Trying To Catch Invalid User Input

[Albert Hopkins]

On Sun, 2009-08-23 at 16:36 +0100, MRAB wrote:
> Victor Subervi wrote:
> > Hi;
> > I have the following:
> > 
> > style = raw_input('What style is this? (1 = short, 2 = long): ')
> > flag = 0
> > while flag == 0:
> >   if (style != 1) or (style != 2):
> >     style = raw_input('There was a mistake. What style is this? (1 = 
> > short, 2 = long): ')
> >   else:
> >     flag = 1
> > 
> > I would think this would catch errors and permit valid values, but it 
> > doesn't. If I enter an erroneous value the first time, and the second 
> > time a good value, it doesn't break the loop. Why?
> > 
> This is wrong:
> 
>      (style != 1) or (style != 2)
> 
> For example, if style is 1 (which should be a valid value):
> 
>      (style != 1) or (style != 2)
>   => (1     != 1) or (1     != 2)
>   => False        or True
>   => True
> 
> What you mean is:
> 
>      (style != 1) and (style != 2)

Or (perhaps) better:

VALID_STYLES = {1: 'short', 2: 'long'}

style = None
while style not in VALID_STYLES:
    style = raw_input('What style is this? %s: ' %
        str(VALID_STYLES).replace(':', ' ='))
    
    # also, raw_input() returns a string
    try:
        style = int(style)
    except ValueError:
        pass