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

Thomas Lee tom at vector-seven.com
Tue Jun 13 02:49:27 CEST 2006

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.

> 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.

Cheers, Tom

-- Tom Lee http://www.vector-seven.com

----- End forwarded message -----

-- Tom Lee http://www.vector-seven.com

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