[Python-ideas] pattern matching proof-of-concept

Michael Selik michael.selik at gmail.com
Mon May 30 02:52:33 EDT 2016

I'm sure y'all are too busy with PyCon to be checking email. But if you're
curious, I added attribute unpacking and post-binding guards (

The object unpacking works as most folks have suggested, via kwargs.
The guards unfortunately need to be functions that take no parameters.

    match(schema=Foo(a=1, b=bind.x), data=Foo(a=1, b=2), lambda : bind.x >

On Sat, May 28, 2016 at 6:26 AM Paul Moore <p.f.moore at gmail.com> wrote:

> On 28 May 2016 at 08:22, Michael Selik <michael.selik at gmail.com> wrote:
> > My original dict unpacking proposal was very short and lacked a
> motivating
> > usage. Toy examples made my proposal look unnecessarily verbose and
> > suggested obvious alternatives with easy current syntax.
> >
> > Nested/recursive unpacking is much more troublesome, especially when
> > combined with name-binding. I wrote an example to compare my proposal
> with
> > current syntax.
> >
> > Example usage.
> > https://github.com/selik/destructure/blob/master/examples/fips.py
> >
> > Implementation.
> > https://github.com/selik/destructure/blob/master/destructure.py
> >
> > The design of my module I'm least happy with is the name-binding. I
> extended
> > a SimpleNamespace to create an Erlang-style distinction between bound and
> > unbound names. Though the API is a bit awkward, now that the module is
> > built, I'm less enthusiastic about introducing new syntax. Funny how that
> > works.
> >
> > I haven't yet decided how to add post-binding guards to the cases.
> Interesting! Thanks for taking the time to make a real-world use case.
> I haven't looked at the module yet, just the example, but the code
> does look pretty clean and readable. The example is certainly complex
> enough that I'd probably end up with pretty messy and fragile code if
> I just tried to put something together with pure Python code.
> And yes, it's interesting how finding a good API for a module can make
> the need for a dedicated syntax less pressing. But working out that
> good API can be really hard (I don't think I'd ever have thought of
> doing it the way you did).
> Paul
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