Python File Handling - Read, Write, Append Files Tutorial

Working with Files

Python makes file I/O straightforward with the built-in open() function. Always use the with statement - it automatically closes the file even if an error occurs.

File Modes

Mode	Description
`'r'`	Read (default) - error if file doesn't exist
`'w'`	Write - creates file or overwrites existing
`'a'`	Append - adds to end of file
`'x'`	Create - error if file already exists
`'r+'`	Read and write
`'b'`	Binary mode (e.g., `'rb'`, `'wb'`)
`'t'`	Text mode (default)

Reading Files

# Read entire file as a string
with open("data.txt", "r") as f:
    content = f.read()
    print(content)

# Read line by line (memory-efficient for large files)
with open("data.txt", "r") as f:
    for line in f:
        print(line.strip())   # strip() removes trailing newline

# Read all lines into a list
with open("data.txt", "r") as f:
    lines = f.readlines()
    print(lines)   # ['line1\n', 'line2\n', ...]

# Read one line at a time
with open("data.txt", "r") as f:
    first_line = f.readline()
    second_line = f.readline()

# Specify encoding (important for non-ASCII text)
with open("data.txt", "r", encoding="utf-8") as f:
    content = f.read()

Writing Files

# Write (overwrites existing content)
with open("output.txt", "w") as f:
    f.write("Hello, World!\n")
    f.write("Second line\n")

# Write multiple lines at once
lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open("output.txt", "w") as f:
    f.writelines(lines)

# Append to existing file
with open("log.txt", "a") as f:
    f.write("New log entry\n")

# Write with print() - convenient for formatted output
with open("report.txt", "w") as f:
    print("Report Title", file=f)
    print(f"Total: {42}", file=f)

Working with JSON Files

JSON Files

import json

# Write JSON to file
data = {
    "name": "Alice",
    "age": 25,
    "hobbies": ["coding", "reading"]
}

with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

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

print(loaded["name"])     # Alice
print(loaded["hobbies"])  # ['coding', 'reading']

# Write list of records
users = [
    {"id": 1, "name": "Alice"},
    {"id": 2, "name": "Bob"},
]
with open("users.json", "w") as f:
    json.dump(users, f, indent=2)

Working with CSV Files

CSV Files

import csv

# Write CSV
students = [
    ["Name", "Age", "Grade"],
    ["Alice", 20, "A"],
    ["Bob", 22, "B"],
    ["Charlie", 21, "A"],
]

with open("students.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(students)

# Read CSV
with open("students.csv", "r") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)

# DictReader - rows as dicts
with open("students.csv", "r") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(f"{row['Name']}: {row['Grade']}")

# DictWriter - write from dicts
with open("output.csv", "w", newline="") as f:
    fieldnames = ["name", "score"]
    writer = csv.DictWriter(f, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerow({"name": "Alice", "score": 95})
    writer.writerow({"name": "Bob", "score": 87})

File System Operations with pathlib

pathlib

from pathlib import Path

# Create path objects
p = Path("data/output.txt")
home = Path.home()
cwd = Path.cwd()

# Path operations
print(p.name)       # output.txt
print(p.stem)       # output
print(p.suffix)     # .txt
print(p.parent)     # data

# Check existence
print(p.exists())
print(p.is_file())
print(p.is_dir())

# Create directories
Path("new_folder/sub").mkdir(parents=True, exist_ok=True)

# Read and write (modern way)
p = Path("hello.txt")
p.write_text("Hello, World!")
content = p.read_text()
print(content)

# List files in directory
for f in Path(".").iterdir():
    print(f)

# Find all Python files recursively
for py_file in Path(".").rglob("*.py"):
    print(py_file)

# Delete file
p.unlink(missing_ok=True)