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.Fram e):

def __init__(self):
wx.Frame.__init __(self, None, wx.ID_ANY, 'Timer',
style=wx.DEFAUL T_FRAME_STYLE ^
wx.RESIZE_BORDE R ^ 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(p anel, wx.ID_ANY, 'Enter a time.')
prompt = wx.StaticText(p anel, wx.ID_ANY, 'Time to wait:')
self.input = wx.TextCtrl(pan el, 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.Disa ble()
self.timer = wx.Timer(self)

mainSizer.Add(s elf.progress, flag=wx.ALIGN_C ENTER | wx.ALL ^
wx.BOTTOM,
border=10)
mainSizer.Add(s elf.status, flag=wx.ALIGN_C ENTER | wx.ALL,
border=10)
mainSizer.Add(i nputSizer, flag=wx.ALIGN_C ENTER | wx.BOTTOM,
border=10)
inputSizer.Add( prompt, flag=wx.ALIGN_C ENTER)
inputSizer.Add( self.input, flag=wx.ALIGN_C ENTER | wx.LEFT |
wx.RIGHT,
border=5)
inputSizer.Add( self.start, flag=wx.ALIGN_C ENTER)
inputSizer.Add( self.reset, flag=wx.ALIGN_C ENTER)

self.Bind(wx.EV T_TEXT_ENTER, self.OnStart, self.input)
self.Bind(wx.EV T_BUTTON, self.OnStart, self.start)
self.Bind(wx.EV T_TIMER, self.OnTimer, self.timer)
self.Bind(wx.EV T_BUTTON, self.OnReset, self.reset)

panel.SetSizer( mainSizer)
mainSizer.Fit(s elf)

def OnStart(self, event):
try:
self.time = int(self.input. GetValue())
self.minutes_pa ssed = 1
self.minutes_re maining = self.time - 1
self.start.Disa ble()
self.input.Disa ble()
self.reset.Enab le()
self.status.Set Label('%s minute(s) remaining.' % self.time)
self.timer.Star t(1000)
except ValueError:
wx.MessageBox(' Enter a valid time.', 'Invalid time entered',
wx.OK | wx.ICON_ERROR)
self.input.Clea r()

def OnReset(self, event):
if self.timer.IsRu nning():
self.timer.Stop ()
self.input.Clea r()
self.input.Enab le()
self.start.Enab le()
self.reset.Disa ble()
self.status.Set Label('Enter a new time.')
self.progress.S etValue(0)
self.minutes_pa ssed = 1

def OnTimer(self, event):
if self.minutes_re maining != 0:
self.progress.S etValue(self.mi nutes_passed * (100.0 /
self.time))
self.status.Set Label('%s minute(s) remaining.' %
self.minutes_re maining)
self.minutes_pa ssed += 1
self.minutes_re maining -= 1
else:
self.timer.Stop ()
self.progress.S etValue(self.mi nutes_passed * (100.0 /
self.time))
self.status.Set Label('%s minute(s) have elapsed.' % self.time)
wx.Sound.Play(w x.Sound(r'C:\Wi ndows\Media\not ify.wav'))


class MyApp(wx.App):

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


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

[Nick Vatamaniuc]

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

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 Salerno wrote:

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.Fram e):
>
def __init__(self):
wx.Frame.__init __(self, None, wx.ID_ANY, 'Timer',
style=wx.DEFAUL T_FRAME_STYLE ^
wx.RESIZE_BORDE R ^ 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(p anel, wx.ID_ANY, 'Enter a time.')
prompt = wx.StaticText(p anel, wx.ID_ANY, 'Time to wait:')
self.input = wx.TextCtrl(pan el, 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.Disa ble()
self.timer = wx.Timer(self)
>
mainSizer.Add(s elf.progress, flag=wx.ALIGN_C ENTER | wx.ALL ^
wx.BOTTOM,
border=10)
mainSizer.Add(s elf.status, flag=wx.ALIGN_C ENTER | wx.ALL,
border=10)
mainSizer.Add(i nputSizer, flag=wx.ALIGN_C ENTER | wx.BOTTOM,
border=10)
inputSizer.Add( prompt, flag=wx.ALIGN_C ENTER)
inputSizer.Add( self.input, flag=wx.ALIGN_C ENTER | wx.LEFT |
wx.RIGHT,
border=5)
inputSizer.Add( self.start, flag=wx.ALIGN_C ENTER)
inputSizer.Add( self.reset, flag=wx.ALIGN_C ENTER)
>
self.Bind(wx.EV T_TEXT_ENTER, self.OnStart, self.input)
self.Bind(wx.EV T_BUTTON, self.OnStart, self.start)
self.Bind(wx.EV T_TIMER, self.OnTimer, self.timer)
self.Bind(wx.EV T_BUTTON, self.OnReset, self.reset)
>
panel.SetSizer( mainSizer)
mainSizer.Fit(s elf)
>
def OnStart(self, event):
try:
self.time = int(self.input. GetValue())
self.minutes_pa ssed = 1
self.minutes_re maining = self.time - 1
self.start.Disa ble()
self.input.Disa ble()
self.reset.Enab le()
self.status.Set Label('%s minute(s) remaining.' % self.time)
self.timer.Star t(1000)
except ValueError:
wx.MessageBox(' Enter a valid time.', 'Invalid time entered',
wx.OK | wx.ICON_ERROR)
self.input.Clea r()
>
def OnReset(self, event):
if self.timer.IsRu nning():
self.timer.Stop ()
self.input.Clea r()
self.input.Enab le()
self.start.Enab le()
self.reset.Disa ble()
self.status.Set Label('Enter a new time.')
self.progress.S etValue(0)
self.minutes_pa ssed = 1
>
def OnTimer(self, event):
if self.minutes_re maining != 0:
self.progress.S etValue(self.mi nutes_passed * (100.0 /
self.time))
self.status.Set Label('%s minute(s) remaining.' %
self.minutes_re maining)
self.minutes_pa ssed += 1
self.minutes_re maining -= 1
else:
self.timer.Stop ()
self.progress.S etValue(self.mi nutes_passed * (100.0 /
self.time))
self.status.Set Label('%s minute(s) have elapsed.' % self.time)
wx.Sound.Play(w x.Sound(r'C:\Wi ndows\Media\not ify.wav'))
>
>
class MyApp(wx.App):
>
def OnInit(self):
frame = MyTimer()
self.SetTopWind ow(frame)
frame.Show()
return True
>
>
if __name__ == '__main__':
app = MyApp(False)
app.MainLoop()

[John McMonagle]

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

On Thu, 2006-07-27 at 20:44 +0000, John Salerno wrote:

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(par ent, id, value="", pos=wx.DefaultP osition,
size=wx.Default Size, style=0, validator=wx.De faultValidator,
name=wx.TextCtr lNameStr)

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



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[John Salerno]

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

John McMonagle wrote:

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]

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

Nick Vatamaniuc wrote:

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.