01.03.19 21:31, Guido van Rossum пише:
On Thu, Feb 28, 2019 at 10:30 PM Serhiy Storchaka <storch...@gmail.com <mailto:storch...@gmail.com>> wrote:And this opens a non-easy problem: how to create a mapping of the same type? Not all mappings, and even not all dict subclasses have a copying constructor.There's a compromise solution for this possible. We already do this for Sequence and MutableSequence: Sequence does *not* define __add__, but MutableSequence *does* define __iadd__, and the default implementation just calls self.update(other). I propose the same for Mapping (do nothing) and MutableMapping: make the default __iadd__ implementation call self.update(other).
This LGTM for mappings. But the problem with dict subclasses still exists. If use the copy() method for creating a copy, d1 + d2 will always return a dict (unless the plus operator or copy() are redefined in a subclass). If use the constructor of the left argument type, there will be problems with subclasses with non-compatible constructors (e.g. defaultdict).
Anyways, the main reason to prefer d1+d2 over {**d1, **d2} is that the latter is highly non-obvious except if you've already encountered that pattern before, while d1+d2 is what anybody familiar with other Python collection types would guess or propose. And the default semantics for subclasses of dict that don't override these are settled with the "d = d1.copy(); d.update(d2)" equivalence.
Dicts are not like lists or deques, or even sets. Iterating dicts produces keys, but not values. The "in" operator tests a key, but not a value.
It is not that I like to add an operator for dict merging, but dicts are more like sets than sequences: they can not contain duplicated keys and the size of the result of merging two dicts can be less than the sum of their sizes. Using "|" looks more natural to me than using "+". We should look at discussions for using the "|" operator for sets, if the alternative of using "+" was considered, I think the same arguments for preferring "|" for sets are applicable now for dicts.
But is merging two dicts a common enough problem that needs introducing an operator to solve it? I need to merge dicts maybe not more than one or two times by year, and I am fine with using the update() method. Perhaps {**d1, **d2} can be more appropriate in some cases, but I did not encounter such cases yet.
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