Using shorter textwidth in comments and docstrings

From the mighty PEP 8:

[P]lease limit all lines to a maximum of 79 characters. For flowing long blocks of text (docstrings or comments), limiting the length to 72 characters is recommended.

When editing Python code in Vim, I set my textwidth to 79, and Vim automatically wraps long lines of Python code for me when I hit the character limit. But in comments and docstrings, I need to wrap text at 72 characters instead.

Is there any way to make Vim automatically set textwidth to 72 when I'm in a comment or docstring, and set it back when I'm done?