Message191896
Problem
=======
Code:
sum([[1,2,3]]*1000000, [])
takes forever to complete.
Suggestion
==========
Patch sum() function so that it did not created 1000000 copies of result, but created just one. Attached patch does that.
Before patch:
$ ./python -mtimeit --setup="x=[[1,2,3]]*10000" "sum(x,[])"
10 loops, best of 3: 915 msec per loop
After patch:
$ ./python -mtimeit --setup="x=[[1,2,3]]*10000" "sum(x,[])"
1000 loops, best of 3: 469 usec per loop
200000% boost! :)
Details
=======
Built-in sum function could look like this:
def sum(seq, start = 0):
for item in seq:
start += item
return start
But that would be bad, becaust in cases like:
empty = []
result = sum(list_of_lists, empty)
content of "empty" would be modified.
So instead it looks like this:
def sum(seq, start = 0):
for item in seq:
start = start + item
return start
it creates a copy of the entire partial result on EVERY "start + item".
While instead it could look like this:
def sum(seq, start = 0):
start = start + seq[0:1]
for item in seq[1:]:
start += item
return start
create just ONE copy, and use it.
That's what attached patch is doing.
An alternative is something like this:
def sum(seq, start = 0):
start = copy.copy(start)
for item in seq:
start += item
return start
But I'm not sure how to make a copy of arbitrary object yet. |
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Date |
User |
Action |
Args |
2013-06-26 06:33:59 | Sergey | set | recipients:
+ Sergey |
2013-06-26 06:33:59 | Sergey | set | messageid: <1372228439.5.0.789865081817.issue18305@psf.upfronthosting.co.za> |
2013-06-26 06:33:59 | Sergey | link | issue18305 messages |
2013-06-26 06:33:58 | Sergey | create | |
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