how best to check a value? (if/else or try/except?)

[John Salerno]

My code is below. The main focus would be on the OnStart method. I want 
to make sure that a positive integer is entered in the input box. At 
first I tried an if/else clause, then switched to try/except. Neither is 
perfect yet, but I was wondering which I should try for in the first 
place. I figure I need to check for an emptry string, non-numeric 
strings (maybe these are the same check), 0 and negative numbers (which 
might also fall into the category of 'anything but a number' because of 
the negative sign).

Thanks.

-------------------------------

import wx


class MyTimer(wx.Frame):

     def __init__(self):
         wx.Frame.__init__(self, None, wx.ID_ANY, 'Timer',
                           style=wx.DEFAULT_FRAME_STYLE ^ 
wx.RESIZE_BORDER ^ wx.MAXIMIZE_BOX)
         panel = wx.Panel(self)

         mainSizer = wx.BoxSizer(wx.VERTICAL)
         inputSizer = wx.BoxSizer(wx.HORIZONTAL)

         self.progress = wx.Gauge(panel, wx.ID_ANY, 100, size=(300, 20))
         self.status = wx.StaticText(panel, wx.ID_ANY, 'Enter a time.')
         prompt = wx.StaticText(panel, wx.ID_ANY, 'Time to wait:')
         self.input = wx.TextCtrl(panel, wx.ID_ANY, size=(20, 20))
         self.start = wx.Button(panel, wx.ID_ANY, 'Start')
         self.reset = wx.Button(panel, wx.ID_ANY, 'Reset')
         self.reset.Disable()
         self.timer = wx.Timer(self)

         mainSizer.Add(self.progress, flag=wx.ALIGN_CENTER | wx.ALL ^ 
wx.BOTTOM,
                       border=10)
         mainSizer.Add(self.status, flag=wx.ALIGN_CENTER | wx.ALL, 
border=10)
         mainSizer.Add(inputSizer, flag=wx.ALIGN_CENTER | wx.BOTTOM, 
border=10)
         inputSizer.Add(prompt, flag=wx.ALIGN_CENTER)
         inputSizer.Add(self.input, flag=wx.ALIGN_CENTER | wx.LEFT | 
wx.RIGHT,
                        border=5)
         inputSizer.Add(self.start, flag=wx.ALIGN_CENTER)
         inputSizer.Add(self.reset, flag=wx.ALIGN_CENTER)

         self.Bind(wx.EVT_TEXT_ENTER, self.OnStart, self.input)
         self.Bind(wx.EVT_BUTTON, self.OnStart, self.start)
         self.Bind(wx.EVT_TIMER, self.OnTimer, self.timer)
         self.Bind(wx.EVT_BUTTON, self.OnReset, self.reset)

         panel.SetSizer(mainSizer)
         mainSizer.Fit(self)

     def OnStart(self, event):
         try:
             self.time = int(self.input.GetValue())
             self.minutes_passed = 1
             self.minutes_remaining = self.time - 1
             self.start.Disable()
             self.input.Disable()
             self.reset.Enable()
             self.status.SetLabel('%s minute(s) remaining.' % self.time)
             self.timer.Start(1000)
         except ValueError:
             wx.MessageBox('Enter a valid time.', 'Invalid time entered',
                           wx.OK | wx.ICON_ERROR)
             self.input.Clear()

     def OnReset(self, event):
         if self.timer.IsRunning():
             self.timer.Stop()
         self.input.Clear()
         self.input.Enable()
         self.start.Enable()
         self.reset.Disable()
         self.status.SetLabel('Enter a new time.')
         self.progress.SetValue(0)
         self.minutes_passed = 1

     def OnTimer(self, event):
         if self.minutes_remaining != 0:
             self.progress.SetValue(self.minutes_passed * (100.0 / 
self.time))
             self.status.SetLabel('%s minute(s) remaining.' % 
self.minutes_remaining)
             self.minutes_passed += 1
             self.minutes_remaining -= 1
         else:
             self.timer.Stop()
             self.progress.SetValue(self.minutes_passed * (100.0 / 
self.time))
             self.status.SetLabel('%s minute(s) have elapsed.' % self.time)
             wx.Sound.Play(wx.Sound(r'C:\Windows\Media\notify.wav'))


class MyApp(wx.App):

     def OnInit(self):
         frame = MyTimer()
         self.SetTopWindow(frame)
         frame.Show()
         return True


if __name__ == '__main__':
     app = MyApp(False)
     app.MainLoop()