Python 2 versus Python 3 ======================== .. currentmodule:: patsy The biggest difference between Python 2 and Python 3 is in their string handling, and this is particularly relevant to Patsy since it parses user input. We follow a simple rule: input to Patsy should always be of type `str`. That means that on Python 2, you should pass byte-strings (not unicode), and on Python 3, you should pass unicode strings (not byte-strings). Similarly, when Patsy passes text back (e.g. :attr:`DesignInfo.column_names`), it's always in the form of a `str`. In addition to this being the most convenient for users (you never need to use any b"weird" u"prefixes" when writing a formula string), it's actually a necessary consequence of a deeper change in the Python language: in Python 2, Python code itself is represented as byte-strings, and that's the only form of input accepted by the :mod:`tokenize` module. On the other hand, Python 3's tokenizer and parser use unicode, and since Patsy processes Python code, it has to follow suit.