While working through my implementation, I've come across a couple of inconsistencies with the current proposal:> The merge operator will have the same relationship to the dict.update method as the list concatenation operator has to list.extend, with dict difference being defined analogously.I like this premise. += for lists *behaves* like extend, and += for dicts *behaves* like update.However, later in the PEP it says:> Augmented assignment will just call the update method. This is analogous to the way list += calls the extend method, which accepts any iterable, not just lists.In your Python implementation samples from the PEP, dict subclasses will behave differently from how list subclasses do. List subclasses, without overrides, return *list* objects for bare "+" operations (and "+=" won't call an overridden "extend" method). So a more analogous pseudo-implementation (if that's what we seek) would look like:def __add__(self, other):
if isinstance(other, dict):
new = dict.copy(self)
dict.update(new, other)
return new
return NotImplemented
def __radd__(self, other): if isinstance(other, dict): new = dict.copy(other) dict.update(other, self) return new return NotImplemented
def __iadd__(self, other): if isinstance(other, dict): dict.update(self, other) return self return NotImplementedThis is what my C looks like right now. We can choose to update these semantics to be "nicer" to subclasses, but I don't see any precedent for it (lists, sets, strings, etc.).Brandt--On Fri, Mar 1, 2019 at 11:41 AM Brett Cannon <brett@python.org> wrote:On Fri, Mar 1, 2019 at 8:50 AM Brandt Bucher <brandtbucher@gmail.com> wrote:I’ve never been part of this process before, but I’m interested in learning and helping any way I can.Thanks!My addition implementation is attached to the bpo, and I’m working today on bringing it in line with the PEP in its current form (specifically, subtraction operations).When your proposed patch is complete, Brandt, just ask Steven to update the PEP to mention that there's a proposed implementation attached to the issue tracking the idea.-Brett_______________________________________________BrandtAttached is a draft PEP on adding + and - operators to dict for
discussion.
This should probably go here:
https://github.com/python/peps
but due to technical difficulties at my end, I'm very limited in what I
can do on Github (at least for now). If there's anyone who would like to
co-author and/or help with the process, that will be appreciated.
--
Steven<dict_addition_pep.txt>_______________________________________________
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