Get exception description and stack trace which caused an exception, all as a string

See the traceback module, specifically the format_exc() function. Here.

import traceback

try:
    raise ValueError
except ValueError:
    tb = traceback.format_exc()
else:
    tb = "No error"
finally:
    print tb

Let's create a decently complicated stacktrace, in order to demonstrate that we get the full stacktrace:

def raise_error():
    raise RuntimeError('something bad happened!')

def do_something_that_might_error():
    raise_error()

Logging the full stacktrace

A best practice is to have a logger set up for your module. It will know the name of the module and be able to change levels (among other attributes, such as handlers)

import logging
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger(__name__)

And we can use this logger to get the error:

try:
    do_something_that_might_error()
except Exception as error:
    logger.exception(error)

Which logs:

ERROR:__main__:something bad happened!
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in do_something_that_might_error
  File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!

And so we get the same output as when we have an error:

>>> do_something_that_might_error()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in do_something_that_might_error
  File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!

Getting just the string

If you really just want the string, use the traceback.format_exc function instead, demonstrating logging the string here:

import traceback
try:
    do_something_that_might_error()
except Exception as error:
    just_the_string = traceback.format_exc()
    logger.debug(just_the_string)

Which logs:

DEBUG:__main__:Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 2, in do_something_that_might_error
  File "<stdin>", line 2, in raise_error
RuntimeError: something bad happened!

With Python 3, the following code will format an Exception object exactly as would be obtained using traceback.format_exc():

import traceback

try: 
    method_that_can_raise_an_exception(params)
except Exception as ex:
    print(''.join(traceback.format_exception(etype=type(ex), value=ex, tb=ex.__traceback__)))

The advantage being that only the Exception object is needed (thanks to the recorded __traceback__ attribute), and can therefore be more easily passed as an argument to another function for further processing.

>>> import sys
>>> import traceback
>>> try:
...   5 / 0
... except ZeroDivisionError as e:
...   type_, value_, traceback_ = sys.exc_info()
>>> traceback.format_tb(traceback_)
['  File "<stdin>", line 2, in <module>\n']
>>> value_
ZeroDivisionError('integer division or modulo by zero',)
>>> type_
<type 'exceptions.ZeroDivisionError'>
>>>
>>> 5 / 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

You use sys.exc_info() to collect the information and the functions in the traceback module to format it. Here are some examples for formatting it.

The whole exception string is at:

>>> ex = traceback.format_exception(type_, value_, traceback_)
>>> ex
['Traceback (most recent call last):\n', '  File "<stdin>", line 2, in <module>\n', 'ZeroDivisionError: integer division or modulo by zero\n']