tuple unpacking in argument list.

[Chris Barker]

Folks,

I am setting out to Cythonize a function, and found that Cython does
not appear to support tuple unpacking in function arguments. This is
perfectly legal Python:

def test_tuple_unpack((x_1, y_1), (x_2, y_2)):
print x_1, y_1, x_2, y_2

but when I try to run Cython on it:

$ cython tuple_unpack_args.pyx

Error compiling Cython file:
------------------------------------------------------------
...
def test_nested_tuples((x_1, y_1), (x_2, y_2)):
^
------------------------------------------------------------

tuple_unpack_args.pyx:1:27: Expected ')', found ','

$ cython --version
Cython version 0.17.1

Whether we want to support this I don't know, but if the goal is to be
able to compile all legal python, then this is a missing feature.

-Chris



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[Stefan Behnel]

Chris Barker, 09.11.2012 01:51:

> I am setting out to Cythonize a function, and found that Cython does
> not appear to support tuple unpacking in function arguments. This is
> perfectly legal Python:

Python 2, to be exact.

> def test_tuple_unpack((x_1, y_1), (x_2, y_2)):
> print x_1, y_1, x_2, y_2
>
> but when I try to run Cython on it:
>
> $ cython tuple_unpack_args.pyx
>
> Error compiling Cython file:
> ------------------------------------------------------------
> ...
> def test_nested_tuples((x_1, y_1), (x_2, y_2)):
> ^
> ------------------------------------------------------------
>
> tuple_unpack_args.pyx:1:27: Expected ')', found ','
>
> $ cython --version
> Cython version 0.17.1
>
> Whether we want to support this I don't know, but if the goal is to be
> able to compile all legal python, then this is a missing feature.

Well, it's a known limitation at least.

http://trac.cython.org/cython_trac/ticket/692

Given that it's a dead feature that requires a dedicated, non-trivial
implementation (unnamed function arguments) but that is very easy to work
around, there hasn't been any move to support it yet.

Stefan

[Chris Barker]

On Thu, Nov 8, 2012 at 10:57 PM, Stefan Behnel <stef...@behnel.de> wrote:

>> not appear to support tuple unpacking in function arguments. This is
>> perfectly legal Python:
>
> Python 2, to be exact.

I see -- I've paid far too little attention to Python 3 -- I didn't
realize this was depreciated.

> Well, it's a known limitation at least.
>
> http://trac.cython.org/cython_trac/ticket/692

I should have searched the tickets, clearly!

But maybe it should be mentioned here:

http://docs.cython.org/src/userguide/limitations.html

(which has nothing in it now!)

or at least the ticket could be assigned to the "Python Semantics"
component, so it will show up here:

http://trac.cython.org/cython_trac/query?status=assigned&status=new&status=reopened&component=Python+Semantics&component=Python3+Semantics&order=priority&col=id&col=summary&col=component&col=status&col=type&col=priority&col=milestone

Should I do that? I _think_ I have TRAC access.

> Given that it's a dead feature that requires a dedicated, non-trivial
> implementation (unnamed function arguments) but that is very easy to work
> around, there hasn't been any move to support it yet.

Fair enough.

-thx.

[Stefan Behnel]

Chris Barker, 09.11.2012 20:33:

> On Thu, Nov 8, 2012 at 10:57 PM, Stefan Behnel wrote:
>>> not appear to support tuple unpacking in function arguments. This is
>>> perfectly legal Python:
>>
>> Python 2, to be exact.
>
> I see -- I've paid far too little attention to Python 3 -- I didn't
> realize this was depreciated.
>
>> Well, it's a known limitation at least.
>>
>> http://trac.cython.org/cython_trac/ticket/692
>
> I should have searched the tickets, clearly!
>
> But maybe it should be mentioned here:
>
> http://docs.cython.org/src/userguide/limitations.html
>
> (which has nothing in it now!)

AFAIR, there was some content a while ago, but it became outdated and was
removed, likely around 0.15 when the last major Python features went in.

> or at least the ticket could be assigned to the "Python Semantics"
> component, so it will show up here:
>
> http://trac.cython.org/cython_trac/query?status=assigned&status=new&status=reopened&component=Python+Semantics&component=Python3+Semantics&order=priority&col=id&col=summary&col=component&col=status&col=type&col=priority&col=milestone

Good call, I've changed the "Component".

Stefan