Why make pep8 changes to __init__ or any other part of this if your not also making to the docstring of the class itself.

tbh. I think the pep8 changes of PPField are out of scope of this PR entirely, since you have made no functional changes to the PPField Class until the x_bounds method (~180 lines ahead).  Since you have done now, saving for splitting your PR into two, you might as well PEP8 the bits you have missed.