invert dictionary with list &c

[Des Small]

Des Small <des.small at bristol.ac.uk> writes:

> anton muhin <antonmuhin.REMOVE.ME.FOR.REAL.MAIL at rambler.ru> writes:
[...]

> This, however, has given me ideas.  It was never concision I wanted
> but rather redundancy elimination, and the pattern I wanted _can_ be
> written:
> 
> def dict_cons(iter, func, default):      
>     def process_element(d, (k, v)):
>         val = d.get(k, default)
>         d.update(dict([[k, func(v, val)]]))
>         return d
>     return reduce(process_element, iter, {})

Or rather:

def dict_cons(iter, func, default):
    def process_element(d, (k, v)):
        d[k] = func(v, d.get(k, default))
        return d
    return reduce(process_element, iter, {})

def count(l):
    def pair_pad(l): return [(e, ()) for e in  l]
    return dict_cons(pair_pad(l), lambda k,d: d+1, 0) 

def invert(d):
    def invertitems(l):
        for k,v in l: yield v,k
    def addtolist(k, l): return l+[k]
    return dict_cons(invertitems(d.iteritems()),
                     addtolist, [])

> Which is not to say that it should be, of course.  
> Whereupon, we can say:
> 
> def count(l):
>     def pair_pad(l): return [(e, ()) for e in  l]
>     return dict_cons(pair_pad(l), lambda k,d: d+1, 0) 
> 
> def invert(d):
>     def invertitems(l): for k,v in l: yield v,k
>     def addtolist(k, l): return l+[k]
>     return dict_cons(invertitems(d.iteritems()),
>                      addtolist, [])
> 
> Perhaps I'm terminally unpythonic, but I quite like these.

[...]

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