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older version of the computer programming language python begin with the version number 2 followed by dots and subversion numbers, such as
Python 3.0 is not 100% backward compatible with the python2.x versions. See History_of_Python. Some of the features of python3.x are backported to the newer versions of python 2.6.
During this book, all code examples are referring to python3.x if not written otherwise.