Exception Handling#

Python provides a robust mechanism for handling runtime errors (exceptions) so that programs can recover, clean up resources, or fail gracefully instead of crashing.

Introduction to Exceptions#

Exceptions are errors that occur during execution and interrupt normal program flow.

Common types of exceptions:

  • TypeError – Operation applied to an inappropriate type.

    x = "10" + 5  # TypeError

  • ValueError – Correct type, but inappropriate value.

    int("hello")  # ValueError

  • IndexError – Index out of range in a list or sequence.

    [1, 2, 3][5]  # IndexError

  • KeyError – Dictionary key not found.

    {"name": "Alex"}["age"]  # KeyError

  • FileNotFoundError – Trying to open a non‑existent file.

    open("missing.txt")  # FileNotFoundError

Why handle exceptions?#

  • Prevent crashes and display user‑friendly error messages.

  • Allow fallback or recovery for expected errors (e.g. network failure, missing input).

  • Clean up resources (files, network connections, database connections) reliably.

Common practices for exception handling:

  1. Catch specific exceptions first Handle each known error type separately to provide tailored recovery or messaging.

  2. Group related exceptions If multiple exceptions share the same handling logic, catch them together:

    except (FileNotFoundError, PermissionError) as e:
    log_error(e)
    print("Cannot access file.")

  3. Use a general fallback After all specific handlers, add except Exception as e: to catch any unexpected errors. Typically log or re‑raise this exception so developers notice:

    except Exception as e:
    log_traceback(e)
    raise

  4. Use `else` for success logic Code in else runs only if no exception occurred, keeping success and error branches separate.

The try…except Structure#

Basic syntax:

try:
    # risky code
except SomeException:
    # handler code

Catch a specific exception:

try:
    result = 10 / 0
except ZeroDivisionError:
    print("You can't divide by zero!")

Multiple except blocks:

try:
    n = int(input("Enter a number: "))
    result = 100 / n
except ValueError:
    print("That's not a number.")
except ZeroDivisionError:
    print("Can't divide by zero.")
except Exception as e:
    print("Unexpected error:", e)

Else Block#

Code inside else runs only if no exception occurred:

try:
    x = int("123")
except ValueError:
    print("Conversion failed.")
else:
    print("Conversion successful:", x)

Finally Block#

Code inside finally runs regardless of what happened—exception, return, or normal exit. Useful for cleanup:

try:
    conn = open_connection()
except ConnectionError:
    print("Cannot connect.")
finally:
    conn.close()

Raising Exceptions with raise#

You can deliberately throw exceptions:

def set_age(age: int):
    if age < 0:
        raise ValueError("Age cannot be negative.")
    print("Age set to", age)

set_age(-1)

Chaining exceptions:

try:
    1 / 0
except ZeroDivisionError as e:
    raise RuntimeError("Custom error message") from e

Additional Resources#