[Draft] Modernize the Unreachable Except Block query

[joefarebrother]

Replaces use of pointsTo.
Implements modelling for subclass relations of builtin exception types.

[github-actions]

QHelp previews:

python/ql/src/Exceptions/IncorrectExceptOrder.qhelp

Unreachable except block

When handling an exception, Python searches the except blocks in source code order until it finds a matching except block for the exception. An except block, except E:, specifies a class E and will match any exception that is an instance of E.

If a more general except block precedes a more specific except block, then the more general block is always executed and the more specific block is never executed. An except block, except A:, is more general than another except block, except B:, if A is a super class of B.

For example: except Exception: is more general than except Error: as Exception is a super class of Error.

Recommendation

Reorganize the except blocks so that the more specific except is defined first. Alternatively, if the more specific except block is no longer required, then it should be deleted.

Example

In the following example, the except Exception: will handle AttributeError preventing the subsequent handler from ever executing.

def incorrect_except_order(val):
    try:
        val.attr
    except Exception:
        print ("Exception")
    except AttributeError:
        print ("AttributeError")
        

References

• Python Language Reference: The try statement, Exceptions.
• Common Weakness Enumeration: CWE-561.