How to read file N lines at a time?

Question

I need to read a big file by reading at most N lines at a time, until EOF. What is the most effective way of doing it in Python? Something like:

with open(filename, 'r') as infile:
    while not EOF:
        lines = [get next N lines]
        process(lines)

Answer 1

One solution would be a list comprehension and the slice operator:

with open(filename, 'r') as infile:
    lines = [line for line in infile][:N]

After this lines is tuple of lines. However, this would load the complete file into memory. If you don't want this (i.e. if the file could be really large) there is another solution using a generator expression and islice from the itertools package:

from itertools import islice
with open(filename, 'r') as infile:
    lines_gen = islice(infile, N)

lines_gen is a generator object, that gives you each line of the file and can be used in a loop like this:

for line in lines_gen:
    print line

Both solutions give you up to N lines (or fewer, if the file doesn't have that much).

Answer 2

A file object is an iterator over lines in Python. To iterate over the file N lines at a time, you could use grouper() function in the Itertools Recipes section of the documenation. (Also see What is the most “pythonic” way to iterate over a list in chunks?):

try:
   from itertools import izip_longest
except ImportError:  # Python 3
    from itertools import zip_longest as izip_longest

def grouper(iterable, n, fillvalue=None):
    args = [iter(iterable)] * n
    return izip_longest(*args, fillvalue=fillvalue)

Example

with open(filename) as f:
     for lines in grouper(f, N, ''):
         assert len(lines) == N
         # process N lines here

Answer 3

This code will work with any count of lines in file and any N. If you have 1100 lines in file and N = 200, you will get 5 times to process chunks of 200 lines and one time with 100 lines.

with open(filename, 'r') as infile:
    lines = []
    for line in infile:
        lines.append(line)
        if len(lines) >= N:
            process(lines)
            lines = []
    if len(lines) > 0:
        process(lines)