How to close a file after reading it in a function

I’ve got a function that reads a text file and saves all the words into a variable. Someone told me that I should close the file after reading it but I’m not sure how to do it especially when I’m using a function.

import re

def read_file(filename):
    with open(filename, "r", encoding="utf-8") as file:
        lines = file.readlines()
        words = []
        for line in lines:
            words += re.findall(r'w+', line.lower())
    return words


The file is closed due to the with statement. There is 2 ways to use files:

f = open(...)
# do stuff

Or, preferred and as you have done:

with open(...) as f:
  # do stuff
# file is closed once the do stuff block ends

The code above works because of 2 magic functions (magic functions are functions used by the compiler like __str__ that have special meaning)

__enter__ and __exit__. These are used by the with block like this:

with some as f:
  # stuff

Means the same thing as:

f = some.__enter__()
# stuff