从警告链接异常时发生TypeError

Question

I am trying to catch a warning that is raised as an error by applying the 'error' filter of warnings.simplefilter . A minimum working example is given below:

>>> import warnings
>>> warnings.simplefilter('error')
>>> try:
...     warnings.warn('test')
... except UserWarning:
...     raise ValueError
...
ValueError: 

This works fine, but if I want to chain this so that the traceback from the warning is included, I get the following TypeError :

>>> import sys
>>> try:
...     warnings.warn('test')
... except UserWarning:
...     raise ValueError from sys.exc_info()[2]
...
TypeError: exception causes must derive from BaseException

It seems that even though I am raising a class derived from BaseException (ie the ValueError ), the information from the traceback from the UserWarning seems to be tricking Python into thinking I am still raising the UserWarning .

Is this the expected behavior? If so, is there a good workaround?

I am on Python 3.4.3.

Answer 1

You are trying to use the traceback object as the exception:

raise ValueError from sys.exc_info()[2]

sys.exc_info() returns a tuple of (ExceptionType, exception_instance, traceback) ; you'd want index 1 here (remember Python counts indices from 0 !):

raise ValueError from sys.exc_info()[1]

Much better, capture the exception instance in a variable directly:

try:
    warnings.warn('test')
except UserWarning as warning:
    raise ValueError from warning

That way you don't have to count out tuple indices.

When you set the warnings filter to 'error' , the exceptions are subclasses of the Warning exception class , which inherits from Exception , see the Exception Hierachy section .