Exceptions and File Handling in Python — try/except and open()

Real programs face bad input, missing files and network problems. Instead of crashing, a good program handles these errors. This lesson also covers reading and writing files, where errors are common.

Part A — Exceptions

def safe_divide(a, b):
    try:
        result = a / b
    except ZeroDivisionError:
        return "Cannot divide by zero"
    else:
        return result
    finally:
        print("checked", a, b)

print(safe_divide(10, 2))
print(safe_divide(5, 0))

checked 10 2
5.0
checked 5 0
Cannot divide by zero

• try — the code that might fail
• except SomeError — runs if that error happens
• else — runs if no error happened
• finally — always runs (good for cleanup)

Common exceptions

Handling bad input

# Input: abc
# Input: 21
while True:
    try:
        age = int(input())
        break
    except ValueError:
        print("Please enter a number")
print("Age:", age)

Please enter a number
Age: 21

Raising your own errors

def set_marks(m):
    if not 0 <= m <= 100:
        raise ValueError("marks must be between 0 and 100")
    return m

try:
    set_marks(120)
except ValueError as e:
    print("Error:", e)

Error: marks must be between 0 and 100

Catch specific exceptions. A bare except: hides real bugs.

Part B — Files

Always open files with with — it closes the file automatically, even if an error happens.

Write and read a text file

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("Learn Python\n")
    f.write("Practise daily\n")

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

Learn Python
Practise daily

CSV files

import csv

rows = [["name", "marks"], ["Asha", 92], ["Ravi", 74]]
with open("marks.csv", "w", newline="", encoding="utf-8") as f:
    csv.writer(f).writerows(rows)

with open("marks.csv", encoding="utf-8") as f:
    for row in csv.DictReader(f):
        print(row["name"], int(row["marks"]) + 5)

Asha 97
Ravi 79

JSON files

import json

data = {"name": "Asha", "skills": ["python", "sql"]}
with open("profile.json", "w", encoding="utf-8") as f:
    json.dump(data, f)

with open("profile.json", encoding="utf-8") as f:
    loaded = json.load(f)
print(loaded["skills"][1])

sql

Common mistakes

• Opening with "w" when you meant "a" — it wipes the file.
• Forgetting encoding="utf-8" and getting errors with ₹ or Hindi text on Windows.
• Catching every exception with a bare except:.

Quick recap

• try / except / else / finally; catch specific exceptions; raise your own.
• with open(path, mode, encoding="utf-8") as f: — always.
• "r" read, "w" overwrite, "a" append.
• csv and json modules handle structured files.