In python 3.6, how do I catch an exception and raise an exception to be handled later?

Question

Suppose I have 2 exceptions:

class FooError (Exception):
    def __init__(self, *args, **kwargs):
        default_message = 'A foo error has occurred!'
        if not (args or kwargs): args = (default_message,)
        super().__init__(*args, **kwargs)
class BarError (Exception):
    def __init__(self, *args, **kwargs):
        default_message = 'A bar error has occurred!'
        if not (args or kwargs): args = (default_message,)
        super().__init__(*args, **kwargs)

And, I have a function which throws FooError :

def foobar (x):
    if x < 0:
        raise FooError()

Generally, you would handle FooError with a try/except block:

try:
    foobar(-1)
except FooError:
    print('Uh oh, foo error!')
    sys.exit()

However, I would like to throw a BarError which I can handle later. Something like this:

except BarError:
    print('Uh oh, bar error!')
    sys.exit()

When executing this, though, I just get the traceback of both errors:

Traceback (most recent call last):
  File "C:\Users\Maze\Desktop\test2.py", line 17, in <module>
    foobar(-1)
  File "C:\Users\Maze\Desktop\test2.py", line 15, in foobar
    raise FooError()
__main__.FooError: A foo error has occurred!

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\Maze\Desktop\test2.py", line 19, in <module>
    raise BarError()
__main__.BarError: A bar error has occurred!

How do I throw BarError inside of the handler for FooError , and then handle BarError in a different except block?

Answer 1

Not sure whether I understood what you were asking, but you can easily raise new exceptions in other exception handlers:

def call_foobar():
    try:
        foobar(-1)
    except FooError:
        print('Uh oh, foo error!')
        raise BarError()

try:
    call_foobar()
except BarError as e:
    print("Bar Error")

You don't necessarily need a function for this, nesting two try blocks would be possible as well.

Does this answer your question?

Answer 2

You can't. Once you've caught an exception, you can't transfer control to another except block in the same try statement. You can use a nested statement:

try:
    try:
        foobar(-1)
    except FooError:
        raise BarError
except BarError:
    print('Uh oh, bar error!')
    sys.exit()

Some additional work is necessary if you want to distinguish between BarError s raised directly by foobar and the BarError raised as a result of the FooError being caught. You can use exception chaining for this. See PEP-3134 for more details; this example may not be the best way to write this.

try:
    try:
        foobar(-1)
    except FooError as exc:
        raise BarError from exc
except BarError as exc:
    if isinstance(exc.__cause__, FooError):
        print("Caught a Foo-induced BarError")
    else:
        print("Caught a regular BarError")

Answer 3

It sounds like you want to catch a FooError or BarError , but you want to do a little extra work first if it is a FooError . If that's the case, you can catch both kinds of exception and only do the work for a FooError :

try:
    foobar(-1)
except (FooError, BarError) as e:
    if isinstance(e, FooError):
        # do extra work
    # Handle errors