Reading and Writing Files

Why files?

Variables disappear when your program ends. To keep data – notes, scores, settings – save it in a file.

Writing a file

with open("notes.txt", "w") as file:
    file.write("Buy milk\n")
    file.write("Call Asha\n")
print("Saved!")

Output

Saved!

• open("notes.txt", "w") opens the file for writing. If it doesn't exist it's created; if it does, it's emptied first.
• with automatically closes the file when the block ends – even if there's an error.
• write() doesn't add a new line for you, so we add \n ourselves.

Reading a file

with open("notes.txt", "w") as file:
    file.write("Buy milk\nCall Asha\n")

with open("notes.txt") as file:
    content = file.read()
print(content)

Output

Buy milk
Call Asha

Or go through it line by line – strip() removes the new-line character at the end of each line:

with open("notes.txt", "w") as file:
    file.write("Buy milk\nCall Asha\n")

with open("notes.txt") as file:
    for number, line in enumerate(file, start=1):
        print(number, line.strip())

Output

1 Buy milk
2 Call Asha

Adding to a file

Mode "a" (append) adds to the end instead of wiping the file:

with open("log.txt", "w") as file:
    file.write("Day 1: started Python\n")

with open("log.txt", "a") as file:
    file.write("Day 2: learned loops\n")

with open("log.txt") as file:
    print(file.read())

Output

Day 1: started Python
Day 2: learned loops

When the file is missing

try:
    with open("missing.txt") as file:
        print(file.read())
except FileNotFoundError:
    print("That file doesn't exist yet.")

Output

That file doesn't exist yet.

Remember: Always use with open(...). It closes the file for you, so your data is saved properly.