttons to remain in the same place as the user progresses through the process.</p>
  
  <p>I have linked together two panels in an infinite loop by their "Back" and "Next" buttons so you can see what is going on.  The first panel looks great; <a href="https://stackoverflow.com/users/102302/tom10">tom10</a>'s code worked on that level, as it eschewed my initial, over-fancy attempt with borders flying every which way.  And then the second panel seems to have shrunk down to the bare minimum.  As we return to the first panel, the shrinkage has occurred here as well.  Why does it look fine on the first panel, but not after I return there?    Why is calling <code>.Fit()</code> necessary if I do not want a 10 pixel by 10 pixel wad of grey?  And if it is necessary, why does <code>.Fit()</code> give inconsistent results?</p>
  
  <p>This infinite loop seems to characterize my experience with this: I fix the layout on a panel, only to find that switching ruins the layout for other panels.  I fix that problem, by using <code>sizer_h.Add(self.panel1, 0)</code> instead of <code>sizer_h.Add(self.panel1, 1, wx.EXPAND)</code>, and now my layouts are off again.  </p>
  
  <p>So far, my "solution" is to add a <code>mastersizer.SetMinSize((475, 592))</code> to each panel's master sizer (commented out in the code below).  This is a cruddy solution because 1) I have had to find the numbers that work by trial and error (-5 pixels for the width, -28 pixels for the height). 2) I don't understand why the underlying issue still happens.</p>
  
  <p>What's the correct, non-ugly solution?  Instead of adding all of the panels to the frame's sizer at once, should switching panels involve <code>.Detach()</code>ing that panel from the frame's sizer and then <code>.Add()</code>ing the next panel to the frame's sizer?  Is there a <code>.JustMakeThisFillThePanel()</code> method hiding somewhere I have missed in both the wxWidgets and the wxPython documents online?</p>
  
  <p>I'm obviously missing something in my mental model of layout. Minimalist code pasted below.</p>
  
  <p><img src="https://i.stack.imgur.com/WVrcJ.jpg" alt="enter image description here"></p>
  
  <pre><code>import wx
  import sys
  
  
  class My_App(wx.App):
  
      def OnInit(self):
          self.frame = My_Frame(None)
          self.frame.Show()
          self.SetTopWindow(self.frame)
          return True
  
      def OnExit(self):
          print 'Dying ...'
  
  
  class My_Frame(wx.Frame):
  
      def __init__(self, image, parent=None,id=-1, title='Generic Title', pos=wx.DefaultPosition, style=wx.CAPTION | wx.STAY_ON_TOP):     
  
          size = (480, 620)
          wx.Frame.__init__(self, parent, id, 'Program Title', pos, size, style)
  
          sizer_h = wx.BoxSizer(wx.HORIZONTAL)
  
          self.panel0 = User_Interaction0(self)       
          sizer_h.Add(self.panel0, 1, wx.EXPAND)
  
          self.panel1 = User_Interaction1(self)       
          sizer_h.Add(self.panel1, 1, wx.EXPAND)
  
          self.SetSizer(sizer_h)
  
          self.panel0.ShowYourself()
  
      def ShutDown(self):
          self.Destroy()
  
  
  class User_Interaction0(wx.Panel):
  
      def __init__(self, parent, id=-1):
  
          wx.Panel.__init__(self, parent, id)
  
          # master sizer for the whole panel
          mastersizer = wx.BoxSizer(wx.VERTICAL)
          #mastersizer.SetMinSize((475, 592))
          mastersizer.AddSpacer(15)
  
  
          # build the top row
          txtHeader = wx.StaticText(self, -1, 'Welcome to This Boring\nProgram', (0, 0))
          font = wx.Font(16, wx.DEFAULT, wx.NORMAL, wx.BOLD)
          txtHeader.SetFont(font)
          txtOutOf = wx.StaticText(self, -1, '1 out of 7', (0, 0))                
          rowtopsizer = wx.BoxSizer(wx.HORIZONTAL)
          rowtopsizer.Add(txtHeader, 3, wx.ALIGN_LEFT) 
          rowtopsizer.Add((0,0), 1)  
          rowtopsizer.Add(txtOutOf, 0, wx.ALIGN_RIGHT) 
          mastersizer.Add(rowtopsizer, 0, flag=wx.EXPAND | wx.LEFT | wx.RIGHT, border=15) 
  
  
          # build the middle row
          text = 'PANEL 0\n\n'
          text = text + 'This could be a giant blob of explanatory text.\n'
  
          txtBasic = wx.StaticText(self, -1, text)
          font = wx.Font(11, wx.DEFAULT, wx.NORMAL, wx.NORMAL)
          txtBasic.SetFont(font)
          mastersizer.Add(txtBasic, 1, flag=wx.EXPAND | wx.LEFT | wx.RIGHT, border=15)  
  
