[Python-ideas] PEP: Dict addition and subtraction

Christopher Barker pythonchb at gmail.com
Thu Mar 21 22:19:06 EDT 2019

> https://docs.python.org/3.8/library/collections.html has some
> examples using collections.Counter, which is clearly described
> as being a subclass of dict.  Amongst the examples:
>      c + d  # add two counters together:  c[x] + d[x]
> That's the + operator operating on two dicts (don't make me
> quote the Liskov Substitution Principle), but doing something
> really different than the base operator.
> So if I know that c and d (or worse, that one of them) is a
> dict, then interpreting c + d becomes much more interesting,

Killing a use of a common operator with a very common built in data type
because the operator is used in a different way by a specialized object in
the stdlib seems a bit backwards to me.

Frankly, I think considering Counter as a dict subclass is the mistake
here, even if it is true.

Christopher Barker, PhD

Python Language Consulting
  - Teaching
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  - Desktop GUI and Web Development
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