Custom exceptions

Learn to write custom exceptions with class MyError(Exception).

Custom exception class

To define your own error you subclass Exception:

class StokError(Exception):
    pass

raise StokError("stok bitti")

Here a new error type called StokError is created. The pass body lets it inherit all of Exception's behaviour. Now you can raise it with raise StokError(...) and catch it with except StokError.

This lets you clearly report domain-specific errors.

Raising and catching custom exceptions

You raise your custom exception with raise and catch it with except — just like built-in errors:

class StokError(Exception):
    pass

try:
    raise StokError("stok bitti")
except StokError as e:
    print(e)  # stok bitti

Here as e gives you access to the error message. Custom exceptions let calling code distinguish errors by type.

In large projects each module defines its own exception class.

Exercise Define and catch StokError

Define class StokError(Exception): with pass in the body. Inside a try: block write raise StokError("stok bitti"). Inside an except StokError as e: block write print(e).

Show example solution

class StokError(Exception):
    pass

try:
    raise StokError("stok bitti")
except StokError as e:
    print(e)

Test yourself

Complete the custom exception definition

class StokError(____):
    pass

Show answer

class StokError(Exception):
    pass

Which class do custom exceptions derive from?
- BaseException
- Exception
- object
- Error
Show answer

Exception — Custom exceptions derive from Exception; so except Exception catches them.
Match custom exceptions

class StokError(Exception)

Custom exception
pass

Empty body
raise StokError

Raising
except StokError as e

Catching

Next lesson: Summary - Error handling

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