Python Lesson 16: File Handling

Programs often need to save data to files or read data from files. Python makes file handling simple and safe with the with statement.

Writing to a File

# "w" = write mode (creates file, overwrites if exists)
with open("notes.txt", "w") as file:
    file.write("Hello from Python!\n")
    file.write("Second line\n")

# File is automatically closed after the with block
# This is the safest way to work with files!

Reading from a File

# "r" = read mode (file must exist)
with open("notes.txt", "r") as file:
    content = file.read()    # read entire file
    print(content)

# Read line by line
with open("notes.txt", "r") as file:
    for line in file:
        print(line.strip())  # strip() removes newline \n

# Read all lines into a list
with open("notes.txt", "r") as file:
    lines = file.readlines()
print(lines)

Appending to a File

# "a" = append mode (adds to end, does not overwrite)
with open("notes.txt", "a") as file:
    file.write("This line is added at the end\n")

# Now the file has 3 lines

Handling File Errors

# What if the file does not exist?
try:
    with open("missing.txt", "r") as f:
        content = f.read()
except FileNotFoundError:
    print("File not found!")
    content = ""

# Check if file exists before opening
import os
if os.path.exists("notes.txt"):
    with open("notes.txt") as f:
        print(f.read())

Working with CSV Data

# Save a list of students to a file
students = [("Alice", 95), ("Bob", 87), ("Charlie", 92)]

with open("students.csv", "w") as f:
    f.write("Name,Score\n")
    for name, score in students:
        f.write(f"{name},{score}\n")

# Read it back
with open("students.csv", "r") as f:
    for line in f:
        print(line.strip())