Most of the writing are Python 2 and 3 compatible (sure the print statement is enough to throw Python 3 off) and this was written back in 2012 when Python 2 was still the center of the community. Maybe the author thinks writing down the interpreter (Python 2.7.1) on the first chapter is good enough. Also, this was prepared for a training if you read the side. So students who took the author's training probably knew it was for Python 2.
Although I do spot a couple really "non-standard" practice, for example, the classes still using the old Python 2 syntax:
Although I do spot a couple really "non-standard" practice, for example, the classes still using the old Python 2 syntax:
class Name: vs class Name(object):