== Why is this an issue?

In Python 3, attempting to catch in an ``++except++`` statement an object which does not derive from BaseException will raise a ``++TypeError++``. In Python 2 it is possible to raise old-style classes but this shouldn't be done anymore in order to be compatible with Python 3.


In order to catch multiple exceptions in an ``++except++`` statement, a tuple of exception classes should be provided.


If you are about to create a custom Exception class, note that custom exceptions should inherit from ``++Exception++``, not ``++BaseException++``. ``++Exception++`` allows people to catch all exceptions except the ones explicitly asking the interpreter to stop, such as ``++KeyboardInterrupt++`` and https://docs.python.org/3/library/exceptions.html#GeneratorExit[``++GeneratorExit++``] which is not an error. See https://www.python.org/dev/peps/pep-0352/#exception-hierarchy-changes[PEP 352] for more information.


This rule raises an issue when the expression used in an ``++except++`` statement is not a class deriving from ``++BaseException++`` nor a tuple of such classes.


=== Noncompliant code example

[source,python]
----
class CustomException:
    """An Invalid exception class."""

try:
    "a string" * 42
except CustomException:  # Noncompliant
    print("exception")
except (None, list()):  # Noncompliant * 2
    print("exception")

try:
    "a string" * 42
except [TypeError, ValueError]:  # Noncompliant. Lists are not accepted.
    print("exception")
except {TypeError, ValueError}:  # Noncompliant. Sets are not accepted.
    print("exception")
----


=== Compliant solution

[source,python]
----
class MyError(Exception):
    pass

try:
    "a string" * 42
except (MyError, TypeError):
    print("exception")
----


== Resources

* Python documentation - https://docs.python.org/3/tutorial/errors.html[Errors and Exceptions]
* Python documentation - https://docs.python.org/3/reference/compound_stmts.html#except[the ``++try++`` statement]
* https://www.python.org/dev/peps/pep-0352/#exception-hierarchy-changes[PEP 352 - Required Superclass for Exceptions]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

* Change this expression to be a class deriving from BaseException or a tuple of such classes.


=== Highlighting

The expression which does not resolve in a valid exception class or a tuple of such classes.


'''
== Comments And Links
(visible only on this page)

include::../comments-and-links.adoc[]
endif::env-github,rspecator-view[]