Truncate a string without ending in the middle of a word

I am looking for a way to truncate a string in Python that will not cut off the string in the middle of a word.

For example:

Original:          "This is really awesome."
"Dumb" truncate:   "This is real..."
"Smart" truncate:  "This is really..."

I'm looking for a way to accomplish the "smart" truncate from above.