[PEAK] Peak rules example's simplification

[P.J. Eby]

At 06:13 AM 2/27/2009 +0300, Alexander Artemenko wrote:
>Hi All,
>
>I found, that  example at http://pypi.python.org/pypi/PEAK-Rules/ page
>can be simplified by omiting one part of the predicate in
>pprint_long_list:
>
># Skip Skip
> >>> @when(pprint, (list,))
>... def pprint_list(lst):
>...     print "pretty-printing a list"
>
> >>> @when(pprint_list, "len(lst)>50") # Change 'pprint' to 
> 'pprint_list' here and remove type check
>... def pprint_long_list(lst):
>...     print "pretty-printing a long list"
>
># Skip Skip
>
>Is it feature or bug?
>Which are the best practices in function extention, using Peak.Rules?

Your version is still correct, but potentially much slower, since it 
does dispatching *twice*...  once to invoke pprint_list, and then 
once to select whether to run the original pprint_list or 
pprint_long_list.  This is slower because the dispatch algorithm runs 
more than once, and because the dispatcher can't optimize complex 
conditions that are split between the two functions.

The only benefit to the way you're doing it here is that if 
pprint_list is ever used independently, it will still be modified by 
pprint_long_list.  But I'm not sure that even that is really a "benefit".