How to Return a File From a Function in Python? – Be on the Right Side of Change

Do you need to create a function that returns a file but you don’t know how? No worries, in sixty seconds, you’ll know! Go! 👇

A Python function can return any object such as a file object. To return a file, first open the file object within the function body, handle all possible errors if the file doesn’t exist, and return it to the caller of the function using the keyword operation return open(filename, mode='r').

Here’s a minimal example that tries to open a filename that was provided by the user via the input() function. If it fails, it prints an error message and asks for a different user input:

def open_file():
    while True:
        filename = input('filename: ')
        try:
            return open(filename, mode='r')
        except:
            print('Error. Try again')


f = open_file()
print(f.read())

If I type in the correct file right away, I get the following output when storing the previous code snippet in a file named code.py—the code reads itself (meta 🤯):

filename: code.py
def open_file():
    while True:
        filename = input('filename: ')
        try:
            return open(filename, mode='r')
        except:
            print('Error. Try again')


f = open_file()
print(f.read())

Note that you can open the file in writing mode rather than reading mode by replacing the line with the return statement with the following line:

open(filename, mode='w')

A more Pythonic way, in my opinion, is to follow the single-responsibility pattern whereby a function should do only one thing. In that case, provide the relevant input values into the function like so:

def open_file(filename, mode):
        try:
            return open(filename, mode=mode)
        except:
            return None


def ask_user():
    f = open_file(input('filename: '), input('mode: '))
    while not f:
        f = open_file(input('filename: '), input('mode: '))
    return f


f = ask_user()

print(f.read())

Notice how the file handling of a single instance and the user input processing are separated into two functions. Each function does one thing only. Unix style.

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