Raising exceptions, custom exceptions and assertions · CAB106

raise creates an exception on purpose: raise ValueError("bad"). Only objects that derive from BaseException can be raised, so raise "error" fails with TypeError. Inside a handler a bare raise re-raises the current exception, and raise NewError(...) from e chains errors, storing the original in __cause__.

A custom exception is a class that derives from Exception (or a more specific class such as ValueError). It can add attributes:

class AgeError(ValueError):
    def __init__(self, age):
        super().__init__(f"bad age {age}")
        self.age = age

It is caught by its own class or by any parent class. print(e) shows the message while type(e).__name__ shows the class name.

assert condition, message raises AssertionError when the condition is false. Assertions are for checking internal assumptions during development and can be disabled with python -O, so they must not be used to validate user input or handle expected failures such as a missing file.

Exam trap: the message given to assert or to an exception is what print(e) shows, not the exception's class name.

Try two questions

Question 1 · Tier 3, Apply

What is the proper use of an assert statement?

To replace try and except when a file cannot be opened
To validate every password that a user types into a form
To make a slow function run faster in production
To check internal assumptions while developing, not to validate user input

Show the answer

D. Assertions document assumptions and can be disabled with the -O option.

Why not A: File errors should be handled with exceptions, not assertions.

Why not B: Assertions can be switched off, so they are unsuitable for input checks.

Why not C: Assertions do not change speed in a helpful way.

Question 2 · Tier 2, Understand

Which class should a custom exception normally derive from?

The object class
The str class
The Exception class
The list class

Show the answer

C. User-defined exceptions inherit from Exception, directly or through another exception class.

Why not A: object does not make a class usable as an exception.

Why not B: str is a text type, not an exception type.

Why not D: list is a container type, not an exception type.

Key points

Try two questions