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 |.
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.
—Ping