[Python-Dev] Re: switch statement

[Shannon -jj Behrens]

On 4/25/05, M.-A. Lemburg <mal at egenix.com> wrote:
> Shannon -jj Behrens wrote:
> > On 4/20/05, M.-A. Lemburg <mal at egenix.com> wrote:
> >
> >>Fredrik Lundh wrote:
> >>
> >>>PS. a side effect of the for-in pattern is that I'm beginning to feel
> >>>that Python
> >>>might need a nice "switch" statement based on dictionary lookups, so I can
> >>>replace multiple callbacks with a single loop body, without writing too
> >>>many
> >>>if/elif clauses.
> >>
> >>PEP 275 anyone ? (http://www.python.org/peps/pep-0275.html)
> >>
> >>My use case for switch is that of a parser switching on tokens.
> >>
> >>mxTextTools applications would greatly benefit from being able
> >>to branch on tokens quickly. Currently, there's only callbacks,
> >>dict-to-method branching or long if-elif-elif-...-elif-else.
> >
> > I think "match" from Ocaml would be a much nicer addition to Python
> > than "switch" from C.
> 
> PEP 275 is about branching based on dictionary lookups which
> is somewhat different than pattern matching - for which we
> already have lots and lots of different tools.
> 
> The motivation behind the switch statement idea is that of
> interpreting the multi-state outcome of some analysis that
> you perform on data. The main benefit is avoiding Python
> function calls which are very slow compared to branching to
> inlined Python code.
> 
> Having a simple switch statement
> would enable writing very fast parsers in Python -
> you'd let one of the existing tokenizers such as mxTextTools,
> re or one of the xml libs create the token input data
> and then work on the result using a switch statement.
> 
> Instead of having one function call per token, you'd
> only have a single dict lookup.
> 
> BTW, has anyone in this thread actually read the PEP 275 ?

I'll admit that I haven't because dict-based lookups aren't as
interesting to me as an Ocaml-style match statement.  Furthermore, the
argument "Instead of having one function call per token, you'd only
have a single dict lookup" isn't very compelling to me personally,
because I don't have such a performance problem in my applications,
which isn't to say that it isn't important or that you don't have a
valid point.

Best Regards,
-jj

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