The Source menu's Rewrap Text command lets you automatically re-wrap lines of text in the editor. In Python code, only a single logical line of source code will be wrapped, making it easy to reformat comment blocks and doc strings without affecting adjacent code.
This command isn't just a way to split up long lines, but can also be used to rejoin a line that's been split up or that has extra white space in it.
The column at which the Rewrap Text command wraps text is controlled with the Reformatting Wrap Column preference.