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 <table-index>'
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 LOAD_CONST 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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