How to use pep8 module using Spyder

(Spyder maintainer here) You can activate pep8 support in Spyder 3 by going to

Tools > Preferences > Editor > Code Introspection/Analysis

and activating the option called

Real-time code style analysis

This will show pep8 warnings directly in our Editor, so you don't need to run pep8 from the command line to see them.

Amendment to Carlos response: In Spyder 4 this option has been moved to

Tools > Preferences > Completion and linting > Code style

where you have to accept

Enable code style linting

FYI, in this new version specific errors or warnings can be selected to be shown or ignored!