This page is meant to describe the development stack for PyBEL, and should be a useful introduction for contributors.


PyBEL is kept under version control on GitHub. This allows for changes in the software to be tracked over time, and for tight integration of the management aspect of software development. Code is produced following the Git Flow philosophy, which means that new features are coded in branches off of the development branch and merged after they are triaged. Finally, develop is merged into master for releases. If there are bugs in releases that need to be fixed quickly, “hot fix” branches from master can be made, then merged back to master and develop after fixing the problem.

Testing in PyBEL

PyBEL is written with extensive unit testing and integration testing. Whenever possible, PyBEL prefers to practice test- driven development. This means that new ideas for functions and features are encoded as blank classes/functions and directly writing tests for the desired output. After tests have been written that define how the code should work, the implementation can be written.

Test-driven development requires us to think about design before making quick and dirty implementations. This results in better code. Additionally, thorough testing suites make it possible to catch when changes break existing functionality.

Tests are written with the standard unittest library. Some functionality, such as the mock module, are only avaliable as default in Python 3, so backports must be used for testing in Python 2

Unit Testing

Unit tests check that the functionality of the different parts of PyBEL work independently.

An example unit test can be found in tests.test_parse_bel.TestAbundance.test_short_abundance. It ensures that the parser is able to handle a given string describing the abundance of a chemical/other entity in BEL. It tests that the parser produces the correct output, that the BEL statement is converted to the correct internal representation. In this example, this is a tuple describing the abundance of oxygen atoms. Finally, it tests that this representation is added as a node in the underlying BEL graph with the appropriate attributes added.

Integration Testing

Integration tests are more high level, and ensure that the software accomplishes more complicated goals by using many components. An example integration test is found in tests.test_import.TestImport.test_from_fileURL. This test ensures that a BEL script can be read and results in a NetworkX object that contains all of the information described in the script


While IDEs like PyCharm provide excellent testing tools, they are not programmatic. Tox is python package that provides a CLI interface to run automated testing procedures (as well as other build functions, that aren’t important to explain here). In PyBEL, it is used to run the unit tests in the tests folder with the py.test harness. It also runs check-manifest, builds the documentation with sphinx, and computes the code coverage of the tests. The entire procedure is defined in tox.ini. Tox also allows test to be done on many different versions of Python.

Continuous Integration

Continuous integration is a philosophy of automatically testing code as it changes. PyBEL makes use of the Travis CI server to perform testing because of its tight integration with GitHub. Travis automatically installs git hooks inside GitHub so it knows when a new commit is made. Upon each commit, Travis downloads the newest commit from GitHub and runs the tests configured in the .travis.yml file in the top level of the PyBEL repository. This file effectively instructs the Travis CI server to run Tox. It also allows for the modification of the environment variables. This is used in PyBEL to test many different versions of python.

Code Coverage

After building, Travis sends code coverage results to This site helps visualize untested code and track the improvement of testing coverage over time. It also integrates with GitHub to show which feature branches are inadequately tested. In development of PyBEL, inadequately tested code is not allowed to be merged into develop.


PyBEL uses semantic versioning. In general, the project’s version string will has a suffix -dev like in 0.3.4-dev throughout the development cycle. After code is merged from feature branches to develop and it is time to deploy, this suffix is removed and develop branch is merged into master.

The version string appears in multiple places throughout the project, so BumpVersion is used to automate the updating of these version strings. See .bumpversion.cfg for more information.


Code for PyBEL is open-source on GitHub, but it is also distributed on the PyPI (pronounced Py-Pee-Eye) server. Travis CI has a wonderful integration with PyPI, so any time a tag is made on the master branch (and also assuming the tests pass), a new distribution is packed and sent to PyPI. Refer to the “deploy” section at the bottom of the .travis.yml file for more information, or the Travis CI PyPI deployment documentation. As a side note, Travis CI has an encryption tool so the password for the PyPI account can be displayed publicly on GitHub. Travis decrypts it before performing the upload to PyPI.


  1. bumpversion release on development branch
  2. Push to git
  3. After tests pass, merge develop in to master
  4. After tests pass, create a tag on GitHub with the same name as the version number (on master)
  5. Travis will automatically deploy to PyPI after tests pass. After checking deployment has been successful, switch to develop and bumpversion patch