Getting Started

To get started, you should fork the ParmEd Github repository into your own Github account. Push changes there, and submit a pull request (PR) with your changes (no change is too small for a PR!).

This document outlines some of the general guidelines for working on ParmEd to try and make sure that the code base is kept clean, organized, manageable, and reliable (i.e., tested).

Test-driven development

Test-driven development is hard, and often irritating, but I’ve found that it is much better at avoiding bugs down the road than alternatives. So when you are writing code for ParmEd, generate a test case and write at least the beginning of the test first.

Then, before you write any code, run the test suite to make sure that it fails! If the test doesn’t fail before you’ve written any code, then your test is clearly not being run, or not being run correctly (which means there is no way that your test could prevent a breakage in the future).

Once you have a failing test case, write code until the test case passes. Then add more test until the test case fails again. Then update the code until the test passes again. Rinse-and-repeat until the functionality you are adding is complete and fully-tested.

Coding Style

For the most part, the coding style here follows the style guide PEP 8. Of the guidelines listed there, the following I consider very important (and will block any PRs):

  • Indentation is done with 4 spaces. No source code file should contain tabstops, and all indentation must be consistent.

  • Source code should be ASCII encoded

  • Please make sure comments and docstrings are written in English

  • Use only absolute imports or explicit relative imports

  • Always use print as a function (via the print_function futures import) – see below for why this is a hard requirement.

Things I would like to see, but are not as important:

  • Try to keep lines limited to 80 characters, but do not sacrifice readability

  • Class definitions should be done in CamelCase naming style. Method, attribute, and variable names should be done in underscore_separated_style. Constants should be named in ALL_CAPS.

Target Python Version

ParmEd officially supports Python 2.7 and Python 3.3+ in the same code base through the use of the six module (although some earlier versions of Python may work, no effort is made to support Python 2.6 and lower or Python 3.2 and lower). The six module is included as a subpackage in the parmed.utils package, and all components should be imported from there.

In particular, the range and zip builtins should be imported from parmed.utils.six.moves rather than relying on the standard versions. This is because in Python 3, range and zip return efficient iterators, while in Python 2 they return potentially inefficient lists. The Python 2-equivalent versions of the range and zip iterators are xrange and itertools.izip, respectively, which are the actual functions defined within parmed.utils.six.moves for Python 2.

Supporting both Python 2 and Python 3 in the same code base means that you must never use the print statement, implicit integer division, or relative imports (as those will not work in Python 3). You are encouraged to put the following line at the top of each Python module in ParmEd:

from __future__ import division, absolute_imports, print_function

In addition to supporting CPython 2.7 and 3.3+, ParmEd also strives to support the pypy interpreter. Under the pypy interpreter, PDB file parsing is about 3-5x faster using the same code. Since ParmEd parses a substantial amount of metadata from RCSB/wwPDB files, the added efficiency can be useful for mining the PDB. As such, modules that require CPython (such as scipy, scikit-learn, matplotlib, … etc.) must be made optional so that the functionality not relying on those modules can be used within pypy.


Please keep test cases relatively small – Github likes to keep repositories under 1 GB in size, so test files should be relatively small. They should also be kept small enough to run in a short amount of time, but they should also ensure that the function being tested has as close to full coverage as can be managed.

The ParmEd tests are run in the ParmEd/tests directory with the command:

nosetests -vs .

which will automatically find and run all test_*.py files. The tests are implemented using the unittest module in the Python standard library. Look at existing tests for examples.

While developing new functionality, I find it helpful to run the tests directly from the ParmEd directory with the following command:

nosetests -vs test/

Obviously in this case, is replaced with whichever test module you are working on. You can select specific tests using the -m flag specifying a regex that matches the test case method. For example:

nosetests -vs test/ -m add_atom

will test both the test_add_atom and test_add_atom_to_residue methods. This is an easy way to run tests quickly while working on new methods without having to run python install after every change. Note that when you run tests from the root ParmEd directory, however, the imported ParmEd repository will not have any Python extensions installed (meaning that the tests relying on them – like the test for the Amber optimized reader – will fail).

ParmEd utilizes the Travis continuous integration server to perform automatic tests of all pull requests. Tests generally must pass these tests before being considered for merge into the master branch.


Just as important as the code is documenting the code so that users can learn how to use it. Documentation is tracked as reStructuredText (.rst), and Sphinx is used to render the documentation. Changes to the documentation can be made part of the pull request adding new functionality, or it can be its own pull request dedicated entirely to documentation improvements.

The actual web docs are generated periodically and pushed directly to the gh-pages branch. This process is not handled by PRs.