Python re.split() vs split()

Question

In my quests of optimization, I discovered that that built-in split() method is about 40% faster that the re.split() equivalent.

A dummy benchmark (easily copy-pasteable):

import re, time, random 

def random_string(_len):
    letters = "ABC"
    return "".join([letters[random.randint(0,len(letters)-1)] for i in range(_len) ])

r = random_string(2000000)
pattern = re.compile(r"A")

start = time.time()
pattern.split(r)
print "with re.split : ", time.time() - start

start = time.time()
r.split("A")
print "with built-in split : ", time.time() - start

Why this difference?

Answer 1

re.split is expected to be slower, as the usage of regular expressions incurs some overhead.

Of course if you are splitting on a constant string, there is no point in using re.split().

Answer 2

When in doubt, check the source code. You can see that Python s.split() is optimized for whitespace and inlined. But s.split() is for fixed delimiters only.

For the speed tradeoff, a re.split regular expression based split is far more flexible.

>>> re.split(':+',"One:two::t h r e e:::fourth field")
['One', 'two', 't h r e e', 'fourth field']
>>> "One:two::t h r e e:::fourth field".split(':')
['One', 'two', '', 't h r e e', '', '', 'fourth field']
# would require an addition step to find the empty fields...
>>> re.split('[:\d]+',"One:two:2:t h r e e:3::fourth field")
['One', 'two', 't h r e e', 'fourth field']
# try that without a regex split in an understandable way...

That re.split() is only 29% slower (or that s.split() is only 40% faster) is what should be amazing.