This rule raises an issue when a class derives from one of the following exception classes: ``++BaseException++``, ``++KeyboardInterrupt++``, ``++SystemExit++`` or ``++GeneratorExit++``.

== Why is this an issue?

https://docs.python.org/3/library/exceptions.html#SystemExit[``++SystemExit++``] is raised when https://docs.python.org/3/library/sys.html#sys.exit[``++sys.exit()++``] is called. https://docs.python.org/3/library/exceptions.html#KeyboardInterrupt[``++KeyboardInterrupt++``] is raised when the user asks the program to stop by pressing interrupt keys. Both exceptions are expected to propagate up until the application stops.

In order to avoid catching ``++SystemExit++`` and ``++KeyboardInterrupt++`` by mistake, https://www.python.org/dev/peps/pep-0352/#exception-hierarchy[PEP-352] created the root class ``++BaseException++`` from which ``++SystemExit++``, ``++KeyboardInterrupt++`` and ``++Exception++`` derive. Thus developers can use ``++except Exception:++`` without preventing the software from stopping.

The ``++GeneratorExit++`` class also derives from ``++BaseException++`` as it is not really an error and is not supposed to be caught by user code.

As said in https://docs.python.org/3/library/exceptions.html#BaseException[Python's documentation], user-defined exceptions are not supposed to inherit directly from ``++BaseException++``. They should instead inherit from ``++Exception++`` or one of its subclasses.

=== Code examples

==== Noncompliant code example

[source,python,diff-id=1,diff-type=noncompliant]
----
class MyException(BaseException):  # Noncompliant
    pass
----

==== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
class MyException(Exception):
    pass
----

== Resources

=== Documentation

* PEP 352 – https://www.python.org/dev/peps/pep-0352/#exception-hierarchy-changes[Required Superclass for Exceptions]
* Python Documentation - https://docs.python.org/3/library/exceptions.html#BaseException[BaseException class]

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

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

=== Message

Derive this class from "Exception" instead of BaseException/GeneratorExit/KeyboardInterrupt/SystemExit


=== Highlighting

The forbidden parent class reference.

[source,text]
----
class Foo(BaseException)
          ^^^^^^^^^^^^^
----

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