Extract all words that begin with x

[Bryan]

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.

--
--Bryan