Custom exceptions
- Python course
- Beginner
- Lesson 104/116
- 5 min
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 bittiHere 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.
Practice 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).
Starter code
# derive StokError from Exception
# raise it and catch with exceptSample solution
class StokError(Exception):
pass
try:
raise StokError("stok bitti")
except StokError as e:
print(e)Expected output
stok bitti
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the custom exception definition
class StokError(____):
passclass StokError(Exception):
passExplanation: Custom exceptions derive from Exception.
2 Which class do custom exceptions derive from?
- BaseException
- Exception Correct answer
- object
- Error
Explanation: Custom exceptions derive from Exception; so except Exception catches them.
3 Match custom exceptions
| Term | Meaning |
|---|---|
class StokError(Exception) |
Custom exception |
pass |
Empty body |
raise StokError |
Raising |
except StokError as e |
Catching |