[Python-ideas] PEP: Dict addition and subtraction

Antoine Pitrou solipsis at pitrou.net
Sat Mar 16 06:39:22 EDT 2019

On Sat, 16 Mar 2019 01:41:59 +1100
Steven D'Aprano <steve at pearwood.info> wrote:
> Matrix multiplication is a perfect example: adding the @ operator could 
> have been done in Python 0.1 if anyone had thought of it, but it took 15 
> years of numerical folk "whinging" about the lack until it happened:

Not so perfect, as the growing use of Python for scientific computing
has made it much more useful to promote a dedicated matrix
multiplication operator than, say, 15 or 20 years ago.

This is precisely why I worded my question this way: what has changed
in the last 20 years that make a "+" dict operator more compelling
today than it was?  Do we merge dicts much more frequently than we
did?  I don't think so.

> Or the infamous := operator, which ultimately is a useful but minor 
> syntactic and semantic change but generated a huge amount of debate, 
> argument and negativity.

... and is likely to be a mistake as well.  Justifying future mistakes
with past mistakes doesn't sound very reasonable ;-)

> I still remember being told in no uncertain terms by the core devs that 
> adding a clear() method to lists was a waste of time because there was 
> already a perfectly good way to spell it with slicing. And then ABCs 
> came along and now lists have a clear method. So opinions change too.

Not really the same problem.  The "+" dict operator is not
intuitively obvious in its meaning, while a "clear()" method on lists

I wouldn't mind the new operator if its meaning was clear-cut.  But
here we have potential for confusion, both for writers and readers of

> > Besides, if I have two dicts with e.g. lists as values, I *really*
> > dislike the fact that the + operator will clobber the values rather than
> > concatenate them.  It's a recipe for confusion.  
> Are you confused that the update method clobbers list values rather than 
> concatenate them? I doubt that you are.
> So why would it be confusing to say that + does a copy-and-update?

Because it's named "+" precisely.  You know, names are important. ;-)



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