rspec/rules/S1045/python/rule.adoc

Exceptions handlers (``except:``) are evaluated in the order they are written. Once a match is found, the evaluation stops.

In some contexts an except block is dead code as it will never catch any exception:

* If there is a handler for a base class followed by a handler for class derived from that base class, the second handler will never trigger: The handler for the base class will match the derived class, and will be the only executed handler.
* When multiple ``except`` statements try to catch the same exception class, only the first one will be executed.
* In python 3, ``BaseException`` is the parent of every exception class. When ``BaseException`` is caught and the same try-except block has a bare ``except:`` statement, i.e. an ``except`` with no expression, the bare except will never catch anything.

This rule raises an issue when an ``except`` block catches every exception before a later ``except`` block could catch it.

== Noncompliant Code Example

----
def foo():
    try:
        raise FloatingPointError()
    except (ArithmeticError, RuntimeError) as e: 
        print(e)
    except FloatingPointError as e: # Noncompliant. FloatingPointError is a subclass of ArithmeticError
        print("Never executed")
    except OverflowError as e: # Noncompliant. OverflowError is a subclass of ArithmeticError
        print("Never executed")

    try:
        raise TypeError()
    except TypeError as e: 
        print(e)
    except TypeError as e: # Noncompliant. Duplicate Except.
        print("Never executed")

    try:
        raise ValueError()
    except BaseException as e:  
        print(e)
    except: # Noncompliant. This is equivalent to "except BaseException" block
        print("Never executed")
----

== Compliant Solution

----
def foo():
    try:
        raise FloatingPointError()
    except FloatingPointError as e:
        print("Executed")
    except OverflowError as e:
        print("Executed")
    except (ArithmeticError, RuntimeError) as e: 
        print(e)

    try:
        raise TypeError()
    except TypeError as e: 
        print(e)

    try:
        raise ValueError()
    except BaseException as e:  
        print(e)
----

== See

* Python Documentation - https://docs.python.org/3/reference/compound_stmts.html#the-try-statement[The ``try`` statement]