[Python-Dev] Switch statement

M.-A. Lemburg mal at egenix.com
Wed Jun 14 18:36:54 CEST 2006

Thomas Lee wrote:
> On Mon, Jun 12, 2006 at 11:33:49PM +0200, Michael Walter wrote:
>> Maybe "switch" became a keyword with the patch..
>> Regards,
>> Michael
> That's correct.
>> On 6/12/06, M.-A. Lemburg <mal at egenix.com> wrote:
>>> Could you upload your patch to SourceForge ? Then I could add
>>> it to the PEP.
> It's already up there :) I thought I sent that through in another
> e-mail, but maybe not:
> http://sourceforge.net/tracker/index.php?func=detail&aid=1504199&group_id=5470&atid=305470
> Complete with documentation changes and a unit test.

Thanks. Please CC me on these emails.

It would also help if your mailer wouldn't add
	Mail-Followup-To: python-dev at python.org
to the messages, since then a Reply-All will not include
any other folks on CC in this thread.

>>> Thomas wrote a patch which implemented the switch statement
>>> using an opcode. The reason was probably that switch works
>>> a lot like e.g. the for-loop which also opens a new block.
> No, Skip explained this in an earlier e-mail: apparently some
> programming languages use a compile-time generated lookup table
> for switch statements rather than COMPARE_OP for each case. The
> restriction is, of course, that you're stuck with constants for each
> case statement.
> In a programming language like Python, where there are no named
> constants, the usefulness of such a construct might be questioned.
> Again, see Skip's earlier e-mails.
>>> Could you explain how your patch works ?
> 1. Evaluate the "switch" expression so that it's at the top of the stack
> 2. For each case clause:
> 2.1. Generate a DUP_TOP to duplicate the switch value for a comparison
> 2.2. Evaluate the "case" expression
> 2.3. COMPARE_OP(PyCmp_EQ)
> 2.4. Jump to the next case statement if false
> 2.5. Otherwise, POP_TOP and execute the suite for the case clause
> 2.6. Then jump to 3
> 3. POP_TOP to remove the evaluated switch expression from the stack
> As you can see from the above, my patch generates a COMPARE_OP for each
> case, so you can use expressions - not just constants - for cases.
> All of this is in the code found in Python/compile.c.

Thanks for the explanation, but the original motivation
for adding a switch statement was to be able to use
a lookup table for the switching in order to speed
up branching code, e.g. in parsers which typically
use constants to identify tokens (see the "Problem" section
of the PEP for the motivation).

The standard

if ...
elif ...
efif ...

scheme already provides the above logic. There's really
no need to invent yet another syntax to write such
constructs, IMHO.

Marc-Andre Lemburg

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