How to count lines in a text file ?

[Christos TZOTZIOY Georgiou]

On Mon, 20 Sep 2004 15:29:18 +0200, rumours say that aleaxit at yahoo.com
(Alex Martelli) might have written:

>Ling Lee <janimal at mail.trillegaarden.dk> wrote:

>> Oh I just did it.
>> 
>> Just used the line:
>> 
>> print "%d lines in your choosen file" % len(open("test.txt").readlines())
>> 
>> Thanks though :)

[Alex]
>You're welcome;-).  However, this approach reads all of the file into
>memory at once.  If you must be able to deal with humungoug files, too
>big to fit in memory at once, try something like:
>
>numlines = 0
>for line in open('text.txt'): numlines += 1

And a short story of premature optimisation follows...

Saw the plain code above and instantly the programmer's instinct of
optimisation came into action... we all know that C loops are faster
than python loops, right?  So I spent 2 minutes of my time to write the
following 'clever' function:

def count_lines(filename):
    fp = open(filename)
    count = 1 + max(enumerate(fp))[0]
    fp.close()
    return count

Proud of my programming skills, I timed it against another function
containing Alex' code.  Guess what?  My code was slower... (and I should
put a try: except Value: clause to cater for empty files)

Of course, on second thought, the reason must be that enumerate
generates one tuple for every line in the file; in any case, I'll mark
this rule:

C loops are *always* faster than python loops, unless the loop does
something useful ;-) in the latter case, timeit.py is your friend.
-- 
TZOTZIOY, I speak England very best,
"Tssss!" --Brad Pitt as Achilles in unprecedented Ancient Greek