How to read specific lines from a file (by line number)?

python

prompt-engineering

best-practices

memory-optimization

byAlex Kataev·Feb 20, 2025

To read lines 2 and 4 from my_file.txt, use a list to specify line numbers and enumerate within a loop to filter them:

lines_to_read = [2, 4]  # only these lines please!
result = [line.strip() for i, line in enumerate(open('my_file.txt'), 1) if i in lines_to_read]
print(result)  # voila! we have got the lines!

Here, enumerate keeps track of the line numbers, the condition if i in lines_to_read ensures we read only the required lines, and line.strip() removes all that undesired leading whitespace.

Reading lines smarter, not harder: optimum approach for large files

For larger files, we want to minimize both memory usage and run-time. A for loop along with enumerate plays this out by stopping the loop as soon as we read all targeted lines:

lines_to_read = set([2, 4])  # only these and nothing more!
with open('large_file.txt') as file:  # let's dive into the ocean!
    result = []  # an empty container for our precious lines
    for i, line in enumerate(file, 1):  # here we go, counting lines...
        if i in lines_to_read:  # bingo! found one of the lines!
            result.append(line.strip())  # "bag 'em and tag 'em," as gamers would say
        elif i > max(lines_to_read):  # nothing more we need, time to break the loop
            break
    print(result)  # tasty lines served!

The if-elif logic ensures that unnecessary lines are skipped over, making the reading quicker and more memory-efficient.

Taking control: reading and processing specific lines dynamically

Need to process lines as they come or wish to select lines based on complex criteria? Generator expressions offer a compact and memory-efficient solution:

lines_to_read = [2, 4]
with open('file.txt') as file:
    lines = (line.strip() for i, line in enumerate(file, 1) if i in lines_to_read)  # generator, please stand by
    for line in lines:  # iterate through the generator
        print(line)  # make it speak!

This code yields one line at a time, perfect for processing lines on-the-fly rather than collecting all at once.

When you need a hand: using external libraries for efficiency

Handling multiple files or frequently accessing different lines? linecache can be your best friend, fetching any line from any file with minimal code:

import linecache

line_number = 3
desired_line = linecache.getline('file.txt', line_number).strip()
print(desired_line)  # jackpot! here's your line

With linecache, remember it counts lines starting from 1, just like us humans.

Range reading: when you need more than one

Need to read a range of lines? Use islice from itertools. It's like slicing a pizza, but with files:

from itertools import islice

start_line = 2
end_line = 4
with open('file.txt') as file:
    selected_lines = islice(file, start_line - 1, end_line)  # and sliceeeee...
    for line in selected_lines:
        print(line.strip())  # sweet lines, hot out of the oven!

Using islice, you can extract lines without reading the entire file into memory — an efficient 'read-once' approach.

Memory or speed: knowing when to opt for what

Deciding between simplicity and efficiency can often depend on the size of your file and nature of your task.

with open('small_file.txt') as file:
    all_lines = file.readlines()  # read it all in one go.
    print(all_lines[1], all_lines[3])  # print exactly what we need.