On Friday, March 1, 2019 at 5:47:06 AM UTC-5, Steven D'Aprano wrote:
>
> On Fri, Mar 01, 2019 at 08:47:36AM +0200, Serhiy Storchaka wrote:
>
> > Currently Counter += dict works and Counter + dict is an error. With
> > this change Counter + dict will return a value, but it will be different
> > from the result of the += operator.
>
> That's how list.__iadd__ works too: ListSubclass + list will return a
> value, but it might not be the same as += since that operates in place
> and uses a different dunder method.
>
> Why is it a problem for dicts but not a problem for lists?
>
>
> > Also, if the custom dict subclass implemented the plus operator with
> > different semantic which supports the addition with a dict, this change
> > will break it, because dict + CustomDict will call dict.__add__ instead
> > of CustomDict.__radd__.
>
> That's not how operators work in Python or at least that's not how they
> worked the last time I looked: if the behaviour has changed without
> discussion, that's a breaking change that should be reverted.
>
> Obviously I can't show this with dicts, but here it is with lists:
>
> py> class MyList(list):
> ... def __radd__(self, other):
> ... print("called subclass first")
> ... return "Something"
> ...
> py> [1, 2, 3] + MyList()
> called subclass first
> 'Something'
>
>
> This is normal, standard behaviour for Python operators: if the right
> operand is a subclass of the left operand, the reflected method __r*__
> is called first.
>
>
> > Adding support of new operators to builting
> > types is dangerous.
>
> Explain what makes new operators more dangerous than old operators
> please.
>
>
> > > 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). Is not this contradicts the Zen?
> > >
> > >
> > >But (as someone else pointed out) {**d1, **d2} always returns a dict,
> > >not the type of d1 and d2.
> >
> > And this saves us from the hard problem of creating a mapping of the
> > same type.
>
> What's wrong with doing this?
>
> new = type(self)()
>
> Or the equivalent from C code. If that doesn't work, surely that's the
> fault of the subclass, the subclass is broken, and it will raise an
> exception.
>
> I don't think it is our responsibility to do anything more than call
> the subclass constructor. If that's broken, then so be it.
>
>
> Possibly relevant: I've always been frustrated and annoyed at classes
> that hardcode their own type into methods. E.g. something like:
>
> class X:
> def spam(self, arg):
> return X(eggs)
> # Wrong! Bad! Please use type(self) instead.
>
> That means that each subclass has to override every method:
>
> class MySubclass(X):
> def spam(self, arg):
> # Do nothing except change the type returned.
> return type(self)( super().spam(arg) )
>
>
> This gets really annoying really quickly. Try subclassing int, for
> example, where you have to override something like 30+ methods and do
> nothing but wrap calls to super.
>
I agree with you here. You might want to start a different thread with
this idea and possibly come up with a PEP. There might be some pushback
for efficiency's sake, so you might have to reel in your proposal to
collections.abc mixin methods and UserDict methods.
Regarding the proposal, I agree with the reasoning put forward by Guido and
I like it. I think there should be:
* d1 + d2
* d1 += d2
* d1 - d2
* d1 -= d2
which are roughly (ignoring steve's point about types)
* {**d1, **d2}
* d1.update(d2)
* {k: v for k, v in d1.items() if k not in d2}
* for k in list(d1): if k not in d2: del d1[k]
Seeing this like this, there should be no confusion about what the
operators do.
I understand the points people made about the Zen of Python. However, I
think that just like with lists, we tend to use l1+l2 when combining lists
and [*l1, x, *l2, y] when combining lists and elements. Similarly, I think
{**d1, **d2} should only be written when there are also key value pairs,
like {**d1, k: v, **d2, k2: v2}.
Best,
Neil
>
> --
> Steven
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