User-defined Exceptions
You can also define your own exception classes. To do this, use the mighty power of OOP – you should inherit from the Exception class.
class StupidInputError(Exception):
""" Raised when a user enters a stupid input """
pass
You can proceed to use this Exception, for example like this (not the best example of EAFP, I know!)
try:
user_input = input("Don't enter 'python': ")
if user_input.lower() == "python":
raise StupidInputError
except StupidInputError:
print("I told you not to enter 'python'. That's taboo!")
It is good practice to place all your custom exception classes into a separate file e.g. exceptions.py.
Handling subclasses
Python will match an Exception to the corresponding except clause if it is the same class or a parent/ancestor class.
The example below should explain this clearer.
class SuperClass(Exception):
pass
class SubClass(SuperClass):
pass
class SubSubClass(SubClass):
pass
for cls in [SuperClass, SubClass, SubSubClass]:
try:
raise cls()
except SubSubClass:
print("SubSubClass")
except SubClass:
print("SubClass")
except SuperClass:
print("SuperClass")
## SuperClass
## SubClass
## SubSubClass
What happens when we reverse the order of the except clauses? Try to guess, and then check!
for cls in [SuperClass, SubClass, SubSubClass]:
try:
raise cls()
except SuperClass:
print("SuperClass")
except SubClass:
print("SubClass")
except SubSubClass:
print("SubSubClass")
Tasks:
- Try to create your own user-defined Exception class, and another that subclasses your custom class
- Try to test out where a raised Exception gets matched in an except clause (as above)