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