Developer’s guide#

The source code for sire is available on GitHub.

Setting up your computer#

You first need to create an anaconda, miniconda or miniforge environment as described in the installation page.

We recommend using miniforge, as this sets the right priority for the conda-forge channel.

Virtual environments#

We recommend that you develop sire in its own conda environment. For example, you could call this environment openbiosim.

It is worth activating this environment during development and testing, e.g. via

$ source /path/to/environment/bin/activate

(where /path/to/environment is the file path to your environment)

or by running

$ conda activate environment_name

(where environment_name should be replaced by the name of your environment - e.g. openbiosim).

This will update your shell so that all python commands (such as python, conda etc.) will use the virtual environment. You can deactivate the environment and return to the “standard” Python using;

$ conda deactivate

Python Coding Style#

Sire is written predominantly in C++. This was for speed and memory efficiency. The Sire C++ objects are wrapped into Python using Py++.

The legacy API was very C++, and thus not very pythonic in nature.

We have engaged in a modernisation program, and now (nearly?) all Python-exposed or Python-native code in the public API is written in a Pythonic style. We aim to be fully PEP8-compliant and ask that all new Python code contributed to sire is written to be PEP8-compliant. We code using flake8 as a linter, and use black as a code autoformatter (format on save). We encourage all developers to use flake8 and black. These are easy to configure and use via your IDE (i.e. we run them automatically in VSCode). The will help ensure we have a consistent code style.

C++ Coding Style#

C++ code style is used for names, with the code written to be strictly C++ 2014 conformant (although we welcome requests to move to a newer C++ standard, if this is justifiable). The code is very portable and should remain so. We ourselves are running production code on X86-64 and ARM64 processors, on Linux, MacOS and Windows. We know of people who compile and use sire on PowerPC.

We have a strict C++ coding style, which is described here.

For ease of installation and support, we require that dependencies are available in conda-forge. As a last resort, we will vendor dependencies, but this does introduce a significant extra support burden.


With this in mind, we use the following conventions:

  • Packages: Lowercase, singleword

  • Classes: CamelCase

  • Methods: snake_case for pure Python, lowerCamelCase for C++

  • Functions: snake_case for pure Python, lowerCamelCase for C++

  • Variables: snake_case for pure Python, should not be used in C++ (variables should be private). But public Python variables are discouraged. Private variables should be preferred, named using a leading underscore.

  • Source Files: snake_case with a leading underscore for pure Python, lowerclassname.cpp / lowerclassname.h for C++

  • __all__ should be used in Python to expose the public API of a file or module. This is used to control what is seen using tab completion in ipython / notebooks, and what is extracted by sphinx to form the API documentation on the website.

  • Documentation - use doxygen style comments for C++ and numpy-style documentation for Python. All functions / classes in the public API should be documented.

Functions or variables in Python that are private should be named with a leading underscore. This prevents them from being prominantly visible in Python’s help and tab completion. Any C++ code should only use private variables, and should use private or protected as much as possible to reduce the API of C++ classes.


Feature branches#

First make sure that you are on the development branch of sire

git checkout devel

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

git checkout -b feat-process


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 tests directory, and should be designed to be run using pytest. Note that you should not place any input files or structure files in the tests directory. Instead, they should be placed on the web, and downloaded using sire.load() via their URL. When we accept your pull request we will move your input files onto the main website and will update your test to download the files from there.

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():
   # code to perform tests...

def func_test():
   # code to perform tests...

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

Running tests#

To run the full test suite, simply run pytest pointing to the tests directory, e.g.

pytest tests

Tests for each module are in a directory named after that module.

