Double-clicking a Python script (lesson01.py) now launches a command-line window (cmd) where it runs
Python 3 is incompatible with Python 2.7 in a few important ways. If you learned Python 3, this page lists the differences. The relevant ones for our course are print, raw_input(), range(), set(), and integer division.
If you are a Windows user, make sure to get the 32-bit version of Python 2.7. It is usually the top download link, labeled "Windows x86 MSI installer". Avoid the 64-bit version, which is marked with "X86-64".
If you are installing on Mac OS X, make sure to download and install the appropriate version of ActiveTcl. If you don't, your Python IDLE will use the older version of Tcl/Tk that comes pre-installed with your OS X, but it is known to be buggy and will crash your IDLE window. If your IDLE displays this WARNING message, it is definitely affected. See this FAQ for further details. Also, see this page for details on which version of ActiveTcl to download.
Windows users: Unless you changed the default setting, your system will display file names without the proper extension, e.g., you see "slides06" instead of "slides06.ppt". This leads to a lot of confusion while programming. Follow "option one" shown on this page to make file extensions visible.
If you work with multiple Windows machines (such as Pitt lab computers), consider getting Portable Python. This distribution requires no system installation: all you need to do is unzip the downloaded archive. You can carry with you the entire Python program, libraries and your program on a USB stick.