“Inner exception” (with traceback) in Python?

Question

My background is in C# and I've just recently started programming in Python. When an exception is thrown I typically want to wrap it in another exception that adds more information, while still showing the full stack trace. It's quite easy in C#, but how do I do it in Python?

Eg. in C# I would do something like this:

try
{
  ProcessFile(filePath);
}
catch (Exception ex)
{
  throw new ApplicationException("Failed to process file " + filePath, ex);
}

In Python I can do something similar:

try:
  ProcessFile(filePath)
except Exception as e:
  raise Exception('Failed to process file ' + filePath, e)

...but this loses the traceback of the inner exception!

Edit: I'd like to see both exception messages and both stack traces and correlate the two. That is, I want to see in the output that exception X occurred here and then exception Y there - same as I would in C#. Is this possible in Python 2.6? Looks like the best I can do so far (based on Glenn Maynard's answer) is:

try:
  ProcessFile(filePath)
except Exception as e:
  raise Exception('Failed to process file' + filePath, e), None, sys.exc_info()[2]

This includes both the messages and both the tracebacks, but it doesn't show which exception occurred where in the traceback.

Answer 1

Assuming:

• you need a solution, which works for Python 2 (for pure Python 3 see raise ... from solution)
• just want to enrich the error message, e.g. providing some additional context
• need the full stack trace

you can use a simple solution from the the docs https://docs.python.org/3/tutorial/errors.html#raising-exceptions:

try:
    raise NameError('HiThere')
except NameError:
    print 'An exception flew by!' # print or log, provide details about context
    raise # reraise the original exception, keeping full stack trace

The output:

An exception flew by!
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
NameError: HiThere

It looks like the key piece is the simplified 'raise' keyword that stands alone. That will re-raise the Exception in the except block.

Answer 2

Python 3 has the raise ... from clause to chain exceptions. Glenn's answer is great for Python 2.7, but it only uses the original exception's traceback and throws away the error message and other details. Here are some examples in Python 2.7 that add context information from the current scope into the original exception's error message, but keep other details intact.

Known Exception Type

try:
    sock_common = xmlrpclib.ServerProxy(rpc_url+'/common')
    self.user_id = sock_common.login(self.dbname, username, self.pwd)
except IOError:
    _, ex, traceback = sys.exc_info()
    message = "Connecting to '%s': %s." % (config['connection'],
                                           ex.strerror)
    raise IOError, (ex.errno, message), traceback

That flavour of raise statement takes the exception type as the first expression, the exception class constructor arguments in a tuple as the second expression, and the traceback as the third expression. If you're running earlier than Python 2.2, see the warnings on sys.exc_info().

Any Exception Type

Here's another example that's more general purpose if you don't know what kind of exceptions your code might have to catch. The downside is that it loses the exception type and just raises a RuntimeError. You have to import the traceback module.

except Exception:
    extype, ex, tb = sys.exc_info()
    formatted = traceback.format_exception_only(extype, ex)[-1]
    message = "Importing row %d, %s" % (rownum, formatted)
    raise RuntimeError, message, tb

Modify the Message

Here's another option if the exception type will let you add context to it. You can modify the exception's message and then reraise it.

import subprocess

try:
    final_args = ['lsx', '/home']
    s = subprocess.check_output(final_args)
except OSError as ex:
    ex.strerror += ' for command {}'.format(final_args)
    raise

That generates the following stack trace:

Traceback (most recent call last):
  File "/mnt/data/don/workspace/scratch/scratch.py", line 5, in <module>
    s = subprocess.check_output(final_args)
  File "/usr/lib/python2.7/subprocess.py", line 566, in check_output
    process = Popen(stdout=PIPE, *popenargs, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory for command ['lsx', '/home']

You can see that it shows the line where check_output() was called, but the exception message now includes the command line.

Answer 3

It's simple; pass the traceback as the third argument to raise.

import sys
class MyException(Exception): pass

try:
    raise TypeError("test")
except TypeError, e:
    raise MyException(), None, sys.exc_info()[2]

Always do this when catching one exception and re-raising another.