How to Read and Write Files in Python

Matthew

1. Opening Files: The Basics

Python uses the built-in open() function to work with files.

file = open("example.txt", "r")  # "r" = read mode

Common modes:

ModeMeaning
"r"Read (default)
"w"Write (overwrite)
"a"Append
"x"Create (error if exists)
"b"Binary mode (e.g. images)

Don’t forget to close the file when done:

file.close()

But a better way is to use with blocks, which auto-close the file:

with open("example.txt", "r") as file:
    content = file.read()

2. Reading From a File

Read the entire file:

with open("example.txt", "r") as file:
    content = file.read()
    print(content)

Read line-by-line:

with open("example.txt", "r") as file:
    for line in file:
        print(line.strip())  # .strip() removes newlines

Read lines into a list:

with open("example.txt", "r") as file:
    lines = file.readlines()

3. Writing to a File

Overwrite mode ("w"): 

with open("output.txt", "w") as file:
    file.write("Hello, world!\n")
    file.write("This is Python.\n")

Note: "w" mode erases existing content.

Append mode ("a"):

with open("output.txt", "a") as file:
    file.write("Adding another line.\n")

4. Working with Paths

If your files are in different directories, use os.path or pathlib:

from pathlib import Path

file_path = Path("data") / "output.txt"

with file_path.open("w") as file:
    file.write("Organized files are good files.")

5. Checking If a File Exists

from pathlib import Path

file_path = Path("data.txt")
if file_path.exists():
    print("File found.")
else:
    print("File not found.")

6. Read & Write Structured Files (Bonus)

JSON

import json

# Write
data = {"name": "Matthew", "score": 100}
with open("data.json", "w") as file:
    json.dump(data, file)

# Read
with open("data.json", "r") as file:
    loaded = json.load(file)

CSV

import csv

# Write
with open("scores.csv", "w", newline='') as file:
    writer = csv.writer(file)
    writer.writerow(["name", "score"])
    writer.writerow(["Matthew", 100])

# Read
with open("scores.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)

7. Common Pitfalls to Avoid

❌ Forgetting to close files (use with blocks!)

❌ Assuming a file exists (check with Path.exists())

❌ Writing in "w" mode when you meant to append

❌ Mixing binary and text modes (e.g., using "rb" vs "r")

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