Reading specific lines only

Question

I\'m using a for loop to read a file, but I only want to read specific lines, say line #26 and #30. Is there any built-in feature to achieve this?

Thanks

Answer 1

For the sake of completeness, here is one more option.

Let's start with a definition from python docs:

slice An object usually containing a portion of a sequence. A slice is created using the subscript notation, [] with colons between numbers when several are given, such as in variable_name[1:3:5]. The bracket (subscript) notation uses slice objects internally (or in older versions, __getslice__() and __setslice__()).

Though the slice notation is not directly applicable to iterators in general, the itertools package contains a replacement function:

from itertools import islice

# print the 100th line
with open('the_file') as lines:
    for line in islice(lines, 99, 100):
        print line

# print each third line until 100
with open('the_file') as lines:
    for line in islice(lines, 0, 100, 3):
        print line

The additional advantage of the function is that it does not read the iterator until the end. So you can do more complex things:

with open('the_file') as lines:
    # print the first 100 lines
    for line in islice(lines, 100):
        print line

    # then skip the next 5
    for line in islice(lines, 5):
        pass

    # print the rest
    for line in lines:
        print line

And to answer the original question:

# how to read lines #26 and #30
In [365]: list(islice(xrange(1,100), 25, 30, 4))
Out[365]: [26, 30]