[Python-ideas] Dict joining using + and +=

Guido van Rossum guido at python.org
Fri Mar 1 14:31:30 EST 2019

On Thu, Feb 28, 2019 at 10:30 PM Serhiy Storchaka <storchaka at gmail.com>

> 28.02.19 23:19, Greg Ewing пише:
> > Serhiy Storchaka wrote:
> >> I do not understand why we discuss a new syntax for dict merging if we
> >> already have a syntax for dict merging: {**d1, **d2} (which works with
> >> *all* mappings).
> >
> > But that always returns a dict. A '+' operator could be implemented
> > by other mapping types to return a mapping of the same type.
> 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

Looking at the code for Counter, its __iadd__ and __add__ behave subtly
different than Counter.update(): __iadd__ and __add__ (and __radd__) drop
values that are <= 0, while update() does not. That's all fine -- Counter
is not bound by the exact same semantics as dict (starting with its
update() method, which adds values rather than overwriting).

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.

--Guido van Rossum (python.org/~guido)
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