This is how I do it:

>>> import traceback
>>> try:
...   int('k')
... except:
...   var = traceback.format_exc()
... 
>>> print(var)
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ValueError: invalid literal for int() with base 10: 'k'

You should however take a look at the traceback documentation, as you might find there more suitable methods, depending to how you want to process your variable afterwards...

The __traceback__ attribute is available only since Python 3.0, so if you're looking to make your code compatible with Python 2 you should use sys.exc_info() instead; otherwise, per PEP-3134, the introduction of the __traceback__ attribute is indeed meant to fully replace sys.exc_info(), and possibly deprecate it:

In today's Python implementation, exceptions are composed of three parts: the type, the value, and the traceback. The sys module, exposes the current exception in three parallel variables, exc_type, exc_value, and exc_traceback, the sys.exc_info() function returns a tuple of these three parts, and the raise statement has a three-argument form accepting these three parts. Manipulating exceptions often requires passing these three things in parallel, which can be tedious and error-prone. Additionally, the except statement can only provide access to the value, not the traceback. Adding the __traceback__ attribute to exception values makes all the exception information accessible from a single place.