How to read a file line-by-line into a list?

This code will read the entire file into memory:

with open(filename) as file:
    lines = file.readlines()

If you want to remove all whitespace characters (newlines and spaces) from the end of each line, use this instead:

with open(filename) as file:
    lines = [line.rstrip() for line in file]

(This avoids allocating an extra list from file.readlines().)

If you’re working with a large file, then you should instead read and process it line-by-line:

with open(filename) as file:
    for line in file:
        print(line.rstrip())

In Python 3.8 and up you can use a while loop with the walrus operator like so:

with open(filename) as file:
    while line := file.readline():
        print(line.rstrip())

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