Writing/Catching Exceptions in Python (PEP8)

Writing Exceptions

Derive exceptions from Exception rather than BaseException. Direct inheritance from BaseException is reserved for exceptions where catching them is almost always the wrong thing to do.

Design exception hierarchies based on the distinctions that code catching the exceptions is likely to need, rather than the locations where the exceptions are raised. Aim to answer the question "What went wrong?" programmatically, rather than only stating that "A problem occurred" (see PEP 3151 for an example of this lesson being learned for the builtin exception hierarchy).

Class naming conventions apply here, although you should add the suffix "Error" to your exception classes if the exception is an error. Non-error exceptions that are used for non-local flow control or other forms of signaling need no special suffix.

Catching Exceptions

When catching exceptions, mention specific exceptions whenever possible instead of using a bare except: clause:

try:
    import platform_specific_module
except ImportError:
    platform_specific_module = None

A bare except: clause will catch SystemExit and KeyboardInterrupt exceptions, making it harder to interrupt a program with Control-C, and can disguise other problems. Use except Exception: if you want to catch all exceptions that signal program errors. (Bare except is equivalent to except BaseException:.)

A good rule of thumb is to limit use of bare 'except' clauses to two cases:

1. If the exception handler will be printing out or logging the traceback; at least the user will be aware that an error has occurred.
2. If the code needs to do some cleanup work, but then lets the exception propagate upwards with raise. try...finally can be a better way to handle this case.