startswith( prefix[, start[, end]]) Query

Duncan Booth duncan.booth at invalid.invalid
Fri Sep 7 10:07:25 CEST 2007


"Tim Williams" <listserver at tdw.net> wrote:

> Isn't slicing still faster than startswith?    As you mention timeit,
> then you should probably add slicing to the pot too :)
> 

Possibly, but there are so many other factors that affect the timing 
that writing it clearly should be your first choice.

Some timings:

@echo off
setlocal
cd \python25\lib
echo "startswith"
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra2'" s.startswith(t)
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra1'" s.startswith(t)
echo "prebound startswith"
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra2';startswith=s.startswith" startswith(t)
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra1';startswith=s.startswith" startswith(t)
echo "slice with len"
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra2'" s[:len(t)]==t
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra1'" s[:len(t)]==t
echo "slice with magic number"
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra2'" s[:12]==t
..\python timeit.py -s "s='abracadabra1'*1000;t='abracadabra1'" s[:12]==t

and typical output from this is:

"startswith"
1000000 loops, best of 3: 0.542 usec per loop
1000000 loops, best of 3: 0.514 usec per loop
"prebound startswith"
1000000 loops, best of 3: 0.472 usec per loop
1000000 loops, best of 3: 0.474 usec per loop
"slice with len"
1000000 loops, best of 3: 0.501 usec per loop
1000000 loops, best of 3: 0.456 usec per loop
"slice with magic number"
1000000 loops, best of 3: 0.34 usec per loop
1000000 loops, best of 3: 0.315 usec per loop

So for these particular strings, the naive slice wins if the comparison is 
true, but loses to the pre-bound method if the comparison fails. The slice is 
taking a hit from calling len every time, so pre-calculating the length
(which should be possible in the same situations as pre-binding startswith)
might be worthwhile, but I would still favour using startswith unless I knew
the code was time critical.



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