position of widget in a panel using wxpython

am trying to put the radioBox in the center and a button below it, but no matter i try it the radioBox is positioned on top left corner of frame and button doesn't even show! . I don't understand why this is happening

class Panelchoose(wx.Panel):
    """
     panel for calculating separately without saving
    """
    def __init__(self ,*args, **kw):
        super(Panelchoose, self).__init__(*args, **kw)
        self.SetBackgroundColour('#48C9B0')
        topsizer=wx.BoxSizer(wx.VERTICAL)
        radiosizer=wx.BoxSizer(wx.VERTICAL)
        btnsizer=wx.BoxSizer(wx.HORIZONTAL)

        lblist=['calculationGrid','calculations','xyz2comcat']
        self.rbox=wx.RadioBox(self,label="choose from the models below:   ",
                              choices=lblist,majorDimension=3,
                              style=wx.RA_SPECIFY_ROWS)


        btn_next=wx.Button(self ,label=" NEXT  ")
        #self.Bind(wx.EVT_BUTTON,self.next_, btn_next)

        topsizer.Add(self.rbox,0,wx.CENTER|wx.ALIGN_CENTER_VERTICAL,10)
        btnsizer.Add(btn_next,0,wx.ALIGN_CENTER_VERTICAL|wx.BOTTOM|wx.EXPAND,5)

        topsizer.Add(btnsizer,4,wx.CENTER|wx.BOTTOM,10)

The part your forgot was to tell your panel what its sizer was. If you add self.SetSizer(topsizer) to the end of your class's __init__, it will work as you expect.

Here's a full runnable version:

import wx


class Panelchoose(wx.Panel):
    """
     panel for calculating separately without saving
    """
    def __init__(self ,*args, **kw):
        super(Panelchoose, self).__init__(*args, **kw)
        self.SetBackgroundColour('#48C9B0')
        topsizer=wx.BoxSizer(wx.VERTICAL)
        radiosizer=wx.BoxSizer(wx.VERTICAL)
        btnsizer=wx.BoxSizer(wx.HORIZONTAL)

        lblist=['calculationGrid','calculations','xyz2comcat']
        self.rbox=wx.RadioBox(self,label="choose from the models below:   ",
                              choices=lblist,majorDimension=3,
                              style=wx.RA_SPECIFY_ROWS)


        btn_next=wx.Button(self ,label=" NEXT  ")
        #self.Bind(wx.EVT_BUTTON,self.next_, btn_next)

        topsizer.Add(self.rbox,0,wx.CENTER|wx.ALIGN_CENTER_VERTICAL,10)
        btnsizer.Add(btn_next,0,wx.ALIGN_CENTER_VERTICAL|wx.BOTTOM|wx.EXPAND,5)

        topsizer.Add(btnsizer,0,wx.CENTER|wx.BOTTOM,10)

        self.SetSizer(topsizer)

if __name__ == '__main__':
    app = wx.App(False)
    frame = wx.Frame(None, title='Test')
    Panelchoose(parent=frame)
    frame.Show()
    app.MainLoop()