File Handling

Python makes reading and writing files straightforward. Whether you need to read configuration files, process logs, or store program output, Python's built-in file handling has you covered.

Opening Files

Use the built-in open() function. Always pair it with a with statement to ensure the file is closed automatically:

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

The second argument is the mode:

Mode	Meaning
r	Read (default)
w	Write (creates or overwrites)
a	Append (adds to end)
x	Create (fails if exists)
b	Binary mode (combine: rb, wb)

Reading Files

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

# Read all lines into a list
with open("data.txt", "r") as f:
    lines = f.readlines()   # includes newline characters

# 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

Writing Files

# Write mode — creates or overwrites the file
with open("output.txt", "w") as f:
    f.write("Hello, file!\n")
    f.write("Second line\n")

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

Working with File Paths

import os

# Check if a file exists
if os.path.exists("data.txt"):
    print("File exists")

# Get the file size
size = os.path.getsize("data.txt")
print(f"Size: {size} bytes")

# Join paths safely (works on Windows and Unix)
path = os.path.join("data", "reports", "summary.txt")

Reading and Writing CSV-style Data

For simple structured data, you can parse it manually:

# Write CSV-style data
with open("students.csv", "w") as f:
    f.write("name,age,grade\n")
    f.write("Alice,20,A\n")
    f.write("Bob,22,B\n")

# Read and parse it
with open("students.csv", "r") as f:
    header = f.readline().strip().split(",")
    for line in f:
        values = line.strip().split(",")
        row = dict(zip(header, values))
        print(row)
# {'name': 'Alice', 'age': '20', 'grade': 'A'}

Handling File Errors

try:
    with open("missing.txt", "r") as f:
        content = f.read()
except FileNotFoundError:
    print("File not found!")
except PermissionError:
    print("Permission denied!")

The with statement is the recommended pattern for all file operations — it guarantees the file is closed even if an exception occurs. This prevents file descriptor leaks in long-running programs.