Catching Exceptions with try…except

The try...except block lets you intercept an exception before it crashes your program. Code that might fail goes in try; your recovery plan goes in except.

try…except…else…finally — exception handling flow in Python

python

try:
    num = int(input("Enter a number: "))
    result = 100 / num
    print(f"100 ÷ {num} = {result}")
except ZeroDivisionError:
    print("❌ Can't divide by zero!")
except ValueError:
    print("❌ That's not a valid number.")

Output (if user enters 0)

Enter a number: 0
❌ Can't divide by zero!

Output (if user enters "abc")

Enter a number: abc
❌ That's not a valid number.

Catching Multiple Exceptions in One Block

If you want to handle several exception types the same way, group them in a tuple:

python

try:
    data = [10, 20, 30]
    index = int(input("Pick an index: "))
    print(f"Value: {data[index]}")
except (IndexError, ValueError):
    print("⚠️ Invalid input — enter a number between 0 and 2.")

Output (if user enters 9)

Pick an index: 9
⚠️ Invalid input — enter a number between 0 and 2.

Accessing the Exception Object

Use as e to grab the exception and inspect its message:

python

try:
    with open("config.json") as f:
        content = f.read()
except FileNotFoundError as e:
    print(f"File error: {e}")
except PermissionError as e:
    print(f"Permission denied: {e}")

Output

File error: [Errno 2] No such file or directory: 'config.json'

The else Clause

Code inside else runs only when the try block succeeds without raising any exception. It keeps your success logic cleanly separated from your error logic:

python

try:
    score = int(input("Enter your score (0–100): "))
except ValueError:
    print("Please enter a number.")
else:
    # This only runs if int() succeeded
    grade = "Pass" if score >= 50 else "Fail"
    print(f"Your result: {grade}")

Output (score: 72)

Enter your score (0–100): 72
Your result: Pass

The finally Clause

finally runs no matter what — whether the try block succeeded, raised an error, or even returned. It's the perfect place for cleanup code like closing files or database connections:

python

def read_config(path):
    f = None
    try:
        f = open(path)
        return f.read()
    except FileNotFoundError:
        print(f"'{path}' not found — using defaults.")
        return "{}"
    finally:
        if f:
            f.close()          # always runs, even after return
        print("Cleanup done.")

read_config("settings.json")

Output

'settings.json' not found — using defaults.
Cleanup done.

Full try…except…else…finally Structure

python

try:
    # risky code here
    result = 10 / int(input("Divisor: "))
except ZeroDivisionError:
    print("Can't divide by zero.")
except ValueError:
    print("That wasn't a number.")
else:
    print(f"Result: {result:.2f}")   # runs only on success
finally:
    print("--- calculation attempt complete ---")  # always runs

Output (divisor: 4)

Divisor: 4
Result: 2.50
--- calculation attempt complete ---

Raising Exceptions Manually

You can trigger an exception yourself with raise — useful for enforcing rules and validating inputs in your own functions:

python

def set_age(age):
    if not isinstance(age, int):
        raise TypeError(f"Age must be an integer, got {type(age).__name__}")
    if age < 0 or age > 150:
        raise ValueError(f"Age {age} is not a realistic value.")
    print(f"Age set to {age}")

set_age(25)        # ✅ works fine
set_age(-5)        # ❌ raises ValueError

Output

Age set to 25

Error

ValueError: Age -5 is not a realistic value.

💡Best practice: Always catch the most specific exception you can. Avoid bare except: blocks — they silently swallow every error including keyboard interrupts, making bugs very hard to track down.

Chaining Exceptions with raise from

When you catch one exception and raise a different one, use raise ... from ... to preserve the original cause — this keeps your traceback informative:

python

def load_user_data(user_id):
    try:
        records = {1: "Alice", 2: "Bob"}
        return records[user_id]
    except KeyError as e:
        raise ValueError(f"User ID {user_id} not found.") from e

try:
    load_user_data(99)
except ValueError as e:
    print(f"Error: {e}")

Output

Error: User ID 99 not found.

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