The PyDy source code is spread across both the SymPy and PyDy repositories. The two projects are very intimately tied together in terms of development. Much of our development practices come from SymPy's methods so reading their documentation for contributing is the best place to start:
https://github.com/sympy/sympy/wiki/introduction-to-contributing
Code written in python should roughly adhere to PEP8:
http://www.python.org/dev/peps/pep-0008/.
There are some exceptions, such as naming of variables. Use your best judgment.
When submitting, please follow the commit guidelines used for Git:
http://git-scm.com/book/ch5-2.html#Commit-Guidelines
The first line should have a soft limit of 50 characters, and subsequent lines limitted to 72 characters. The commit messages should be written in the imperative present tense.