Count lines in a file in Python

To count the lines in a file, loop over the file and add 1 for each line. This reads one line at a time, so it also works for very large files.

The program

with open("story.txt", "w", encoding="utf-8") as f:
    f.write("Margaret wrote the code.\n")
    f.write("The code went to the Moon.\n")
    f.write("The landing was a success.\n")

with open("story.txt", encoding="utf-8") as f:
    line_count = sum(1 for line in f)

print(f"Lines: {line_count}")

Lines: 3

How it works

• sum(1 for line in f) gives 1 for every line in the file and adds them up. It is the same as a loop with count += 1.
• The common version, len(f.readlines()), gives the same answer, but readlines() first builds a list of every line in memory. For a small file that is fine; for a file of several gigabytes it is not.
• A last line without a \n at the end still counts as a line.

Lines, words and characters

The Unix command wc prints these three numbers. Here is the same thing in Python:

with open("story.txt", "w", encoding="utf-8") as f:
    f.write("Margaret wrote the code.\n")
    f.write("The code went to the Moon.\n")
    f.write("The landing was a success.\n")

lines = words = chars = 0
with open("story.txt", encoding="utf-8") as f:
    for line in f:
        lines += 1
        words += len(line.split())
        chars += len(line)

print(f"Lines: {lines}, words: {words}, characters: {chars}")

Lines: 3, words: 15, characters: 79

line.split() with no argument splits on any run of spaces and ignores the \n, so it gives the words of the line. The character count includes the three \n characters.