What is the appropriete way to handling exceptions inside a python method?

Question

Suppose I have a function and, depending on its inputs, it must "advise" the caller function that something went wrong:

def get_task(msg, chat):
    task_id = int(msg)
    query = db.SESSION.query(Task).filter_by(id=task_id, chat=chat)
    try:
        task = query.one()
    except sqlalchemy.orm.exc.NoResultFound:
        return "_404_ error"
    return task

Notice at the except block I want to pass something that the caller function can handle and stop its execution if it's necessary, otherwise, it will return the right object.

def something_with_the_task(msg, chat):
   task = get_task(msg, chat)
   if task == "_404_ error":
       return
   #do some stuff with task

You should raise an exception and catch it in the caller function. — Chris Mueller
– Chris Mueller, Commented Apr 30, 2018 at 15:22
Define a higher-level more appropriate exception for your interface and raise it, given you have no way to recover from the lower-level failure. — AChampion
– AChampion, Commented Apr 30, 2018 at 15:24

fferri · Accepted Answer · 2018-04-30 15:43:39Z

You already seem to know how exceptions work.

The best thing to do in case of an error is to raise an exception.

Returning some magic value is considered a bad practice, because it requires the caller to explicitly check for it, and a hundred of other reasons.

You can simply let the sqlalchemy.orm.exc.NoResultFound exception escape (by removing the try: and the except: block in get_task()), and let the caller handle it with a try: ... except: ... block, or, if you prefer to do some hiding, you can define a custom exception:

class YourException(Exception):
    pass

and use it like this:

def get_task(msg, chat):
    try:
        task = ...
    except sqlalchemy.orm.exc.NoResultFound:
        raise YourException('explanation')
    return task

def something_with_the_task(msg, chat):
    try:
        task = get_task(msg, chat)
        # do some stuff with task
    except YourException as e:
        # do something with e
        # e.args[0] will contain 'explanation'

Feel free to make the YourException class more informative by explicitly adding some attributes and a constructor to set those, if needed.

The default constructor makes a decent job however:

>>> e = YourException('Program made a boo boo', 42, 'FATAL')
>>> e
YourException('Program made a boo boo', 42, 'FATAL')
>>> e.args[0]
'Program made a boo boo'
>>> e.args[1]
42
>>> e.args[2]
'FATAL'

You could also use except sqlalchemy.orm.exc.NoResultFound as err: raise YourException('explanation') from err to include the original exception in the traceback.

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