A note about Macintoshes and the IDLE debugger
In short, you cannot set breakpoints in IDLE on the Mac. This has been true for a long time and the usual solution is to "use the command-line debugger pdb". This is not very easy to use.
One solution which is not ideal but works: the IDLE debugger always stops at the first executable line of code, regardless of where any breakpoints are set. You can take control at that point, and use Step or Over to get to the lines you want to debug.
A solution which I cannot verify is here. It looks complicated. At the bottom of that page, there is a footnote which says that the bug (of not being able to set breakpoints on a Mac) was fixed in Python versions 3.2.2 and 2.7.2. So you might check your version and upgrade to the latest one.