[Python-Dev] Switch statement (original) (raw)

Guido van Rossum guido at python.org
Wed Jun 21 07:14:51 CEST 2006

On 6/20/06, Phillip J. Eby <pje at telecommunity.com> wrote:

At 12:26 PM 6/21/2006 +1200, Greg Ewing wrote: >Guido van Rossum wrote: > > > But it would be easy enough to define a dict-filling function that > > updates only new values. > >Or evaluate the case expressions in reverse order.

-1; stepping through the code in a debugger is going to be weird enough, what with the case statements being executed at function definition time, without the reverse order stuff.

Agreed.

I'd rather make it an error to list the same value more than once; we can just check if the key is present before defining that value.

That makes sense too.

I was thinking of a use case where you'd have a couple of sets of cases that need the same treatment per set (sre_compile.py has a few of these) but one of the sets has an exception. With the if/elif style you could write this as

if x is exception: ...exceptional case... elif x in set1: ...case for set1... elif x in set2: ..case for set2... etc.

But the prospect of something like this passing without error:

switch x: case 1: ...case 1... case 1: ...another case 1?!?!...

makes me think that it's better to simply reject overlapping cases.

BTW I think the several-sets use case above is important and would like to have syntax for it. Earlier it was proposed to allow saying

case 1, 2, 3: ...executed if x==1 or x==2 or x==3...

but now that we're agreed to evaluate the expression at function definition time, I want to support

case S: ...executed if x in S...

but that would be ambiguous. So, thinking aloud, here are a few possibilities:

case A: ... if x == A... cases S: ...if x in A...

or perhaps (saving a keyword):

case A: ... if x == A... case in S: ...if x in A...

This would also let us write cases for ranges:

case in range(10): ...if x in range(10)...

I propose that the expression used for a single-value should not allow commas, so that one is forced to write

case (1, 2): ...if x == (1, 2)...

if you really want a case to be a tuple value, but you should be able to write

case in 1, 2: ...if x in (1, 2)...

since this really doesn't pose the same kind of ambiguity. If you forget the 'in' it's a syntax error.

Hm, so this still doesn't help if you write

case S: ...

(where S is an immutable set or sequence) when you meant

case in S: ...

so I'm not sure if it's worth the subtleties.

Time for bed here,

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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