Python Files and Error Handling: Read, Write and Never Crash

A program that crashes the moment a user types "abc" instead of a number, or when a file is missing, is not finished. This lesson covers two skills that make your code dependable: working with files and handling errors.

Writing and reading a text file

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("DBMS: revise normalization\n")
    f.write("OS: revise deadlocks\n")

with open("notes.txt", "r", encoding="utf-8") as f:
    for line in f:
        print(line.strip())

• "w" writes (and replaces any existing content), "a" appends, "r" reads.
• with closes the file automatically, even if an error happens inside the block. Always use it.
• encoding="utf-8" avoids problems with special characters like ₹ or Hindi text, especially on Windows.

Reading a CSV file

import csv

with open("marks.csv", newline="", encoding="utf-8") as f:
    reader = csv.DictReader(f)          # first row = column names
    for row in reader:
        print(row["name"], row["marks"])

DictReader gives each row as a dictionary — exactly the list-of-dictionaries pattern from the previous lesson. (Later, in data analytics, you will use pandas for bigger files.)

Handling errors with try / except

while True:
    try:
        age = int(input("Enter your age: "))
        break
    except ValueError:
        print("Please enter a number, like 21.")

If int() fails, Python jumps to the except block instead of crashing, and the loop asks again.

Catch specific errors

try:
    with open("marks.csv", encoding="utf-8") as f:
        data = f.read()
except FileNotFoundError:
    print("marks.csv not found — check the file name and folder.")
except PermissionError:
    print("No permission to read marks.csv.")
else:
    print("Loaded", len(data), "characters")
finally:
    print("Done")

• else runs only if no error happened.
• finally always runs — useful for clean-up.

Common exceptions to know

Raising your own errors

def set_cgpa(value):
    if not 0 <= value <= 10:
        raise ValueError("CGPA must be between 0 and 10")
    return value

Raising a clear error early is far better than letting a wrong value travel through your program and cause a confusing problem later.