The wx.ComboBox is a combination of an editbox and a (dropdown) listbox. It let's the user slect an item from the listbox and puts it in the editbox. In this snippet we use two comboboxes, each used to select an area unit of measurement. Once selected, an area value can be converted from a unit like acres to another unit, for instance squaremiles. Additional wxPython widgets like wx.Frame, wx.Panel, wx.ToolTip, wx.StaticText, wx.TextCtrl, and wx.Button are part of this demonstration. The layout is done entirely with upper left corner position (x, y) tuples and size (width, height) tuples. This demo is supposed to illustrate the layout too, so I used plenty of key-words like pos=wx.Point(x, y) and size=wx.Size(width, height).

# use wxPython's wx.ComboBox() to select different units of measurement
# then convert the area associated with the selections
# tested with Python24 and wxPython26     vegaseat     21oct2005

import wx

class MyPanel(wx.Panel):
    class MyPanel creates a panel with 2 comboboxes and more, inherits wx.Panel
    (putting your components/widgets on a panel gives additional versatility)
    def __init__(self, parent, id):
        # no pos and size given, so panel defaults to fill the parent frame
        wx.Panel.__init__(self, parent, id)
        self.SetBackgroundColour((255,228,196))  # bisque

         # no size given, so the text determines the needed label size       
        wx.StaticText(self, -1, "convert from:", (10, 10))
        # create a combo box to select units of measurement to convert from
        self.combo1 = wx.ComboBox(self, -1, value=areaList[0], pos=wx.Point(10, 30),
            size=wx.Size(120, 150), choices=areaList)
        # optional tooltip
        self.combo1.SetToolTip(wx.ToolTip("select unit from dropdown-list"))
        wx.StaticText(self, -1, "convert to:", pos=wx.Point(150, 10))
        # create a combo box to select units of measurement to convert to
        self.combo2 = wx.ComboBox(self, -1, value=areaList[0], pos=wx.Point(150, 30),
            size=wx.Size(120, 150), choices=areaList)
        wx.StaticText(self, -1, "value to convert:", pos=wx.Point(10, 70))
        self.edit1 = wx.TextCtrl(self, -1, value="1", pos=wx.Point(10, 90), size=wx.Size(175, 25))
        self.edit1.SetBackgroundColour((255,255,197))  # suds yellow
        self.button1 = wx.Button(self, -1, label="Do the Conversion ...",
            pos=wx.Point(10, 130), size=wx.Size(175, 28))
        # respond to button click event
        self.button1.Bind(wx.EVT_BUTTON, self.button1Click, self.button1)
        wx.StaticText(self, -1, "result:", (10, 170))
        self.edit2 = wx.TextCtrl(self, -1, value="", pos=wx.Point(10, 190), size=wx.Size(350, 25))
        self.edit2.SetBackgroundColour((217,255,219))  # vegaseat green
    def button1Click(self,event):
        """Conversion button has been clicked"""
        unit1 = self.combo1.GetValue()
        unit2 = self.combo2.GetValue()
        x = float(self.edit1.GetValue())
        y = convertArea(x, unit1, unit2)
        if y < 0.001:
            str1 = "%f %s = %0.12f %s" % (x, unit1, y, unit2)  # very small y
        elif y > 1000:
            str1 = "%f %s = %0.3f %s" % (x, unit1, y, unit2)   # very large y
            str1 = "%f %s = %f %s" % (x, unit1, y, unit2)      # 6 decimals is default

def convertArea(x, unit1, unit2):
    """convert area x of unit1 to area of unit2 and return area, on error return False"""
    if (unit1 in areaD) and (unit2 in areaD):
        factor1 = areaD[unit1]
        factor2 = areaD[unit2]
        return factor2*x/factor1
        return False
#create an empty dictionary
areaD = {}
# populate dictionary with units and conversion factors relative to sqmeter = 1.0
# this minimizes the total number of conversion factors
areaD['sqmeter']      = 1.0
areaD['sqmillimeter'] = 1000000.0
areaD['sqcentimeter'] = 10000.0
areaD['sqkilometer']  = 0.000001
areaD['hectare']      = 0.0001
areaD['sqinch']       = 1550.003
areaD['sqfoot']       = 10.76391
areaD['sqyard']       = 1.19599
areaD['acre']         = 0.0002471054
areaD['sqmile']       = 0.0000003861022

# create a sorted list for the combo boxes
areaList = sorted(areaD.keys())

app = wx.PySimpleApp()
# create a window/frame, no parent, -1 is default ID, title, size
frame = wx.Frame(None, -1, "Convert Area ...", size = (400, 300))
# call the derived class, -1 is default ID, can also use wx.ID_ANY
# show the frame
# start the event loop