On Fri, Oct 4, 2019 at 3:59 PM Caleb Donovick <donov...@cs.stanford.edu>
wrote:
> While there is no restriction on passing dicts to getitem. Doing so tends
> to be a bit ugly. I have two main use cases in mind for this syntax.
>
> The first and perhaps the most obvious, is doing relational queries.
> ```
> where_x_1 = db[x=1]
> ```
> is more beautiful than
> ```
> where_x_1 = db[dict(x=1)]
> where_x_1 = db[{'x': 1}]
> # or by abusing slices
> where_x_1 = db['x':1]
> # or in the style of Pandas
> where_x_1 = db[db['x'] == 1]
> ```
>
> Beyond relational queries my own personal use case is a shorthand for
> dataclasses / protocols.
> ```
> foo: ProtoRecord[x=int, y=int] = DataRecord[x=int, y=int](0, 1)
> ```
> where `DataRecord[field0=T0, ..., fieldk=Tk]` generates
> ```
> @dataclass
> class Record:
> field0: T0
> ...
> fieldk: Tk
> ```
> and `ProtoRecord[field0=T0, ..., fieldk=Tk]` generates a similar protocol.
>
> Allowing key value pairs in geitem need not change the interface of
> getitem. All the key value pairs could be collected as a dict and passed
> to getitem as the index. Similar to how the all the positional arguments
> are gather into a single tuple.
> ```
> class Foo:
> def __getitem__(self, idx):
> print(idx)
>
> f = Foo()
> f[x=1, y=2] # {'x': 1, 'y': 2}
> ```
> This would make any legacy code using normal dicts as keys (I don't know
> how prevalent that is) automatically work with the new syntax.
>
> There doesn't necessarily need to be support for mixing of tuple based
> indexing and keyword indexing. i.e.
> ```
> obj[0, x=1] # SyntaxError
> ```
>
> I don't really know anything about parsers but I think the grammar could
> be extended without issue with the following rule:
> ```
> subscriptlist: ... | kwargsubscript (',' kwargsubscript )* [',']
> kwargsubscript: NAME '=' test
> ```
> if `NAME '=' test` would result in ambiguity similar to argument it could
> be `test '=' test` with a block in ast.c
>
>
> - Caleb Donovick
>
>
I thought about this in terms of labelled arrays like pandas and xarray,
but I haven't talked to them about whether they would actually want to use
such a feature so I hesitated to post it here.
One possible approach I thought of for backwards-compatibility would be to
be to create a new "kwslice" object, that would be API-incompatible and not
derived from "slice". Anything that isn't explicitly programmed to deal
with it would simply choke on the unknown object. The downside is the
error message would be somewhat cryptic.
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/FL3F7MWZKHDVV6DJVWKARAG5X4VLAZY7/
Code of Conduct: http://python.org/psf/codeofconduct/
