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()

 

[Nick Vatamaniuc]

John,

The way I do it is, is I ask myself 'is there a more common (basic)
base between the code paths or do they both have about a 50/50 chance
on average?' or 'will one code path be taken during an error and the
other one during the normal processing?'. If there is a clear single
common/usual/basic case or I try to detect an error I use 'try/except',
if it could go either way on average I use 'if'.

In general in Python we follow the 'it is better to ask forgiveness
than to ask permission'. In other words if you don't know if a piece
of data you have is a function and is callable you just call it and if
it raises an exception then you'll know it's not. This is the opposite
of C where you have to spend a good deal of time validating your input
arguments before you can safely continue.

That said, Python is also a practical language and quite often if you
do just one check then an 'if' may take 2 lines (if no need for else)
while a try/except will take 4 lines. So just apply common sense. For
example, I want to find out if I can call f then do some stuff if that
is the case:
try:
f(arg)
#...do stuff because f is callable...
except TypeError:
pass
# ... if f is not callable, then I don't care...

But of course it is much shorter to do:
if callable(f):
#...do stuff because f is callable...

Hope this helps,
Nick Vatamaniuc

 

[John McMonagle]

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).

Have a look at using a wx.Validator in your wx.TextCtrl.


Syntax for wx.TextCtrl:

wx.TextCtrl(parent, id, value="", pos=wx.DefaultPosition,
size=wx.DefaultSize, style=0, validator=wx.DefaultValidator,
name=wx.TextCtrlNameStr)

This will enable you to automagically check the TextCtrl if it's empty,
has wrong data type, does not meet a prescribed condition (eg: no
negative numbers) and do something if it does not pass validation (eg:
colour the background of the TextCtrl red, initiate a system beep, etc).

Regards,

John

 

[John Salerno]

Have a look at using a wx.Validator in your wx.TextCtrl.

Ah, good thinking! I wasn't even considering another option like that,
which probably makes more sense anyway!

 

[John Salerno]

John,

The way I do it is, is I ask myself 'is there a more common (basic)
base between the code paths or do they both have about a 50/50 chance
on average?' or 'will one code path be taken during an error and the
other one during the normal processing?'. If there is a clear single
common/usual/basic case or I try to detect an error I use 'try/except',
if it could go either way on average I use 'if'.

Thanks, that's something good to think about. This issue came up once
before and I remember someone saying something like "you're using a
try/except statement, but i wouldn't really consider that situation
'exceptional'", so thinking of it in your terms might help a lot.