How to Write and Append to Files in Python

Reading data is useful, but most programs also need to save something: a diary entry, a high score, a list of orders. In this lesson, you'll learn how to write to a file in Python, how to add to a file without destroying what's already there, and the small details (like newlines) that trip up almost every beginner.

How do you write to a file in Python?

Writing uses the same open() function you met in How to Read Files in Python, but with a writing mode as the second argument. Then you call the file object's write() method with a string:

file = open("diary.txt", "w", encoding="utf-8")
file.write("Dear diary,\n")
file.write("Today I learned how to write files in Python.\n")
file.close()

file = open("diary.txt", encoding="utf-8")
print(file.read())
file.close()

Dear diary,
Today I learned how to write files in Python.

If diary.txt doesn't exist yet, mode "w" creates it. After the program runs, you can open the file in any text editor and see the two lines.

The write() method also returns the number of characters it wrote. You usually ignore it, but it's handy to know:

file = open("greeting.txt", "w", encoding="utf-8")
count = file.write("Hello, world!\n")
file.close()

print(count)

14

Writing modes at a glance

Mode "w" overwrites the whole file

This is the most important rule in this lesson. Opening a file with "w" empties it immediately, even before you write anything. Run this after the diary example above:

file = open("diary.txt", "w", encoding="utf-8")
file.write("Oops, the old entry is gone.\n")
file.close()

file = open("diary.txt", encoding="utf-8")
print(file.read())
file.close()

Oops, the old entry is gone.

How do you append to a file in Python?

Mode "a" (append) keeps the existing content and adds new text at the end. Here a coffee shop records orders as they come in:

file = open("orders.txt", "a", encoding="utf-8")
file.write("Latte, $4.25\n")
file.close()

file = open("orders.txt", "a", encoding="utf-8")
file.write("Croissant, $3.10\n")
file.close()

file = open("orders.txt", encoding="utf-8")
print(file.read())
file.close()

Latte, $4.25
Croissant, $3.10

Run the program a second time and you'll see four lines, because each run appends two more. That's exactly what you want for a log or a history.

Why doesn't write() add a new line?

Unlike print(), write() writes exactly the characters you give it. If you forget \n, everything ends up on one line:

file = open("todo.txt", "w", encoding="utf-8")
file.write("Buy milk")
file.write("Call mom")
file.close()

file = open("todo.txt", encoding="utf-8")
print(file.read())
file.close()

Buy milkCall mom

Add \n to the end of each line ("Buy milk\n") and the lines stay separate. You learned about \n in escape characters.

write() vs writelines()

writelines() takes a list (or any sequence) of strings and writes them one after another. Despite its name, it does not add newlines either:

playlist = ["Morning Coffee", "Road Trip", "Rainy Day"]

file = open("playlist.txt", "w", encoding="utf-8")
file.writelines(playlist)
file.close()

file = open("playlist.txt", encoding="utf-8")
print(file.read())
file.close()

Morning CoffeeRoad TripRainy Day

Add the newlines with a list comprehension first:

playlist = ["Morning Coffee", "Road Trip", "Rainy Day"]

file = open("playlist.txt", "w", encoding="utf-8")
file.writelines([song + "\n" for song in playlist])
file.close()

file = open("playlist.txt", encoding="utf-8")
print(file.read())
file.close()

Morning Coffee
Road Trip
Rainy Day

Mode "x": create a file only if it's new

Mode "x" is a safety net. It creates a new file, but refuses to touch one that already exists:

file = open("report.txt", "x", encoding="utf-8")
file.write("Monthly report\n")
file.close()

file = open("report.txt", "x", encoding="utf-8")

FileExistsError: [Errno 17] File exists: 'report.txt'

The first open() works; the second raises FileExistsError instead of wiping the report. You can catch it with try and except and ask the user for a different name.

How do you write numbers to a file?

write() only accepts strings. Passing a number is an error:

file = open("score.txt", "w", encoding="utf-8")
file.write(42)

TypeError: write() argument must be str, not int

Convert it first with str(42), or use an f-string such as f"{score}\n". When you read it back later, remember to turn it into a number again with int().

Real-world example: logging orders

An online store wants a permanent record of every order. A small function appends one line per order, and a second part of the program reads the log to calculate the day's sales:

def log_order(item, price):
    file = open("order_log.txt", "a", encoding="utf-8")
    file.write(f"{item},{price:.2f}\n")
    file.close()

log_order("Cappuccino", 3.8)
log_order("Blueberry muffin", 2.95)
log_order("Iced tea", 3.25)

total = 0
file = open("order_log.txt", encoding="utf-8")
for line in file:
    item, price = line.strip().split(",")
    print(f"{item:<18} ${price}")
    total += float(price)
file.close()

print(f"{'Total':<18} ${total:.2f}")

Cappuccino         $3.80
Blueberry muffin   $2.95
Iced tea           $3.25
Total              $10.00

Because the log uses "a", orders from yesterday stay in the file and new orders are simply added below them.

Common mistakes when writing files

1. Using "w" when you meant "a"

Your log only ever contains the last entry. Every open(..., "w") wiped the previous one. Use "a" for anything that should grow over time.

2. Forgetting \n

All your lines run together. Remember that neither write() nor writelines() adds line breaks.

3. Forgetting to close the file

Python collects written text in memory and saves it to disk in batches. If you never call close(), some of your text may not reach the file yet. The next lesson shows how the with statement closes files for you.

4. Writing numbers or lists directly

write() needs a string. Convert values with str() or an f-string first. For whole lists and dictionaries, formats like JSON are a better fit.

Practice exercise

items = ["Eggs", "Bread", "Apples"]

file = open("shopping.txt", "w", encoding="utf-8")
for item in items:
    file.write(item + "\n")
file.close()

file = open("shopping.txt", "a", encoding="utf-8")
file.write("Milk\n")
file.close()

file = open("shopping.txt", encoding="utf-8")
number = 1
for line in file:
    print(f"{number}. {line.strip()}")
    number += 1
file.close()

1. Eggs
2. Bread
3. Apples
4. Milk

What's next?

Every example so far had to remember close(). Next, learn the cleaner, safer way to open files that closes them automatically: the Python with statement.