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Write this accumuator in a functional style

On 7/11/17, Steven D'Aprano <steve at pearwood.info> wrote:
> I have a colleague who is allergic to mutating data structures. Yeah, I
> know, he needs to just HTFU but I thought I'd humour him.
> Suppose I have an iterator that yields named tuples:
> Parrot(colour='blue', species='Norwegian', status='tired and shagged out')
> and I want to collect them by colour:
> accumulator = {'blue': [], 'green': [], 'red': []}
> for parrot in parrots:
>     accumulator[parrot.colour].append(parrot)
> That's pretty compact and understandable, but it require mutating a bunch
> of pre-allocated lists inside an accumulator. Can we re-write this in a
> functional style?
> The obvious answer is "put it inside a function, then pretend it works by
> magic" but my colleague's reply to that is "Yes, but I'll know that its
> actually doing mutation inside the function".
> Help me humour my colleague.

This seems to be philosophical question to me:

How to (or "what colleague could accept" or "what do you mean by")
collect elements if not adding them to lists?