On Wed, Mar 6, 2019 at 4:01 PM Chris Angelico <rosuav@gmail.com> wrote:
On Thu, Mar 7, 2019 at 10:52 AM Josh Rosenberg
<shadowranger+pythonideas@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@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.