I'm working on a mail-sending library, and I want to be able to catch exceptions produced by the senders (SMTP, Google AppEngine, etc.) and wrap them in easily catchable exceptions specific to my library (ConnectionError, MessageSendError, etc.), with the original traceback intact so it can be debugged. What is the best way to do this in Python 2?
3 Answers
Use raise_from from the future.utils package.
Relevant example copied below:
from future.utils import raise_from
class FileDatabase:
def __init__(self, filename):
try:
self.file = open(filename)
except IOError as exc:
raise_from(DatabaseError('failed to open'), exc)
Within that package, raise_from is implemented as follows:
def raise_from(exc, cause):
"""
Equivalent to:
raise EXCEPTION from CAUSE
on Python 3. (See PEP 3134).
"""
# Is either arg an exception class (e.g. IndexError) rather than
# instance (e.g. IndexError('my message here')? If so, pass the
# name of the class undisturbed through to "raise ... from ...".
if isinstance(exc, type) and issubclass(exc, Exception):
e = exc()
# exc = exc.__name__
# execstr = "e = " + _repr_strip(exc) + "()"
# myglobals, mylocals = _get_caller_globals_and_locals()
# exec(execstr, myglobals, mylocals)
else:
e = exc
e.__suppress_context__ = False
if isinstance(cause, type) and issubclass(cause, Exception):
e.__cause__ = cause()
e.__suppress_context__ = True
elif cause is None:
e.__cause__ = None
e.__suppress_context__ = True
elif isinstance(cause, BaseException):
e.__cause__ = cause
e.__suppress_context__ = True
else:
raise TypeError("exception causes must derive from BaseException")
e.__context__ = sys.exc_info()[1]
raise e
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