Throwing a custom error in a module meant to be imported by arbitrary modules (library)

Question

I'm building a python library that implements a task queue. If for whatever reason the thread that processes the tasks dies, and the option to restart the thread isn't set, I need to throw an exception.

Is this something that I should just throw a RuntimeException for? I don't want to throw a custom exception since they'd have to import that, but I'm not sure how throw exceptions to arbitrary calling code is best implemented.

If `ValueError` or `TypeError` or any other built-in exception is correct, use it. If not, don't be afraid of creating another type of exception. — Chris Morgan, Dec 20 '10 at 04:04
If the exception doesn't fit into an existing category, create a new type an raise it instead -- that's their purpose. — martineau, Dec 20 '10 at 13:27

score 2 · Accepted Answer · answered Dec 20 '10 at 03:31

Throw a custom exception. If the user of the library needs to catch that exception, they can import it.

For instance, take pickle.PicklingError:

>>> import pickle
>>> pickle.dumps(type(None))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\pickle.py", line 1374, in dumps
    Pickler(file, protocol).dump(obj)
  File "C:\Python27\lib\pickle.py", line 224, in dump
    self.save(obj)
  File "C:\Python27\lib\pickle.py", line 286, in save
    f(self, obj) # Call unbound method with explicit self
  File "C:\Python27\lib\pickle.py", line 748, in save_global
    (obj, module, name))
pickle.PicklingError: Can't pickle <type 'NoneType'>: it's not found as __builtin__.NoneType
>>> try:
...     pickle.dumps(type(None))
... except pickle.PicklingError:
...     print 'Oops.'
...
Oops.

Throwing a custom error in a module meant to be imported by arbitrary modules (library)

1 Answers1