Write data to files

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Writing to a file lets your program save results, keep logs, or persist data between runs. Python offers two main modes for putting data into a file: write and append.

Writing to a file with "w"

Opening a file in write mode ("w") creates a new file—or completely overwrites any existing file with the same name. Use the .write() method to add text:

with open("greeting.txt", "w") as file:
    file.write("Hello, world!\n")
    file.write("Welcome to Python.\n")

Warning

Write mode erases the file the moment it's opened. Only use "w" when you're sure you want to replace the file's contents.

Notice the \n at the end of each string. Unlike print(), .write() doesn't add a newline for you.

Appending to a file with "a"

Opening a file in append mode ("a") adds to the end of the file, keeping anything that's already there. If the file doesn't exist yet, Python creates it.

with open("log.txt", "a") as file:
    file.write("New entry added.\n")

Append mode is a good fit for log files, notes, and any data you want to build up over time.

Writing multiple lines

To write a list of strings at once, use .writelines(). Just remember that it doesn't add newlines—you need to include them yourself:

tasks = ["Buy groceries\n", "Walk the dog\n", "Finish homework\n"]

with open("tasks.txt", "w") as file:
    file.writelines(tasks)

Choosing between write and append

Use "w" when...	Use "a" when...
You want to start fresh each time	You want to keep the existing content
You're saving the final result of a process	You're recording new events or entries
Overwriting is intentional	Preserving history matters