Reading and writing text files

Until now every program you wrote forgot everything the moment it ended. Files fix that. A program can write results to disk and read them back the next time it runs, or read data someone else prepared. This lesson covers plain text files: the simplest and most useful kind.

The examples read a small file called notes.txt, which sits in the same folder as the scripts. Its content is three lines:

Ada Lovelace wrote the first program.
Café au lait costs 3 euros.
Pi is roughly 3.14159.

Python looks for a file name like notes.txt in the current working directory, which is the folder your terminal is in when you run the script. If you get a FileNotFoundError, check where your terminal is first.

Open, read, close

The standard way to read a whole file is with open(...).

with open("notes.txt", encoding="utf-8") as file:
    text = file.read()

print(text)
print(len(text), "characters")
print(repr(text[:12]))

Ada Lovelace wrote the first program.
Café au lait costs 3 euros.
Pi is roughly 3.14159.

89 characters
'Ada Lovelace'

Three things are going on:

• open("notes.txt", encoding="utf-8") opens the file for reading, since reading is the default mode. It returns a file object.
• with ... as file: gives that object the name file and guarantees the file is closed when the indented block ends, even if an error happens inside. This is the job a finally block would do by hand, as the try and except lesson mentioned.
• file.read() returns the whole content as one string, including the newline character \n at the end of each line. That is why print(text) ends with a blank line: the text already ends in a newline and print adds another.

The encoding argument

A file stores bytes, and the encoding says how bytes map to characters. The Unicode and bytes lesson explained this. The é in Café takes two bytes in UTF-8, so reading it with the wrong encoding gives garbled text such as Café.

3.15+Requires version 3.15 or later UTF-8 is now the default for open() on every platform (PEP 686). Before 3.15 the default came from the operating system, and on many Windows machines it was not UTF-8, so the same program could read a file correctly on one computer and garble it on another. You can still switch the default off with PYTHONUTF8=0 or -X utf8=0.

Even so, keep writing encoding="utf-8". It costs nothing, your code then behaves the same on Python 3.14 and earlier, and the reader can see the intention. The examples in this course always include it.

Reading line by line

A file object is iterable: looping over it gives one line at a time. For big files, this is better than read(), because it does not load everything at once.

with open("notes.txt", encoding="utf-8") as file:
    for number, line in enumerate(file, start=1):
        print(number, line.rstrip("\n"))

1 Ada Lovelace wrote the first program.
2 Café au lait costs 3 euros.
3 Pi is roughly 3.14159.

enumerate(file, start=1) numbers the lines from 1, as in the for loops lesson. rstrip("\n") removes the newline from the end of each line, so print does not add a second one. Two related methods are worth knowing: file.readline() reads one line, and file.readlines() returns a list of all lines.

Writing and appending

The second argument of open is the mode:

import os

with open("shopping.txt", "w", encoding="utf-8") as file:
    file.write("milk\n")
    file.write("café crème\n")
    print("bread", file=file)

with open("shopping.txt", "a", encoding="utf-8") as file:
    file.write("eggs\n")

with open("shopping.txt", encoding="utf-8") as file:
    items = file.read().splitlines()

print(items)
print(len("café crème"), "characters,", len("café crème".encode("utf-8")), "bytes")
os.remove("shopping.txt")

['milk', 'café crème', 'bread', 'eggs']
10 characters, 12 bytes

file.write writes exactly the string you give it, so you must add \n yourself. Passing file=file to print is a neat alternative: print adds the newline for you. The second with block uses mode "a", so eggs goes at the end rather than replacing the content. splitlines() turns the text back into a list without the newline characters. The final lines show that the two accented letters each take two bytes in UTF-8, so ten characters occupy twelve bytes on disk. os.remove deletes the file again, so you can rerun the example. It comes from the os module, brought in with import as in the previous lesson.

When the file is missing

Opening a file that does not exist raises FileNotFoundError. It is an error you can expect, so it is a good fit for try and except.

def count_lines(filename):
    try:
        with open(filename, encoding="utf-8") as file:
            return len(file.readlines())
    except FileNotFoundError:
        print("Cannot find", filename)
        return 0


print(count_lines("notes.txt"))
print(count_lines("nothing_here.txt"))

3
Cannot find nothing_here.txt
0

The with sits inside the try: if open fails, there is no file to close, and the handler runs. A return inside with is fine, because the file still closes on the way out.

Common pitfalls

• Forgetting encoding="utf-8". On Python 3.14 and earlier this can corrupt accented letters on Windows. Always include it.
• Using "w" when you meant "a". It silently erases the file.
• Forgetting the newline. write("milk") followed by write("eggs") produces milkeggs on one line.
• Reading a file twice with one read(). After read() the position is at the end, so a second read() returns "". Open the file again, or loop once.
• Wrong working directory. FileNotFoundError for a file you can see usually means the terminal is in another folder.
• Leaving out with. A file you open by hand must be closed by hand, and an error in between leaves it open.

Lines: 3
Longest: Ada Lovelace wrote the first program.
Length: 37

with open("notes.txt", encoding="utf-8") as file:
    lines = [line.rstrip("\n") for line in file]

longest = max(lines, key=len)
print("Lines:", len(lines))
print("Longest:", longest)
print("Length:", len(longest))

Lines: 3
Longest: Ada Lovelace wrote the first program.
Length: 37

Total: 55

import os

with open("squares.txt", "w", encoding="utf-8") as file:
    for n in range(1, 6):
        file.write(f"{n * n}\n")

total = 0
with open("squares.txt", encoding="utf-8") as file:
    for line in file:
        total += int(line)

print("Total:", total)
os.remove("squares.txt")

Total: 55

with open("notes.txt", encoding="utf-8") as file:
    for line in file:
        print("first pass:", line.rstrip("\n"))
    for line in file:
        print("second pass:", line.rstrip("\n"))

with open("notes.txt", encoding="utf-8") as file:
    lines = file.read().splitlines()

for line in lines:
    print("first pass:", line)
for line in lines:
    print("second pass:", line)

Summary

• Use with open(name, encoding="utf-8") as file:. The block closes the file for you, even after an error.
• file.read() gives the whole text, looping over the file gives lines, and "w" and "a" write and append. write does not add a newline.
• 3.15+Requires version 3.15 or later UTF-8 is the default encoding for open(), but write encoding="utf-8" anyway so your code behaves the same on older versions.
• A missing file raises FileNotFoundError, which you can catch with try and except.

Next: debugging habits.