[Python-Dev] Simple Switch statement

[Michael Urman]

On 6/26/06, Raymond Hettinger <rhettinger at ewtllc.com> wrote:
> With the simplified proposal, this would be coded with an inverse mapping:
>
>     for event in pygame.event.get():
>         switch eventmap[event.type]:
>         case 'KEYDOWN': ...
>         case 'KEYUP': ...
>         case 'QUIT': ...

Ah, here's where it gets interesting. While that definitely works on
the surface, it can run into some difficulties of scope. SDL (on which
pygame is based) allows user-defined events, also integers, but
without a predefined meaning. If pygame provides the inverse mapping,
it won't contain the user-defined events. If you construct it, you
have to choose between a local lookup and a global one, and then we
start seeing a global store for an essentially local construct, or we
risk differences when there's more than one locality for using it.

While you're right; it should be simple to ensure that the inverse map
handles at least the set the switch handles, and it keeps evaluation
simpler, I still find the limitation ugly. As mentioned, the early
error checking is poor, and it just doesn't feel like the rest of
python.

> >I also would like to see a way to use 'is' [...]for the comparison
>
> [If] the goal is having several distinct cases that are equal but
> not identical, then that's another story.  I suggest leave the initial
> switch syntax as simple as possible and just switch on id(object).

Switching on id(object) only sounds palatable if we're not bound by
the simple switch's limitations. Having a second (if inverted) mapping
merely of identifier to object smells particularly rancid.

What if I want some cases done as identity, but some as equality?
Since I'm having no luck finding a real use case for this, perhaps I
should assume a nested switch would be adequate. Assuming static or
capture it doesn't look too bad, so I think I'll go with Guido's
hypothesis that it's a red herring.

    switch id(value):
    case id(object): ...
    case id(None): ...
    else:
        switch value:
        case 1: ...
        case 'orange':
        else: raise ValueError

Michael
-- 
Michael Urman  http://www.tortall.net/mu/blog