So far my favorite additions and changes in Python 2.7.1 since upgrading from the default Python 2.6.1 installation in Mac OS X Snow Leopard are the following:

1. Dictionary and Set Comprehensions.
   List comprehensions are one of my favorite language features in Python, they are incredibly useful for processing and building lists.  So I am very excited to see dictionary and set comprehensions back ported from Python 3 to Python 2.7.1.
2. The ArgParse Module.
   As a C/C++ programmer I original did command line argument processing in Python manually using sys.argv, then I discovered the C-style getopt module.  I always found myself wondering if there was a more concise Pythonic way to handle command line parameters.  The argparse module is the solution, it replaces the optparse module.  I particularly like how argparse (and optparse) will generate the command line help for you!
3. csv.DictWriter.writeheader method.
   While this is a very minor change  (in Python 2.7 to be precise), I am a big fan of the csv module’s DictWriter class as a way to easily dump lists of dictionaries to a file for easy analysis and debugging with Excel.  The addition of the DictWriter class of an new writeheader method makes this class even easier to use.

You can find the full release notes for Python 2.7.1 here, there are so many more changes than I’ve covered here so its well worth checking out the release notes.  What are your favorite changes in Python 2.7.1?