[Python-Dev] switch-based programming in Python (original) (raw)

Thomas Wouters thomas@xs4all.net
Thu, 8 Nov 2001 16:24:25 +0100

On Thu, Nov 08, 2001 at 12:32:39PM +0100, M.-A. Lemburg wrote:

Thomas Wouters wrote:

> Would a lengthy if/elif/elif/else construct work better ? Why ?

Yes, because it doesn't involve calling methods, no execution frames have to be setup, no arguments need to be passed in, state can be managed in local variables, etc.

> I'm not sure what you want the if/else to actually do. Personally, I > either need a function call in there (in which case a dispatch table calling > the function directly, sometimes with apply() or lambda-wrapper tricks, does > fine) or some kind of variable assignment, in which case a simple dict > lookup works just as fine. Then again, I don't write that much Python code.

You don't ?

Is that sarcasm ? :) No, I don't. My actual job, the part I get paid for, doesn't (yet) involve writing Python. It's part C and Perl, part system design, and part administration. So Python is just a hobby.

> I personally wouldn't be adverse to a switch-like syntax, as long as we > define it like a dict dispatch (the argument is evaluated once, it should be > hashable, and all the 'cases' should be hashable -- preferably even > compile-time constants.) I like the idea, I'm just not sure if there's > enough use for it.

That's the idea.

There's enough need in it for my applications, so I'd go through the trouble of writing the code for it, provided I get the OK and help from python-dev.

The writing part would be very tricky. I don't think you can do it without syntax support, at least not reliably, even if 'without syntax support' is some kind of directive statement to signal that a particulare if/elif/elif/else chain should be converted to a jump table behind the scene.

For new syntax, I'd imagine something like this:

switch EXPR: case CONSTANT: [suite] case CONSTANT: [suite] ... else:

EXPR is a normal Python expression. CONSTANT should be a hashable constant (with a persistant hash value, duh) so we don't have to re-hash all the cases when entering the switch. The 'else' would function like a 'default:' case in C's switch. I'm not sure on the naming of 'switch' and 'case', nor about the indentation-level of the 'cases'. And what to do about fallthrough? It's commonly accepted (or at least argued :) as a design flaw that C's switch() defaults to fallthrough. Bytecodewise it should probably turn something like:

def whatis(x): switch(x): case 'one': print '1' case 'two': print '2' case 'three': print '3' else: print "D'oh!"

Into (ommitting POP_TOP's and SET_LINENO's):

6 LOAD_FAST 0 (x) 9 LOAD_CONST 1 (switch-table-1) 12 SWITCH 26 (to 38) # or maybe 'SWITCH '

14 LOAD_CONST 2 ('1') 17 PRINT_ITEM 18 PRINT_NEWLINE 19 JUMP 43

22 LOAD_CONST 3 ('2') 25 PRINT_ITEM 26 PRINT_NEWLINE 27 JUMP 43

30 LOAD_CONST 4 ('3') 33 PRINT_ITEM 34 PRINT_NEWLINE 35 JUMP 43

38 LOAD_CONST 5 ("D'oh!") 41 PRINT_ITEM 42 PRINT_NEWLINE

43 LOADCONST 0 (None) 46 RETURN_VALUE

Where the 'SWITCH' opcode would jump to 14, 22, 30 or 38 depending on 'x'. PEP, anyone ? :)

-- Thomas Wouters <thomas@xs4all.net>

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