Steven Bethard <[EMAIL PROTECTED]> wrote:
> map = {}
> for key, value in sequence:
> map.setdefault(key, []).append(value)
I was thinking about exactly that the other day, when converting some
perl to python.
[ digression: In perl, you do
push @{$map->{$key}}, $value
If $map->{$key} doesn't exist is is autovivified into an array (since
its in array context). Now thats exactly the sort of magic that gives
perl a bad name ;-) ]
However, one thing I noticed is that a list is created and destroyed
as the second parameter to setdefault whether or not it is used in
map, which strikes me as a bit wasteful. You obviously can't use the
same list there either. If it was an object with a more expensive
constructor then you'd notice it more.
It would be nice if setdefault didn't evaluate the second argument
unless it needed it. However I guess it would have to be a language
feature to do that.
Here are some timings
$ /usr/lib/python2.3/timeit.py -s 'sequence=zip(range(1000),range(1000))' 'map
= {}
for key, value in sequence:
map.setdefault(key, []).append(value)'
1000 loops, best of 3: 1.42e+03 usec per loop
$ /usr/lib/python2.3/timeit.py -s 'sequence=zip(range(1000),[ i%11 for i in
range(1000)])' 'map = {}
for key, value in sequence:
map.setdefault(key, []).append(value)'
1000 loops, best of 3: 1.57e+03 usec per loop
$ /usr/lib/python2.3/timeit.py -s 'sequence=zip(range(1000),range(1000))' 'map
= {}
for key, value in sequence:
if map.has_key(key):
map[key].append(value)
else:
map[key] = [ value ]'
1000 loops, best of 3: 1.1e+03 usec per loop
$ /usr/lib/python2.3/timeit.py -s 'sequence=zip(range(1000),[ i%11 for i in
range(1000)])' 'map = {}
for key, value in sequence:
if map.has_key(key):
map[key].append(value)
else:
map[key] = [ value ]'
1000 loops, best of 3: 1.11e+03 usec per loop
Not that timing is everything of course ;-)
--
Nick Craig-Wood <[EMAIL PROTECTED]> -- http://www.craig-wood.com/nick
--
http://mail.python.org/mailman/listinfo/python-list
