Surprised no mention of JetBrains PyCharm. It's incredibly easy to use for debugging, even supports debugging gevent based code.
I agree though, outputting print statements at different levels of severity, i.e. warning, error, info, etc. is a great way to see if things are working on a high level. For more granularity, debuggers are invaluable to figure out why a specific portion of code isn't working as expected.
And now that they have released a community edition -- open source (Apache Licensed) -- copy of PyCharm, I cannot imagine any sane person continuing to use a "dumb" editor.
All of the JetBrains products are super smart and efficient, and the community editions are no exception.
I agree though, outputting print statements at different levels of severity, i.e. warning, error, info, etc. is a great way to see if things are working on a high level. For more granularity, debuggers are invaluable to figure out why a specific portion of code isn't working as expected.