[Python-ideas] PEP: Dict addition and subtraction

Josh Rosenberg shadowranger+pythonideas at gmail.com
Wed Mar 6 07:20:50 EST 2019

On Wed, Mar 6, 2019 at 11:52 AM Rhodri James <rhodri at kynesim.co.uk> wrote:

> On 06/03/2019 10:29, Ka-Ping Yee wrote:
> > len(dict1 + dict2) does not equal len(dict1) + len(dict2), so using the +
> > operator is nonsense.
> I'm sorry, but you're going to have to justify why this identity is
> important.  Making assumptions about length where any dictionary
> manipulations are concerned seems unwise to me, which makes a nonsense
> of your claim that this is nonsense :-)

It's not "nonsense" per se. If we were inventing programming languages in a
vacuum, you could say + can mean "arbitrary combination operator" and it
would be fine. But we're not in a vacuum; every major language that uses +
with general purpose containers uses it to mean element-wise addition or
concatenation, not just "merge". Concatenation is what imposes that
identity (and all the others people are defending, like no loss of input
values); you're taking a sequence of things, and shoving another sequence
of things on the end of it, preserving order and all values.

The argument here isn't that you *can't* make + do arbitrary merges that
don't adhere to these semantics. It's that adding yet a third meaning to +
(and it is a third meaning; it has no precedent in any existing type in
Python, nor in any other major language; even in the minor languages that
allow it, they use + for sets as well, so Python using + is making Python
itself internally inconsistent with the operators used for set), for
limited benefit.

- Josh Rosenberg
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