I need newbie help

[Sean Berry]

Sorry for any confusion.

I didn't write this page...
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/269708
I was just trying to point out what they were trying to show.



"Andrew Bennetts" <andrew-pythonlist at puzzling.org> wrote in message
news:mailman.3.1084513865.4157.python-list at python.org...
> On Thu, May 13, 2004 at 10:16:01PM -0700, Sean Berry wrote:
> [...]
> >
> > Since Python doesn't have a switch statement, you can make your own.
> > In Python you could do something like this though:
> >
> > selector={
> >    x<0    : 'return None',
> >    0<=x<1 : 'return 1',
> >    1<=x<2 : 'return 2',
> >    2<=x<3 : 'return 3',
> >    3<=x   : 'return None'
> >    }[a]
> >
> > if a == 1, it would "return 1".
>
> You seem to be a little bit confused, which is likely to make a newbie
> really confused.
>
> What is x in your example?  Your dictionary's definition depends on
multiple
> expressions involving it -- you'd only get the result of selector ==
'return
> 1' when a is 1 if 0 <= x < 1.
>
> For instance:
>
> >>> x = 0.5
> >>> {
> ...    x<0    : 'return None',
> ...    0<=x<1 : 'return 1',
> ...    1<=x<2 : 'return 2',
> ...    2<=x<3 : 'return 3',
> ...    3<=x   : 'return None'
> ... }
> {False: 'return None', True: 'return 1'}
>
> But:
>
> >>> x = 2
> >>> {
> ...    x<0    : 'return None',
> ...    0<=x<1 : 'return 1',
> ...    1<=x<2 : 'return 2',
> ...    2<=x<3 : 'return 3',
> ...    3<=x   : 'return None'
> ...    }
> {False: 'return None', True: 'return 3'}
>
> So looking up 1 in that dictionary would return whatever string was
> associated with the key True (because 1 == True), which is entirely
> dependent on the value of x when the dictionary was defined.
>
> Also, your use of strings containing python statements is misleading --
> retrieving a value from a dictionary doesn't implicitly execute it as a
> string of Python source code too.  Less confusing values would have been
> 'less than 0', 'between 0 and 1', and so on, because at least that is less
> likely to confuse a newbie.
>
> Finally, as someone else pointed out, often the best way to do a switch
> statement in Python is not to be clever at all, but instead to just simply
> do:
>
>     if x < 0:
>         return None
>     elif 0 <= x < 1:
>         return 1
>     elif 1 <= x < 2:
>         return 2
>     elif 2 <= x < 3:
>         return 3
>     else:
>         return None
>
> Although that's a very contrived example (what's the point of doing that?)
> -- more convincing would be something like:
>
>     if colour == 'red':
>         r, g, b = 255, 0, 0
>     elif colour == 'purple':
>         r, g, b = 255, 0, 255
>
>     # and so on ...
>
> Which I think turns out to be a much better candidate for the dictionary
> trick anyway:
>
>    rgbValues = {
>        'red': (255, 0, 0),
>        'purple': (255, 0, 255),
>        # ...
>    }
>
>    r, g, b = rgbValues[colour]
>
> -Andrew.
>
>