Background: Python is one of the most popular modern programming languages. In 2008 its authors introduced a new version of the language, Python 3.0, that was not backward compatible with Python 2, initiating a transitional phase for Python software developers. Aims: The study described in this paper investigates the degree to which Python software developers are making the transition from Python 2 to Python 3. Method: We have developed a Python compliance analyser, PyComply, and have assembled a large corpus of Python applications. We use PyComply to measure and quantify the degree to which Python 3 features are being used, as well as the rate and context of their adoption. Results: In fact, Python software developers are not exploiting the new features and advantages of Python 3, but rather are choosing to retain backward compatibility with Python 2. Conclusions: Python developers are confining themselves to a language subset, governed by the diminishing intersection of Python 2, which is not under development, and Python 3, which is under development with new features being introduced as the language continues to evolve.