[Python-ideas] Fwd: Allow a group by operation for dict comprehension

[MRAB]

On 2018-06-29 05:14, David Mertz wrote:
> 
> Mike Selik asked for my opinion on a draft PEP along these lines. I 
> proposed a slight modification to his idea that is now reflected in his 
> latest edits. With some details fleshed out, I think this is a promising 
> idea. I like the a collections class better, of course, but a dict 
> classmethod is still a lot smaller change than new syntax change in 
> comprehension.
> 
> On Thu, Jun 28, 2018, 8:15 PM David Mertz <mertz at gnosis.cx 
> <mailto:mertz at gnosis.cx>> wrote:
> 
[snip]
>     For example (this typed without testing, forgive any typos or thinkos):
> 
>     >>> from collections import Grouper # i.e. in Python 3.8+
>     >>> grouped = Grouper(range(7), key=mod_2)
>     >>> grouped
>     Grouper({0: [0, 2, 4, 6], 1: [1, 3, 5]})
>     >>> grouped.update([2, 10, 12, 13], key=mod_2)
 >     >>> grouped
 >     Grouper({0: [0, 2, 4, 6, 2, 10, 12], 1: [1, 3, 5, 13]})
 >     >>> # Updating with no key function groups by identity
 >     >>> # ... is there a better idea for the default key function?
 >     >>> grouped.update([0, 1, 2])
 >     >>> grouped
 >     Grouper({0: [0, 2, 4, 6, 2, 10, 12, 0], 1: [1, 3, 5, 13, 1], 2: [2]})

I think that if a Grouper instance is created with a key function, then 
that key function should be used by the .update method.

You _could_ possibly override that key function by providing a new one 
when updating, but, OTOH, why would you want to? You'd be mixing 
different kinds of groupings! So -1 on that.

>     >>> # Maybe do a different style of update if passed a dict subclass
>     >>> #  - Does a key function make sense here?
>     >>> grouped.update({0: 88, 1: 77})
>     >>> grouped
>     Grouper({0: [0, 2, 4, 6, 2, 10, 12, 0, 88],
>     1: [1, 3, 5, 13, 1, 77],
>     2: [2]})
>     >>> # Avoiding duplicates might sometimes be useful
>     >>> grouped.make_unique()   # better name?  .no_dup()?
>     >>> grouped
>     Grouper({0: [0, 2, 4, 6, 10, 12, 88],
>     1: [1, 3, 5, 13, 77],
>     2: [2]})
> 
If you want to avoid duplicates, maybe the grouper should be created 
with 'set' as the default factory (see 'defaultdict'). However, there's 
the problem that 'list' has .append but 'set' has .add...
>     I think that most of the methods of Counter make sense to include
>     here in appropriately adjusted versions. Converting to a plain
>     dictionary should probably just be `dict(grouped)`, but it's
>     possible we'd want `grouped.as_dict()` or something.
> 
>     One thing that *might* be useful is a way to keep using the same key
>     function across updates. Even with no explicit provision, we *could*
>     spell it like this:
> 
>     >>> grouped.key_func = mod_2
>     >>> grouped.update([55, 44, 22, 111], key=grouped.key_func)
> 
>     Perhaps some more official API for doing that would be useful though.
> 
[snip]