In general, I follow strict pep8 and pyflakes. All code must pass these tests. Since we support python 2.4-3.4 and pypy, pyflakes reports unknown names in python 3. pyflakes is run in python 2.7 only in my tests.
Only modules included in the standard library are permitted for use in this application. This application should not be dependent on any 3rd party modules that would need to be installed external to just Python itself.