Wrapping exceptions in Python

Question

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?

Answer 1

The simplest way would be to reraise with the old trace object. The following example shows this:

import sys  def a():     def b():         raise AssertionError("1")     b()  try:     a() except AssertionError: # some specific exception you want to wrap     trace = sys.exc_info()[2]     raise Exception("error description"), None, trace 

Check the documentation of the raise statement for details of the three parameters. My example would print:

Traceback (most recent call last):   File "C:\...\test.py", line 9, in <module>     a()   File "C:\...\test.py", line 6, in a     b()   File "C:\...\test.py", line 5, in b     raise AssertionError("1") Exception: error description 

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For completeness, in Python 3 you'd use the raise MyException(...) from e syntax.

Answer 2

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