open in function using with

If we open the file in the recommended way using the with statement then we can be sure that the close method of the fh object will be called when we leave the context of the with statement.

import sys
import re

def do_something(filename):
    with open(filename) as fh:

        while True:
            line = fh.readline()
            if line is None:
                break
            line = line.rstrip("\n")

            if re.search(r'\A\s*\Z', line):
                return
            print(line)


def main():
    if len(sys.argv) != 2:
        exit(f"Usage: {sys.argv[0]} FILENAME")
    filename = sys.argv[1]
    do_something(filename)

main()