case/switch statement?

[Dan Sommers]

On Sun, 12 Jun 2005 10:35:42 +1000,
Steven D'Aprano <steve at REMOVETHIScyber.com.au> wrote:

> I don't relish the idea of especially long case statements.

> I've never understood why something like:

> if x = 5:
>     do_this
> elif x = 6:
>     do_that
> else:
>     do_something_else

> is supposed to be "bad", but

> case of:
>     x = 5:
>         do_this
>     x = 6:
>         do_that
> otherwise:
>         do_something_else

> is supposed to be "good".

In the truly general case, I agree.

But twist your second example slightly into this:

    case x of:
        5: do_this
        6: do_that
        otherwise: do_something_else

and the goodness is obvious:  we're choosing functionality based on the
value of x, so it's nice to see x in only one place.

> Arguably, a case statement *might* allow the compiler to optimize the
> code, maybe, sometimes. But in general, no such optimization is
> possible, so a case statement is merely equivalent to a series of
> if...elif...  statements.

I agree that in general, optimizing a series of if/elif statements is
tricky, but your first example is very command and exactly the kind of
code that a good optimizer *can* optimize (as long as "x" isn't
pathological in the sense that evaluating it also changes its value or
has other side effects).

> There is no case statement in Python. If you don't care about
> readability, one alternative is to use a dictionary:

> case = {5: do_this, 6: do_that}
> case.get(x, do_something_else)()

I find this very readable.  YMMV.

I also find this easier to extend in the "case" (pardon the pun) that my
program expands and x might now be 7 or 8 (or "foo" or 3j).

The biggest drawback here, as others have noted in previous discussions,
is that the do_* functions execute in a separate scope.

Regards,
Dan

-- 
Dan Sommers
<http://www.tombstonezero.net/dan/>