Writing code that is compatible across versions
Many Python modules these days are designed to run under multiple Python versions, especially supporting Python 2.x as well as Python 3.x versions. We will want to run the same tests in both versions, and to do this, we will need to write our code in such a way that the tests are compatible with both the versions.
Python's import mechanism gives us the flexibility we need to do this. At the top of the file, we import unittest like the following:
try: import unittest2 as unittest except ImportError: import unittest
What this does is to first try and import unittest2. If we are running Python 2.x, then we should have installed this already. If it succeeds, then the module is imported and the module reference is renamed to unittest.
If we get an ImportError, then we are running Python 3.x, in which case we can import the unittest module bundled in the standard library.
Later in the code, we can just reference the unittest module and it...
