[Python-ideas] PEP: Dict addition and subtraction

Ka-Ping Yee zestyping at gmail.com
Thu Mar 7 12:36:46 EST 2019

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

> On Thu, Mar 7, 2019 at 10:52 AM Josh Rosenberg
> <shadowranger+pythonideas at gmail.com> wrote:
> >
> > Allowing dicts to get involved in + means:
> Lots of words that basically say: Stuff wouldn't be perfectly pure.
> But adding dictionaries is fundamentally *useful*. It is expressive.

It is useful.  It's just that + is the wrong name.

Filtering and subtracting from dictionaries are also useful!  Those are
operations we do all the time.  It would be useful if & and - did these
things too—and if we have & and -, it's going to be even more obvious that
the merge operator should have been |.

Josh Rosenberg <shadowranger+pythonideas at gmail.com> wrote:

> 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".

If we were inventing Python from scratch, we could have decided that we
always use "+" to combine collections.  Sets would combine with + and then
it would make sense that dictionaries also combine with + .

But that is not Python.  Lists combine with + and sets combine with |.
Why?  Because lists add (put both collections together and keep
everything), but sets merge (put both collections together and keep some).

So, Python already has a merge operator.  The merge operator is "|".

For lists, += is shorthand for list.extend().
For sets, |= is shorthand for set.update().

Is dictionary merge more like extend() or more like update()?  Python
already took a position on that when it was decided to name the dictionary
method update().  That ship sailed a long time ago.

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