File Input Output in Python

Welcome to the File Input Output in Python page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

def write_file():
    try:
        with open("output.txt", "w") as out:
            out.write("Hi! I'm a line of text in this file!\n")
            out.write("Me, too!\n")
    except OSError as e:
        print(f"Cannot open file: {e}")
        return


def read_file():
    try:
        with open("output.txt", "r") as in_file:
            for line in in_file:
                print(line.strip())
    except OSError as e:
        print(f"Cannot open file to read: {e}")
        return


if __name__ == '__main__':
    write_file()
    read_file()

File Input Output in Python was written by:

• Ganesh Naik
• Jeremy Grifski
• Noah Nichols

This article was written by:

• Jeremy Grifski
• rzuckerm

If you see anything you'd like to change or update, please consider contributing.

How to Implement the Solution

We'll first present the solution in its entirety. Then, we'll go over the code line by line.

Afterward, we'll take a look at how to run the solution.

Writing to a File

As you may have noticed above, our file writing procedure has been broken out into its own function:

def write_file():
    try:
        with open("output.txt", "w") as out:
            out.write("Hi! I'm a line of text in this file!\n")
            out.write("Me, too!\n")
    except OSError as e:
        print(f"Cannot open file: {e}")
        return

First, we setup up a try/except block to catch any exceptions that might occur when we want to open a file:

try:
    with open("output.txt", "w") as out:
        ...
except OSError as e:
    print("Cannot open file: {}", e)

The Python documentation tells us if open() fails to create a new file, it will raise an OSError exception. If we do get an exception, we will exit the method.

Next, if no exceptions occurred, we can now write to the file using the write() method:

out.write("Hi! I'm a line of text in this file!\n")
out.write("Me, too!\n")

As we can see, we attempt to output a couple of lines to the text file. The with statement sets up a context that will automatically close the file when the context is exited.

Reading From a File

After we write to a file, we can read back from that file. Naturally, we've broken the reading procedure into its own function:

    try:
        with open("output.txt", "r") as in_file:
            for line in in_file:
                print(line.strip())
    except OSError as e:
        print(f"Cannot open file to read: {e}")
        return

Just like when we were opening the file for writing purposes, we surround the code that could potentially raise exceptions in a try/except block:

try:
    with open("output.txt", "r") as in_file:
        ...
except OSError as e:
    print(f"Cannot open file to read: {e}")
    return

If an exception occurs, we report the error and exit the function.

Next, we have a while loop that iterates over each line in the file:

for line in in_file:
    print(line.strip())

As we can see, the loop performs some basic processing before we print the line out onto the screen. When we get a line from the file, we also get the newline. If we print it with the newline we print an extra empty line because print automatically adds a newline by default. To fix this problem we use strip() to strip away any newlines at the end of the line. This loop will end when we reach EOF (end of file).

The with statement sets up a context that will automatically close the file when the context is exited.

The Main Function

All of this code would be useless if we didn't execute it somewhere. Thankfully, we can drop everything into the main function:

if __name__ == '__main__':
    write_file()
    read_file()

And, that's it! We've conquered File IO in Python.

How to Run the Solution

As usual, you're free to use an online Python interpreter such as the one on Repl and run the solution there. Alternatively, if you have a Python interpreter installed on your machine, you can use the following command:

python file-input-output.py

After you execute this command, you should be able to find a nearby output.txt file containing the arbitrary text we used earlier. If so, you've successfully run the program.