📁 Topic 09 · Robustness
Files & Exceptions
Read and write files safely, handle errors gracefully with try/except, and create custom exceptions for cleaner, production-ready code.
⏱ ~45 min
🟡 Intermediate
🛡️ Reliability
Reading & Writing Files
Always use the with statement — it guarantees the file is closed even if an error occurs mid-operation.
files.py
# Write to a file
with open("data.txt", "w") as f:
f.write("Hello, file!\n")
f.write("Second line\n")
# Read entire file at once
with open("data.txt", "r") as f:
content = f.read()
# Read line by line (memory-efficient)
with open("data.txt") as f:
for line in f:
print(line.strip())
📌 File modes "r" read (default) · "w" write (overwrites) · "a" append · "b" binary · "x" exclusive create (fails if exists)
try / except / finally
Wrap risky code in a try block and handle specific exceptions so the program stays running.
exceptions.py
try:
num = int(input("Enter number: "))
result = 100 / num
print(f"Result: {result}")
except ValueError:
print("That wasn't a valid number!")
except ZeroDivisionError:
print("Cannot divide by zero!")
except Exception as e:
print(f"Unexpected error: {e}")
else:
print("Success — no exceptions!")
finally:
print("This always runs — great for cleanup")
Custom Exceptions
Inherit from Exception to create domain-specific errors with meaningful names.
custom_exc.py
class InsufficientFundsError(Exception):
def __init__(self, amount, balance):
super().__init__(f"Need {amount}, have {balance}")
def withdraw(balance, amount):
if amount > balance:
raise InsufficientFundsError(amount, balance)
return balance - amount
try:
withdraw(50, 100)
except InsufficientFundsError as e:
print(e) # Need 100, have 50
Working with JSON
Python's built-in json module makes reading and writing structured data trivial.
json_files.py
import json
data = {"name": "Alice", "scores": [95, 87, 92]}
# Write JSON
with open("data.json", "w") as f:
json.dump(data, f, indent=2)
# Read JSON
with open("data.json") as f:
loaded = json.load(f)
print(loaded["name"]) # Alice
