Exception Passing In Python

Question

I have a bit of code that does some functional exception handling and everything works well, exceptions are raised when I want them to be, but when I'm debugging, the line-traces don't always do quite what I want them to.

Example A:

>>> 3/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

Example B:

>>> try: 3/0
... except Exception as e: raise e
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: integer division or modulo by zero

In both of these examples, the exception really occurs in line 1, where we attempt to do 3/0, but in the latter example, we are told it has occurred on line 2, where it is raised.

Is there a way in Python to raise an exception, as if it were another exception, something that would produce the following output:

>>> try: 3/0
... except Exception as e: metaraise(e)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

Falmarri · Accepted Answer · 2011-01-19 00:04:13Z

50

When you re-raise an exception that you caught, such as

except Exception as e: raise e

it resets the stack trace. It's just like re-raising a new exception. What you want is this:

except Exception as e: raise

answered Jan 19, 2011 at 0:04

Falmarri

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4 Comments

Sven Marnach Over a year ago

And you don't need to store in in a variable in this case.

Kevin Dolan Over a year ago

Except that I need to be storing it in a variable somehow and passing it around to somewhere it will eventually be raised.

John Machin Over a year ago

@Kevin Dolan: and this is making your code easier to understand and more debuggable, or is it an entry in an obfuscation contest?

Kevin Dolan Over a year ago

It's actually for a decorator abstraction that allows exception swallowing/modification.

Kevin Dolan · Accepted Answer · 2011-01-24 18:40:00Z

6

For reference, the solution is approximately as follows:

def getException():
    return sys.exc_info()

def metaraise(exc_info):
    raise exc_info[0], exc_info[1], exc_info[2]

try: 3/0
except:
    e = getException()
    metaraise(e)

The beautiful part of this is that you can then pass around the variable e and metaraise it somewhere else, even if other exceptions have been encountered along the way.

answered Jan 24, 2011 at 18:40

Kevin Dolan

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1 Comment

Alfe Over a year ago

But then you'll have no information about the trace of the new raise. Any catcher above will be fooled to believe that the exception was raised in the original spot only. The second raiser hides its existence which can be quite confusing in debugging cases.

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