[Python-ideas] PEP: Dict addition and subtraction

Stephen J. Turnbull turnbull.stephen.fw at u.tsukuba.ac.jp
Fri Mar 8 00:11:13 EST 2019


Ka-Ping Yee writes:
 > On Wed, Mar 6, 2019 at 4:01 PM Chris Angelico <rosuav at gmail.com> wrote:

 > > But adding dictionaries is fundamentally *useful*. It is expressive.
 > 
 > It is useful.  It's just that + is the wrong name.

First, let me say that I prefer ?!'s position here, so my bias is made
apparent.  I'm also aware that I have biases so I'm sympathetic to
those who take a different position.

Rather than say it's "wrong", let me instead point out that I think
it's pragmatically troublesome to use "+".  I can think of at least
four interpretations of "d1 + d2"

1.  update
2.  multiset (~= Collections.Counter addition)
3.  addition of functions into the same vector space (actually, a
    semigroup will do ;-), and this is the implementation of
    Collections.Counter
4.  "fiberwise" set addition (ie, of functions into relations)

and I'm very jet-lagged so I may be missing some.

There's also the fact that the operations denoted by "|" and "||" are
often implemented as "short-circuiting", and therefore not
commutative, while "+" usually is (and that's reinforced for
mathematicians who are trained to think of "+" as the operator for
Abelian groups, while "*" is a (possibly) non-commutative operator.  I
know commutativity of "+" has been mentioned before, but the
non-commutativity of "|" -- and so unsuitability for many kinds of
dict combination -- hasn't been emphasized before IIRC.

Since "|" (especially "|=") *is* suitable for "update", I think we
should reserve "+" for some future commutative extension.

In the spirit of full disclosure:
Of these, 2 is already implemented and widely used, so we don't need
to use dict.__add__ for that.  I've never seen 4 in the mathematical
literature (union of relations is not the same thing).  3, however, is
very common both for mappings with small domain and sparse
representation of mappings with a default value (possibly computed
then cached), and "|" is not suitable for expressing that sort of
addition (I'm willing to say it's "wrong" :-).

Steve


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