bluepaperbirds
Posted on February 20, 2020
Python Applications may call custom exceptions by creating a brand new exception class. Custom Exceptions need to be derived from the Exception class.
What is an Exception?
An exception can be thrown with the raise keyword. They can be handled with the try-catch block.
You can try this in the Python shell, by typing raise Exception("your message").
➜ ~ python3
Python 3.7.5 (default, Nov 20 2019, 09:21:52)
[GCC 9.2.1 20191008] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> raise Exception("No can do")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
Exception: No can do
>>>
All built-in exceptions use the Exception class. User-defined exceptions should be derived from this class (inherited).
Custom Exception
The program shown below creates a custom exception HappyError that inherits from the Exception class.
class HappyError(Exception):
pass
raise HappyError("To happy to run")
The above program then raises the HappyError exception:
➜ ~ python3 program.py
Traceback (most recent call last):
File "t.py", line 5, in <module>
raise HappyError("To happy to run")
__main__.HappyError: To happy to run
Many modules define their own exceptions.
Optionally you can override the constructor like this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super().__init__(message)
# Now for your custom code...
self.errors = errors
But with Python 3 Exceptions, you don't need to do override anything. If all you want is an informative message for your exception, do this:
class MyException(Exception):
pass
raise MyException("My car is swimming")
Posted on February 20, 2020
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