Exceptions Lesson

Now that the motivation and some syntax for exceptions has been presented, we can delve a bit deeper into the structure of writing programs containing exceptions. The following lesson is designed to help put into context the rudiments just presented. In addition, one more important component of exception handling is the 'raise' statement which is useful for raising an exception if, for example, it occurs inside an exception handler.

Exceptions

3. Best Practices

BEST PRACTICES

Catch More Specific Exceptions First

Remember, your except handlers are evaluated in order, so be sure to put more specific exceptions first. For example:

>>> try:
... my_value = 3.14 / 0
... except ArithmeticError:
... print("We had a general math error")
... except ZeroDivisionEror:
... print("We had a divide-by-zero error")
...
We had a general math error

When we tried to divide by zero, we inadvertently raised a ZeroDivisionError. However, because ZeroDivisionError is a subclass of ArithmeticError, and except ArithemticError came first, the information about our specific error was swallowed by the except ArithemticError handler, and we lost more detailed information about our error.

Don't Catch `Exception`

It's bad form to catch the general Exception class. This will catch every type of exception that subclasses the Exception class, which is almost all of them. You may have errors that you don't care about, and don't affect the operation of your program, or maybe you're dealing with a flaky API and want to swallow errors and retry. By catching Exception, you run the risk of hitting an unexpected exception that your program actually can't recover from, or worse, swallowing an important exception without properly logging it - a huge headache when trying to debug programs that are failing in weird ways.

Definitely don't catch `BaseException`

Catching BaseException is a really bad idea, because you'll swallow every type of Exception, including KeyboardInterrupt, the exception that causes your program to exit when you send a SIGINT (Ctrl-C). Don't do it.