On 5/11/2010 6:01 PM, Bryan wrote:
Tycho Andersen wrote:
Terry Reedy wrote:
... word[0:1] does the same thing. All Python programmers should learn to
use slicing to extract a char from a string that might be empty.
The method call of .startswith() will be slower, I am sure.
Why? Isn't slicing just sugar for a method call?
Yes, but finding the method doesn't require looking it up by name at
run-time, and startswith is built to work for startings of any length.
Let's timeit:
# -----
from timeit import Timer
from random import choice
from string import ascii_lowercase as letters
strs = [''.join([choice(letters) for _ in range(5)])
for _ in range(5000)]
way1 = "[s for s in strs if s.startswith('a')]"
way2 = "[s for s in strs if s[:1] == 'a']"
assert eval(way1) == eval(way2)
for way in [way1, way2]:
t = Timer(way, 'from __main__ import strs')
print(way, ' took: ', t.timeit(1000))
# -----
On my particular box, I get:
[s for s in strs if s.startswith('a')] took: 5.43566498797
[s for s in strs if s[:1] == 'a'] took: 3.20704924968
So Terry Reedy was right: startswith() is slower. I would,
nevertheless, use startswith(). Later, if users want my program to run
faster and my profiling shows a lot of the run-time is spent finding
words that start with 'a', I might switch.
Thank you for that timing report.
My main point is that there are two ways to fetch a char, the difference
being the error return -- exception IndexError versus error value ''.
This is an example of out-of-band versus in-band error/exception
signaling, which programmers, especially of Python, should understand.
The fact that in Python syntax tends to be faster than calls was
secondary, though good to know on occasion.
.startswith and .endswith are methods that wrap the special cases of
slice at an end and compare to one value. There are not necessary, and
save no keystrokes, but Guido obviously thought they added enough to
more than balance the slight expansion of the language. They were added
after I learned Python and I thought the tradeoff to be a toss-up, but I
will consider using the methods when writing didactic code meant to be
read by others.
Terry Jan Reedy
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