Reading and writing files

2 min read 6 runnable examples Quiz included

Files let a program keep data after it finishes. Python's built-in open() reads and writes them.

Writing a file

Python

with open("notes.txt", "w") as f:
    f.write("Buy milk\n")
    f.write("Call Mei\n")
    f.write("Finish the Python course\n")
print("Saved.")

"w" opens for writing, creating the file or replacing its contents.
write() doesn't add line breaks; include \n yourself.
with closes the file automatically (more on that in the next lesson).

Reading a file

Python

with open("notes.txt", "w") as f:
    f.write("Buy milk\nCall Mei\nFinish the Python course\n")

with open("notes.txt") as f:      # "r" (read) is the default
    content = f.read()
print(content)

Reading line by line

Looping over a file gives one line at a time, which works even for huge files:

Python

with open("notes.txt", "w") as f:
    f.write("Buy milk\nCall Mei\nFinish the Python course\n")

with open("notes.txt") as f:
    for number, line in enumerate(f, start=1):
        print(number, line.strip())     # strip() removes the trailing \n

f.readlines() returns all lines as a list.

File modes

Mode	Means
`"r"`	read (the default); error if the file doesn't exist
`"w"`	write; creates the file or overwrites it
`"a"`	append; adds to the end
`"x"`	create; error if the file already exists

Python

with open("log.txt", "w") as f:
    f.write("Started\n")
with open("log.txt", "a") as f:
    f.write("Step 1 done\n")
    f.write("Step 2 done\n")
with open("log.txt") as f:
    print(f.read())

A missing file

Python

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

CSV files

The csv module reads and writes comma-separated data, the format spreadsheets export:

Python

import csv

rows = [["name", "score"], ["Asha", 92], ["Leo", 78], ["Mei", 85]]
with open("scores.csv", "w", newline="") as f:
    csv.writer(f).writerows(rows)

with open("scores.csv", newline="") as f:
    for row in csv.DictReader(f):
        print(f"{row['name']} scored {row['score']}")

DictReader uses the first line as column names, giving you each row as a dictionary.

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