  
          # build the bottom row
          btnBack = wx.Button(self, -1, 'Back')
          self.Bind(wx.EVT_BUTTON, self.OnBack, id=btnBack.GetId())
          btnNext = wx.Button(self, -1, 'Next')
          self.Bind(wx.EVT_BUTTON, self.OnNext, id=btnNext.GetId())
          btnCancelExit = wx.Button(self, -1, 'Cancel and Exit')
          self.Bind(wx.EVT_BUTTON, self.OnCancelAndExit, id=btnCancelExit.GetId())
          rowbottomsizer = wx.BoxSizer(wx.HORIZONTAL)
          rowbottomsizer.Add(btnBack, 0, wx.ALIGN_LEFT)
          rowbottomsizer.AddSpacer(5)
          rowbottomsizer.Add(btnNext, 0)
          rowbottomsizer.AddSpacer(5)
          rowbottomsizer.AddStretchSpacer(1)
          rowbottomsizer.Add(btnCancelExit, 0, wx.ALIGN_RIGHT)
          mastersizer.Add(rowbottomsizer, flag=wx.EXPAND | wx.LEFT | wx.RIGHT, border=15)
  
          # finish master sizer
          mastersizer.AddSpacer(15)   
          self.SetSizer(mastersizer)
  
          self.Raise()
          self.SetPosition((0,0))
          self.Fit()  
          self.Hide()
  
  
      def ShowYourself(self):
          self.Raise()
          self.SetPosition((0,0))
          self.Fit()
          self.Show()
  
  
      def OnBack(self, event):
          self.Hide()
          self.GetParent().panel1.ShowYourself()
  
      def OnNext(self, event):
          self.Hide()
          self.GetParent().panel1.ShowYourself()
  
      def OnCancelAndExit(self, event):
          self.GetParent().ShutDown()
  
  
  class User_Interaction1(wx.Panel):
  
      def __init__(self, parent, id=-1):
  
          wx.Panel.__init__(self, parent, id)
  
          # master sizer for the whole panel
          mastersizer = wx.BoxSizer(wx.VERTICAL)
          #mastersizer.SetMinSize((475, 592))
          mastersizer.AddSpacer(15)
  
  
          # build the top row
          txtHeader = wx.StaticText(self, -1, 'Read about This Boring\nProgram', (0, 0))
          font = wx.Font(16, wx.DEFAULT, wx.NORMAL, wx.BOLD)
          txtHeader.SetFont(font)
          txtOutOf = wx.StaticText(self, -1, '2 out of 7', (0, 0))                
          rowtopsizer = wx.BoxSizer(wx.HORIZONTAL)
          rowtopsizer.Add(txtHeader, 3, wx.ALIGN_LEFT) 
          rowtopsizer.Add((0,0), 1)  
          rowtopsizer.Add(txtOutOf, 0, wx.ALIGN_RIGHT) 
          mastersizer.Add(rowtopsizer, 0, flag=wx.EXPAND | wx.LEFT | wx.RIGHT, border=15) 
  
  
          # build the middle row
          text = 'PANEL 1\n\n'
          text = text + 'This could be a giant blob of boring text.\n'
  
          txtBasic = wx.StaticText(self, -1, text)
          font = wx.Font(11, wx.DEFAULT, wx.NORMAL, wx.NORMAL)
          txtBasic.SetFont(font)
          mastersizer.Add(txtBasic, 1, flag=wx.EXPAND | wx.LEFT | wx.RIGHT, border=15)  
  
  
          # build the bottom row
          btnBack = wx.Button(self, -1, 'Back')
          self.Bind(wx.EVT_BUTTON, self.OnBack, id=btnBack.GetId())
          btnNext = wx.Button(self, -1, 'Next')
          self.Bind(wx.EVT_BUTTON, self.OnNext, id=btnNext.GetId())
          btnCancelExit = wx.Button(self, -1, 'Cancel and Exit')
          self.Bind(wx.EVT_BUTTON, self.OnCancelAndExit, id=btnCancelExit.GetId())
          rowbottomsizer = wx.BoxSizer(wx.HORIZONTAL)
          rowbottomsizer.Add(btnBack, 0, wx.ALIGN_LEFT)
          rowbottomsizer.AddSpacer(5)
          rowbottomsizer.Add(btnNext, 0)
          rowbottomsizer.AddSpacer(5)
          rowbottomsizer.AddStretchSpacer(1)
          rowbottomsizer.Add(btnCancelExit, 0, wx.ALIGN_RIGHT)
          mastersizer.Add(rowbottomsizer, flag=wx.EXPAND | wx.LEFT | wx.RIGHT, border=15)
  
          # finish master sizer
          mastersizer.AddSpacer(15)   
          self.SetSizer(mastersizer)
  
          self.Raise()
          self.SetPosition((0,0))
          self.Fit()  
          self.Hide()
  
  
      def ShowYourself(self):
          self.Raise()
          self.SetPosition((0,0))
          self.Fit()
          self.Show()
  
  
      def OnBack(self, event):
          self.Hide()
          self.GetParent().panel0.ShowYourself()
  
      def OnNext(self, event):
          self.Hide()
          self.GetParent().panel0.ShowYourself()
  
      def OnCancelAndExit(self, event):
          self.GetParent().ShutDown()
  
  
  def main():
      app = My_App(redirect = False)
      app.MainLoop()
  
  
  if __name__ == '__main__':
      main()
  </code></pre>