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From "Nugent, Daniel" <Daniel.Nug...@mlp.com>
Subject RE: [EXTERNAL] Re: Question about memoryviews and array construction
Date Wed, 11 Mar 2020 23:06:26 GMT
Thanks for closing this out!

Sorry I didn't get around to working on this before you ended up putting it in. I had some
difficulty getting the dev environment set up and limited time to work on it.

Is there a list of good first issues to take a crack at? I've really appreciated the project
overall and would like to help out in the time I can.

-Dan Nugent

-----Original Message-----
From: Wes McKinney <wesmckinn@gmail.com> 
Sent: Saturday, March 7, 2020 10:55 AM
To: user@arrow.apache.org
Subject: [EXTERNAL] Re: Question about memoryviews and array construction

There's a couple places to start

* Add PyMemoryView type check to internal::IsPyBinary https://github.com/apache/arrow/blob/master/cpp/src/arrow/python/helpers.h#L80.
I think this is all that's needed to take care of type inference
* Make sure PyMemoryView is handled in the PyBytesView helper in
https://github.com/apache/arrow/blob/master/cpp/src/arrow/python/common.h#L193

On Sat, Mar 7, 2020 at 9:35 AM Daniel Nugent <nugend@gmail.com> wrote:
>
> Great!
>
> If you could provide a smidgen of guidance about where to start making this change, I
would be happy to give it a shot.
>
> Thanks,
>
> -Dan Nugent
> On Mar 7, 2020, 09:18 -0500, Wes McKinney <wesmckinn@gmail.com>, wrote:
>
> hi Dan,
>
> Yes, we should support constructing StringArray directly from 
> memoryview as we do with bytes and unicode -- you're the first person 
> to ask about this so far. I opened 
> https://issues.apache.org/jira/browse/ARROW-8026. This should not be a 
> huge amount of work so would be a good first contribution to the 
> project
>
> Thanks
>
> Wes
>
> On Fri, Mar 6, 2020 at 8:29 PM Nugent, Daniel <Daniel.Nugent@mlp.com> wrote:
>
>
> Hi,
>
>
>
> I have a short program which I’m wondering about the sensibility of. Could anyone let
me know if this is reasonable or not:
>
>
>
> import pyarrow as pa, third_party_library
>
>
> memory_views = third_party_library.get_strings()
>
>
> memory_views
>
>
> [<memory at 0x7f1745cc0870>, <memory at 0x7f1745cc0940>, <memory at 
> 0x7f1745cc0a10>, <memory at 0x7f1745cc0ae0>]
>
> pa.array(memory_views,pa.string())
>
>
> Traceback (most recent call last):
>
> File "<stdin>", line 1, in <module>
>
> File "pyarrow/array.pxi", line 269, in pyarrow.lib.array
>
> File "pyarrow/array.pxi", line 38, in pyarrow.lib._sequence_to_array
>
> File "pyarrow/error.pxi", line 107, in pyarrow.lib.check_status
>
> pyarrow.lib.ArrowTypeError: Expected a string or bytes object, got a 
> 'memoryview' object
>
> pa.array(map(bytes,memory_views),pa.string())
>
>
> <pyarrow.lib.StringArray object at 0x7f1745cbdd00>
>
> [
>
> "this",
>
> "is",
>
> "a",
>
> "sample"
>
> ]
>
>
>
> I have a big list of byte sequences being provided to me as memoryviews from a third
party library. I’d like to create an Arrow StringArray from them as efficiently as possible.
Having to map and consequently copy them through a bytes constructor seems not great (and
the memoryview tobytes function appears to just call the bytes constructor, afaict).
>
>
>
> To me, it seemed like pa.array should be able to use the memoryview objects directly
in order to construct the StringArray, but it seems like Arrow wants them copied into fresh
byte objects first. I don’t know if I understand why and was ultimately wondering if it’s
a reasonable thing to desire.
>
>
>
> Thanks in advance,
>
> -Dan Nugent
>
>
>
>
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