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This rule raises an issue when the expression used in an except statement is neither a class deriving from BaseException
nor a tuple of such classes.
Why is this an issue?
In Python 3’s except statement, attempting to catch an object that does not derive from BaseException will raise a
TypeError.
In order to catch multiple exceptions in an except statement, a tuple of exception classes should be provided. Trying to
catch multiple exceptions with a list or a set will raise a TypeError.
If you are about to create a custom exception class, note that custom exceptions should inherit from Exception, rather than
BaseException.
BaseException is the base class for all built-in exceptions in Python, including system-exiting exceptions like
SystemExit or KeyboardInterrupt, which are typically not meant to be caught. On the other hand, Exception is
intended for exceptions that are expected to be caught, which is generally the case for user-defined exceptions. See PEP 352 for more information.
To fix this issue, make sure the expression used in an except statement is an exception which derives from
BaseException/Exception or a tuple of such exceptions.
Code examples
Noncompliant code example
class CustomException(object):
    """An Invalid exception class."""
    pass

try:
    ...
except CustomException:  # Noncompliant: this custom exception does not derive from BaseException or Exception.
    print("exception")

try:
    ...
except [TypeError, ValueError]:  # Noncompliant: list of exceptions, only tuples are valid.
    print("exception")

Compliant solution
class CustomException(Exception):
    pass

try:
    ...
except CustomException:
    print("exception")

try:
    ...
except (TypeError, ValueError):
    print("exception")

Note: In Python 2 it is possible to raise and catch old-style classes, which do not inherit from
BaseException.
Resources
Documentation

   Python Documentation - Errors and Exceptions 
   Python Documentation - The try statement 
   Python Documentation - Exception hierarchy 

Standards

   PEP 352 - Required Superclass for Exceptions