Read first N lines of a file in python

Question

We have a large raw data file that we would like to trim to a specified size. I am experienced in .net c#, however would like to do this in python to simplify things and out of interest.

How would I go about getting the first N lines of a text file in python? Will the OS being used have any effect on the implementation?

Thanks :)

Answer 1

with open("datafile") as myfile:
    head=[myfile.next() for x in xrange(N)]
print head

Here's another way

from itertools import islice
with open("datafile") as myfile:
    head=list(islice(myfile,N))
print head

Answer 2

There is no specific method to read number of lines exposed by file object.

I guess the easiest way would be following:

lines =[]
with open(file_name) as f:
    lines.extend(f.readline() for i in xrange(N))

Answer 3

If you want something that obviously (without looking up esoteric stuff in manuals) works without imports and try/except and works on a fair range of Python 2.x versions (2.2 to 2.6):

def headn(file_name, n):
    """Like *x head -N command"""
    result = []
    nlines = 0
    assert n >= 1
    for line in open(file_name):
        result.append(line)
        nlines += 1
        if nlines >= n:
            break
    return result

if __name__ == "__main__":
    import sys
    rval = headn(sys.argv[1], int(sys.argv[2]))
    print rval
    print len(rval)

Answer 4

N=10
f=open("file")
for i in range(N):
    line=f.next().strip()
    print line
f.close()