This rule raises an issue when an exception class is duplicated in an except statement, or when an exception class has a parent class in the same except statement.

Why is this an issue?

In Python it is possible to catch multiple types of exception in a single except statement using a tuple of the exceptions.

Repeating an exception class in a single except statement will not fail but it does not have any effect. Either the exception class is not the one which should be caught, or it is duplicated code which should be removed.

Having a subclass and a parent class in the same except statement does not provide any benefit either. It is enough to keep only the parent class.

Code examples

Noncompliant code example

try:
    ...
except (TypeError, TypeError):  # Noncompliant: duplicated code or incorrect exception class.
    print("Foo")

try:
    ...
except (NotImplementedError, RuntimeError):  # Noncompliant: NotImplementedError inherits from RuntimeError.
    print("Foo")

Compliant solution

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

try:
    ...
except RuntimeError:
    print("Foo")

Resources

Documentation

• Python Documentation - Handling Exceptions
• Python Documentation - Exception hierarchy