It is important to understand that two stable versions of Python are available. Python 2 was first released in October 2000, and version 2.7 is the latest in a long line of evolutionary upgrades. The Python developers realized that certain aspects of the Python language could not be improved without breaking compatibility with existing code. The result was Python 3, which is the first release of Python that is intentionally backwards-incompatible. In other words, some code written for Python 2 will have to be modified to run on a Python 3 interpreter. For this reason, the migration from Python 2 to Python 3 has been rather slow, even though Python 3 is mature and stable. Version 2.7 is the final release of Python 2, and new features are being added only to Python 3. Sage uses Python 2.7 (as of Sage version 4.6), so the code in this book is written for Python 2.7. Whenever possible, the examples have been written so that they will continue to run when Sage eventually...