appending to a list via properties

[Larry Bates]

Lonnie Princehouse wrote:
> Here's a curious hack I want to put up for discussion.  I'm thinking of
> writing a PEP for it.
> 
> Observation
> -----------------
> I found myself using this construct for assembling multiple lists:
> 
>     foo = []
>     qux = []
> 
>     while some_condition:
>        a, b = calculate_something()
>        foo.append(a)
>        qux.append(b)
> 
> Not elegant!  It requires temporary variables a and b, which are only
> used to populate the lists.
> 
> 
> Suggestion
> ----------------
> 
>     class better_list (list):
>         tail = property(None, list.append)
> 
>     foo = better_list()
>     qux = better_list()
> 
>     while some_condition:
>         foo.tail, qux.tail = calculate_something()
> 
> Granted, the name tail might not be the best, but you get the idea.
> 
> 
> Alternatives
> ------------------
> 
>     1. Use "append" instead, preserving original list.append behavior.
> 
>         class better_list (list):
>             append = property(lambda l: lambda x: list.append(l,x),
> list.append)
> 
>     2. Use an external wrapper, similar to operator.*getter
> 
>         class propertize (object):
>             def __init__(self, target):
>                 self.__target__ = target
>             def __setattr__(self, attribute, value):
>                 if attribute.startswith('_'): return
> object.__setattr__(self, attribute, value)
>                 else: getattr(self.__target__, attribute)(value)
> 
>         propertize(foo).append, propertize(qux).append =
> calculate_something()
> 
>    
> Well?
> 
Not entirely sure I understand, but here goes.  I normally
use list comprehensions.  I'm assuming that calculate_something()
is acting on a list of items.  If not then maybe some underlying
data refactoring is in order.  If so you write:

results=[calculate_something(x) for x in list_of_items]
foo=result[x[0] for x in results]
qux=result[x[1] for x in results]

Without knowing more about what calculate_something() does
and what condition terminates the loop it is hard to say if
this will work for you.

-Larry Bates