Python Exceptions

1. What is an Exception?

An exception is an error raised when something invalid happens in your program.

int("hello")   # → ValueError: invalid literal for int() with base 10: 'hello'

Common exception types:

When an exception is not handled, the program crashes immediately.

2. Try / Except Block

Wrap code that might fail in a try block. If an exception occurs, the except block runs instead of crashing.

try:
    int("hello")       # this raises a ValueError
except:
    print("Exception occurred!")

print("Done")          # still runs

• If no exception occurs → except block is skipped
• If an exception occurs → except block runs, then program continues

3. Catching Specific Exceptions

Accept only a specific type of exception and store it in a variable.

try:
    int("hello")
except ValueError as e:
    print("Value error:", e)

Multiple except blocks

try:
    result = 1 / 0
except ValueError as e:
    print("Value exception:", e)
except ZeroDivisionError as e:
    print("Zero division exception:", e)

• Only the first matching except block runs
• If the exception type doesn't match any block, the program still crashes

Catch any exception

try:
    int("hello")
except Exception as e:
    print("Any exception:", e)

⚠️ Avoid catching a general Exception unless necessary — it makes bugs harder to find. Prefer catching specific exception types.

4. Finally Block

The finally block always runs, regardless of whether an exception occurred or not. Used for cleanup operations.

try:
    int("hello")
except ValueError as e:
    print("Error:", e)
finally:
    print("This always runs")   # runs no matter what

Use case: Restore state, close files, release resources — anything that must happen whether the try succeeds or fails.

5. Raising Your Own Exceptions

Use the raise keyword to throw an exception manually.

raise ValueError("This is not a valid number!")
raise Exception("Something went wrong.")
raise IndexError("Index out of range.")

Practical example — validate user input

num = input("Enter a number: ")
if not num.isdigit():
    raise ValueError("This is not a valid number!")

6. Practical Example — Loop Until Valid Input

Use try/except inside a while loop to keep asking until valid input is given.

while True:
    num = input("Enter a number: ")
    try:
        num = float(num)
        break                         # valid float → exit loop
    except ValueError:
        print("Not a valid float, try again!")

7. Compile Time vs Runtime Errors

# Compile time error — code won't even run
while True    # SyntaxError: missing colon

# Runtime error — only fails on bad input
float(input("Enter a number: "))   # crashes if user types "hello"

8. Recommended Conventions

• Only wrap code in try/except when you know an exception might occur
• Catch specific exceptions rather than a general Exception
• Use finally for cleanup that must always happen
• Use raise to give helpful error messages when your code is used incorrectly

Cheat Sheet

# Basic try/except
try:
    # risky code
except:
    # runs if any exception occurs

# Catch specific exception
try:
    ...
except ValueError as e:
    print(e)

# Multiple exceptions
try:
    ...
except ValueError as e:
    ...
except ZeroDivisionError as e:
    ...

# Catch any exception
try:
    ...
except Exception as e:
    print(e)

# Finally (always runs)
try:
    ...
except ValueError:
    ...
finally:
    # cleanup — always runs

# Raise an exception
raise ValueError("Invalid input!")
raise Exception("Something went wrong.")