[Python-ideas] dict.merge(d1, d2, ...) (Counter proposal for PEP 584)

[Andrew Svetlov]

Python C API has PyDict_Merge
(https://docs.python.org/3/c-api/dict.html#c.PyDict_Merge) function
which has different behavior than the proposed Python level method
(doesn't copy but merge in-place).
This is a red flag for me.

On Tue, Mar 5, 2019 at 12:24 PM fhsxfhsx <fhsxfhsx at 126.com> wrote:
>
> I agree so much on your opinion that I was just to create a topic about this if you didn't.
> I also propose here a small modification to make it more general which adds an argument `how` (name to be discussed), telling how to merge the dicts, as many have pointed out that there could be different ways to merge dicts.
> So things would be like
>
>     def addition_merge(key, values, exists):
>         """
>         :param key: the key to merge
>         :param values: values of dicts to merge indexed at `key`
>         :param exists: whether each dict contains `key`
>         """
>         if any(exists):
>             return True, sum([value for exist, value in zip(exists, values) if exist])
>         else:
>             return False
>     d1.merge(d2, d3, ..., how=addition_merge)
>
> We could even have
>
>     def discard(key, values, exists):
>         return not any(exists[1:]), values[0]
>     d1.merge(d2, how=discard)
>
> which does the same thing as proposed `d1-d2`.
>
> This would make things like
>     d = d1.merge(iter_of_pairs)
> d = d1.merge(key=value)
> not working, but people could easily wrap a `dict()` over the iterator or key-value stuff and attach no complication.
>
>
> At 2019-03-05 15:39:40, "INADA Naoki" <songofacandy at gmail.com> wrote:
> >I think some people in favor of PEP 584 just want
> >single expression for merging dicts without in-place update.
> >
> >But I feel it's abuse of operator overload.  I think functions
> >and methods are better than operator unless the operator
> >has good math metaphor, or very frequently used as concatenate
> >strings.
> >
> >This is why function and methods are better:
> >
> >* Easy to search.
> >* Name can describe it's behavior better than abused operator.
> >* Simpler lookup behavior. (e.g. subclass and __iadd__)
> >
> >Then, I propose `dict.merge` method.  It is outer-place version
> >of `dict.update`, but accepts multiple dicts.  (dict.update()
> >can be updated to accept multiple dicts, but it's not out of scope).
> >
> >* d = d1.merge(d2)  # d = d1.copy(); d.update(d2)
> >* d = d1.merge(d2, d3)  # d = d1.copy(); d.update(d2); d2.update(d3)
> >* d = d1.merge(iter_of_pairs)
> >* d = d1.merge(key=value)
> >
> >
> >## Merits of dict.merge() over operator +
> >
> >* Easy to Google (e.g. "python dict merge").
> >* Easy to help(dict.merge). (or dict.merge? in IPython)
> >* No inefficiency of d1+d2+d3+...+dN, or sum(list_of_many_dicts)
> >* Type of returned value is always same to d1.copy().  No issubclass,
> >no __iadd__.
> >
> >## Why not dict.updated()?
> >
> >sorted() is a function so it looks different from L.sort()
> >But d.updated() is very similar to d.update() for human eyes.
> >
> >## How about d1 - d2?
> >
> >If it is really useful, it can be implemented as method too.
> >
> >dict.discard(sequence_of_keys)
> >
> >Regards,
> >--
> >INADA Naoki  <songofacandy at gmail.com>
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>
>
>
>
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-- 
Thanks,
Andrew Svetlov