Developer’s guide#

The source code for BioSimSpace is available on GitHub.


BioSimSpace uses the Python programming language. Our aim is to provide a simple and robust API where unnecessary implementation details are hidden from the user.

Hold on a second, this code isn’t very Pythonic!

Indeed it is not, but this is a design choice. BioSimSpace is intended primarily to be used by novices, who may be unfamiliar with Python, or programming in general. We want to make it as easy as possible for these users to get up and running with molecular simulation. BioSimSpace also needs to be robust and portable, hence we need to use encapsulation to shield the user from unintended consequences.

With this in mind, we use the following coding conventions:


We follow a C++ style naming convention.

  • Packages: CamelCase

  • Classes: CamelCase

  • Methods: camelCase

  • Functions: camelCase

  • Variables: snake_case

For example, to instantiate a minimisation protocol from the Protocol package:

import BioSimSpace as BSS
protocol = BSS.Protocol.Minimisation()


BioSimSpace is a collection of packages, e.g. BioSimSpace.Gateway and BioSimSpace.Protocol. Within each package is a set of modules that implement the required functionality. Rather than directly exposing all of the modules we choose to hide implementation details from the user. Instead we use the package to selectively import the required classes and functions.

  • Module files containing implementation details are prefixed with an underscore, i.e.

  • Where possible, external packages are hidden inside each module, e.g. import mdtraj as _mdtraj

  • Each module file contains an __all__ variable that lists the specific items that should be imported.

  • The package can be used to safely expose the required functionality to the user with:

from module import *

This results in a clean API and documentation, with all extraneous information, e.g. external modules, hidden from the user. This is important when working interactively, since IPython and Jupyter do not respect the __all__ variable when auto-completing, meaning that the user will see a full list of the available names when hitting tab. When following the conventions above, the user will only be able to access the exposed names. This greatly improves the clarity of the package, allowing a new user to quickly determine the available functionality. Any user wishing expose further implementation detail can, of course, type an underscore to show the hidden names when searching.


BioSimSpace aims to provide a means of writing robust and portable workflow components (nodes). To this end, we choose to use an object oriented approach where data is encapsulated, with getters used to retrieve data from an object.

To avoid unintended consequences, getters that return mutable data types, e.g. lists and dictionaries, should return a copy of the data. This prevents the user unintentionally modifying the private data contained in the object. Setters should be used to explicitly modify member data.

For example:

# A class that holds a list of numbers.

class MyClass():
    # A private class member variable containing a list of numbers.
    _list = [1, 2, 3, 4, 5]

    def getList(self):
        return self._list

# Create an instance of the class.
c = MyClass()
n = c.getList()
[1, 2, 3, 4, 5]

# Update n.

# The private member data has been modified!
[1, 2, 3, 4, 5, 6]

Instead use:

class MyClass():
    # A private class member variable containing a list of numbers.
    _list = [1, 2, 3, 4, 5]

    def getList(self):
        return self._list.copy()

# Create an instance of the class.
c = MyClass()
n = c.getList()
[1, 2, 3, 4, 5]

# Update n.

# The private member data is untouched.
[1, 2, 3, 4, 5]


BioSimSpace is built on top of the Sire and makes exstensive use of its API. Sire has recently been updated to a new, Python compliant API. Within BioSimSpace, Sire is currently imported in a mixed mode so that we can use both the new and legacy APIs while we work to port things over. A consequence of this is that Sire imports must follow those for BioSimSpace, e.g.:

from sire.mol import AtomIdx
import BioSimSpace as BSS

will fail, while the following will work:

import BioSimSpace as BSS
from sire.mol import AtomIdx


Feature branches#

First make sure that you are on the development branch of BioSimSpace:

git checkout devel

Now create and switch to a feature branch. This should be prefixed with feature, e.g.

git checkout -b feature-process

While working on your feature branch you won’t want to continually re-install in order to make the changes active. To avoid this, you can either make use of PYTHONPATH, e.g.

PYTHONPATH=$HOME/Code/BioSimSpace/python python

or use the develop argument when running the script, i.e.

python develop


When working on your feature it is important to write tests to ensure that it does what is expected and doesn’t break any existing functionality. Tests should be placed inside the test directory, creating an appropriately named sub-directory for any new packages.

The test suite is intended to be run using pytest. When run, pytest searches for tests in all directories and files below the current directory, collects the tests together, then runs them. Pytest uses name matching to locate the tests. Valid names start or end with test, e.g.:

# Files:

# Functions:
def test_func():   def func_test():

We use the convention of test_* when naming files and functions.