To run tests for a specific sub-module, e.g. sire.mol type

pytest tests/mol

To only run the unit tests in a particular file, e.g. tests/mol/, you can type

pytest tests/mol/

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

pytest -v tests

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

Writing tests#


Try to keep individual unit tests clear and fast. The aim is that they should test a single part of the code, and should complete in seconds (if not quicker). Use fixtures to re-use files that have been downloaded and parsed as much as possible. These are all defined in the file tests/

Reading files#

Please place all file reads (and any associated common setup) behind a pytest.fixture. For example, you could add your file read to which is where we define all of the test systems that are loaded and used for the tests.

You can then use the fixture in your test by passing in the name as an argument, e.g.

def test_myfunc(ala_mols):
   # ala_mols is the ala_mols() pytest.fixture() - assign this to `mols`
   mols = ala_mols

   # if you need to edit it, then please make a copy!
   import sire as sr
   mols = sr.system.System(ala_mols)

   # or use copy.deepcopy
   import copy
   mols = copy.deepcopy(ala_mols)

   # or copy.copy (it is already a deepcopy)
   mols = copy.copy(ala_mols)

Writing temporary files#

Please use the tmpdir fixture to write any temporary files into a temporary directory for the test. This ensures that you are not accidentally creating temporary files that make their way into the repo, or that clash with those created by other tests.

You should create a temporary directory in the tmpdir for your test, and then create the files as needed. For example;

def test_myfunc(tmpdir, ala_mols):
   mols = ala_mols

   # create my temporary directory
   d = tmpdir.mkdir("test_myfunc")

   # write a file into this directory
   import sire as sr
   f =, d.join("output"), format="PRM7")

   # read back in again...
   mols2 = sr.load(f[0])

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.

Please do use slow to mark tests that take more than 3 seconds to run, and use veryslow for tests that take more than 10 seconds to run.

To keep tests running quickly, please do not do any setup or initialisation outside the test function. Import modules within the test functions, and make use of fixtures (as described above) to ensure any file loads are shared across all tests.

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):

Continuous integration and delivery#

We use GitHub Actions to run a full continuous integration (CI) on all pull requests to devel and main, and all pushes to devel and main. We will not merge a pull request until all tests pass. We only accept pull requests to devel. Only the release managers and accept pull requests to devel.

Only the release managers can make and accept pull requests from devel to main, and only as part of creating a new release of sire. In addition to CI, we also perform a build of the website on pushes to devel and tags to main. Finally, we have set up continuous delivery (CD) on pushes to main and devel, which build and upload the conda packages.


Sire is fully documented using a combination of hand-written files (in the doc folder) and auto-generated api documentation created from NumPy style docstrings. See here for details. The documentation is automatically built using Sphinx.

To build the documentation locally you will first need to install some additional packages as described in the requirements.txt

conda install sphinx sphinxcontrib-programoutput sphinx-issues furo

Then move to the doc directory and run:


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


If you create new tests, please make sure that they pass locally before commiting. Please also check that all your Python code is formatted to be PEP8-compliant. This will be easier if you use an autoformatter such as black.

When happy, commit your changes, e.g.

git commit -a -m "Implementation and test for new feature."

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

Next, make sure that you have no conflicts with the devel branch. Pull this branch via;

git pull origin devel

and resolve any conflicts that appear (ideally by modifying your code). Please feel free to get in touch if there are many conflicts or you need to modify lots of other code in sire.

Remember to then recompile your code and check that all of the unit tests (including your new tests) pass.

If your edits don’t change the sire source code, or documentation, e.g. fixing typos, then please add ci skip to your commit message, e.g.

git commit -a -m "Updating docs [ci skip]"

This will avoid unnecessarily running the GitHub Actions, e.g. building a new sire` package, updating the website, etc. (the GitHub actions are configured in the file .github/workflows/main.yaml).

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

git push

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.


First, thanks to you for your interest in sire and for reading this far. We hope you enjoy having a play with the code and having a go at adding new functionality, fixing bugs, writing docs etc.

We would also like to thank Lester Hedges and the BioSimSpace team who provided great advice to set up the above, and from whose GitHub repo most of the procedures, scripts and documentation above is derived.