Advanced exceptions - explicitly chained exceptions
This time we'd like to convert an explicit type of exception object to another type of exception object at the moment when the second exception is occurring.
Imagine that your code is responsible for the final checking process before the rocket is launched. The list of checks is a long one, and different checks could result in different exceptions.
But as it is a very serious process, you should be sure that all checks are passed. If any fails, it should be marked in the log book and re-checked next time.
Now you see that it would be convenient to convert each type of exception into its own exception (like RocketNotReadyError) and to log the origin of the exception.
Final check procedure
The captain's name is John
The pilot's name is Mary
The mechanic's name is Mike
Traceback (most recent call last):
File "exceptions#050.py", line 10, in personnel_check
print("\tThe navigator's name is", crew[3])
IndexError: list index out of range
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "exceptions#050.py", line 19, in
personnel_check()
File "exceptions#050.py", line 12, in personnel_check
raise RocketNotReadyError('Crew is incomplete') from e
__main__.RocketNotReadyError: Crew is incomplete
output
Once again, the result of the code execution contains an interesting piece of information indicating that we have just witnessed a chain of exceptions:
The above exception was the direct cause of the following exception:
Code
class RocketNotReadyError(Exception):pass
def personnel_check():
try:
print("\tThe captain's name is", crew[0])
print("\tThe pilot's name is", crew[1])
print("\tThe mechanic's name is", crew[2])
print("\tThe navigator's name is", crew[3])
except IndexError as e:
raise RocketNotReadyError('Crew is incomplete') from e
crew = ['John', 'Mary', 'Mike']
print('Final check procedure')
personnel_check()