> On Mar 1, 2019, at 11:31 AM, Guido van Rossum <gu...@python.org> wrote:
>
> 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).
Usually, it's easy to add methods to classes without creating disruption, but
ABCs are more problematic. If MutableMapping grows an __iadd__() method, what
would that mean for existing classes that register as MutableMapping but don't
already implement __iadd__? When "isinstance(m, MutableMapping)" returns True,
is it a promise that the API is fully implemented? Is this something that mypy
could would or should complain about?
> 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
I concur. The latter is also an eyesore and almost certain to be a stumbling
block when reading code.
That said, I'm not sure we actually need a short-cut for "d=e.copy();
d.update(f)". Code like this comes-up for me perhaps once a year. Having a
plus operator on dicts would likely save me five seconds per year.
If the existing code were in the form of "d=e.copy(); d.update(f); d.update(g);
d.update(h)", converting it to "d = e + f + g + h" would be a tempting but
algorithmically poor thing to do (because the behavior is quadratic). Most
likely, the right thing to do would be "d = ChainMap(e, f, g, h)" for a
zero-copy solution or "d = dict(ChainMap(e, f, g, h))" to flatten the result
without incurring quadratic costs. Both of those are short and clear.
Lastly, I'm still bugged by use of the + operator for replace-logic instead of
additive-logic. With numbers and lists and Counters, the plus operator creates
a new object where all the contents of each operand contribute to the result.
With dicts, some of the contents for the left operand get thrown-away. This
doesn't seem like addition to me (IIRC that is also why sets have "|" instead
of "+").
Raymond
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