Write this accumuator in a functional style

[Wolfgang Maier]

On 07/11/2017 08:11 AM, Steven D'Aprano 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.
> 
> 
> 

Hmm, isn't this just asking for itertools.groupby on the parrots sorted 
by colour?