Running tests#

To run the full test suite, simply type:

pytest tests

To run tests for a specific sub-module, e.g.:

pytest tests/Process

To only run the unit tests in a particular file, e.g.:

pytest tests/Process/

To run a specific unit tests in a particular file, e.g.:

pytest tests/Process/

To get more detailed information about each test, run pytests using the verbose flag, e.g.:

pytest -v

More details regarding how to invoke pytest can be found here.

Writing tests#


Try to keep individual unit tests short and clear. Aim to test one thing, and test it well. Where possible, try to minimise the use of assert statements within a unit test. Since the test will return on the first failed assertion, additional contextual information may be lost.

Floating point comparisons#

Make use of the approx function from the pytest package for performing floating point comparisons, e.g:

from pytest import approx

assert 0.1 + 0.2 == approx(0.3)

By default, the approx function compares the result using a relative tolerance of 1e-6. This can be changed by passing a keyword argument to the function, e.g:

assert 2 + 3 == approx(7, rel=2)
Skipping tests#

If you are using test-driven development it might be desirable to write your tests before implementing the functionality, i.e. you are asserting what the output of a function should be, not how it should be implemented. In this case, you can make use of the pytest skip decorator to flag that a unit test should be skipped, e.g.:

@pytest.mark.skip(reason="Not yet implemented.")
def test_new_feature():
    # A unit test for an, as yet, unimplemented feature.
Parametrizing tests#

Often it is desirable to run a test for a range of different input parameters. This can be achieved using the parametrize decorator, e.g.:

import pytest
from operator import mul

@pytest.mark.parametrize("x", [1, 2])
@pytest.mark.parametrize("y", [3, 4])
def test_mul(x, y):
    """ Test the mul function. """
    assert mul(x, y) == mul(y, x)

Here the function test_mul is parametrized with two parameters, x and y. By marking the test in this manner it will be executed using all possible parameter pairs (x, y), i.e. (1, 3), (1, 4), (2, 3), (2, 4).


import pytest
from operator import sub
@pytest.mark.parametrize("x, y, expected",
                        [(1, 2, -1),
                         (7, 3,  4),
                         (21, 58, -37)])
def test_sub(x, y, expected):
    """ Test the sub function. """
    assert sub(x, y) == -sub(y, x) == expected

Here we are passing a list containing different parameter sets, with the names of the parameters matched against the arguments of the test function.

Testing exceptions#

Pytest provides a way of testing your code for known exceptions. For example, suppose we had a function that raises an IndexError:

def indexError():
    """ A function that raises an IndexError. """
    a = []

We could then write a test to validate that the error is thrown as expected:

def test_indexError():
    with pytest.raises(IndexError):
Custom attributes#

It’s possible to mark test functions with any attribute you like. For example:

def test_slow_function():
    """ A unit test that takes a really long time. """

Here we have marked the test function with the attribute slow in order to indicate that it takes a while to run. From the command line it is possible to run or skip tests with a particular mark.

pytest mypkg -m "slow"        # only run the slow tests
pytest mypkg -m "not slow"    # skip the slow tests

The custom attribute can just be a label, as in this case, or could be your own function decorator.


BioSimSpace is fully documented using NumPy style docstrings. See here for details. The documentation is automatically built using Sphinx whenever a commit is pushed to devel, which will then update this website.

To build the documentation locally you will first need to install some additional packages.

pip install sphinx sphinx_issues sphinx_rtd_theme

Then move to the doc directory and run:

sphinx-build make html

When finished, point your browser to build/html/index.html.


If you create new tests, please make sure that they pass locally before committing. When happy, commit your changes, e.g.

git commit python/BioSimSpace/Feature/ test/Feature/test_feature \
    -m "Implementation and test for new feature."

Remember that it is better to make small changes and commit frequently.

If your edits don’t change the BioSimSpace source code, or documentation, e.g. fixing typos, then please add ci skip to your commit message. This will avoid unnecessarily triggering GitHub actions, e.g. building a new BioSimSpace binary, updating the website, etc. To this end, we have provided a git hook that will append ci skip if the commit only modifies files in a blacklist that is specified in the file .ciignore (analogous to the .gitignore used to ignore untracked files). To enable the hook, simply copy it into the .git/hooks directory:

cp git_hooks/commit-msg .git/hooks

Any additional files or paths that shouldn’t trigger a re-build can be added to the .ciignore file.

Next, push your changes to the remote server, e.g.

# Push to the feature branch on the main BioSimSpace repo, if you have access.
git push origin feature

# Push to the feature branch your own fork.
git push fork feature

When the feature is complete, create a pull request on GitHub so that the changes can be merged back into the development branch. For information, see the documentation here.