Development guide


Strawberry Fields requires the following libraries be installed:

as well as the following Python packages:

If you currently do not have Python 3 installed, we recommend Anaconda for Python 3, a distributed version of Python packaged for scientific computation.


For development purposes, it is recommended to install the Strawberry Fields source code using development mode:

git clone
cd strawberryfields
pip install -e .

The -e flag ensures that edits to the source code will be reflected when importing StrawberryFields in Python.

TensorFlow support

To use Strawberry Fields with TensorFlow, version 2.0 of TensorFlow (or higher) is required. This can be installed alongside Strawberry Fields as follows:

pip install strawberryfields tensorflow

Or, to install Strawberry Fields and TensorFlow with GPU and CUDA support:

pip install strawberryfields tensorflow-gpu

Software tests

The Strawberry Fields test suite requires pytest and pytest-cov for coverage reports. These can both be installed via pip:

pip install pytest pytest-cov

To ensure that Strawberry Fields is working correctly after installation, the test suite can be run by navigating to the source code folder and running

make test

Note that this runs all of the tests, using all available backends, so can be quite slow (it should take around 40 minutes to complete). Alternatively, you can run the full test suite for a particular component by running

make test-[component]

where [component] should be replaced with either frontend for the Strawberry Fields frontend UI, apps for the applications layer, or one of the backends you would like to test (fock, tf, or gaussian).

Pytest can accept a boolean logic string specifying exactly which tests to run, if finer control is needed. For example, to run all tests for the frontend and the Gaussian backend, as well as the Fock backend (but only for pure states), you can run:

make test-"gaussian or frontend or (fock and pure)"

The above syntax also works for the make coverage command, as well as make batch-test command for running the tests in batched mode.

Individual test modules are run by invoking pytest directly from the command line:

pytest tests/


Adding tests to Strawberry Fields

The tests folder is organised into several subfolders:

  • backend for tests that only import a Strawberry Fields backend,

  • frontend for tests that import the Strawberry Fields UI but do not make use of a backend,

  • integration for tests that verify integration of the frontend and backends,

  • apps for tests of the applications layer

  • api for tests that only import and use the strawberryfields.api package

When writing new tests, make sure to mark what components they test.

Certain tests that are related to a specific backend, e.g. test cases for its operations or the states returned by a backend. For a backend test, you can use the backends mark, which accepts the names of the backends:

pytest.mark.backends("fock", "gaussian")

For specific test cases, the decorator can be used to mark only classes and test functions:

@pytest.mark.backends("fock", "gaussian")
def test_fock_and_gaussian_feature():

Adding tests for an engine, operations, parameters and other parts of the user interface can be added as part of the frontend tests. For a frontend-only test, you can use the frontend mark:


This could then be used on the module level to mark not just a single test case, but the entire test file as a frontend test:

mark = pytest.mark.frontend


Run options for Strawberry Fields tests

Several run options can be helpful for testing Strawberry Fields.

Marks mentioned in the previous section are useful also when running tests and selecting only certain tests to be run. They can be specified by using the -m option for pytest.

The following command can be used for example, to run tests related to the "Fock" backend:

pytest -m fock

When running tests, it can also be useful to examine a single failing test. The following command stops at the first failing test:

pytest -x

For further useful options (e.g. -k, -s, --tb=short, etc.) refer to the pytest --help command line usage description or the pytest online documentation.

Test coverage

Test coverage can be checked by running

make coverage

The output of the above command will show the coverage percentage of each file, as well as the line numbers of any lines missing test coverage.

To obtain coverage, the pytest-cov plugin is needed.

The coverage of a specific file can also be checked by generating a report:

pytest tests/backend/ --cov=strawberryfields/location/to/module --cov-report=term-missing

Here the coverage report will be created relative to the module specified by the path passed to the --cov= option.

The previously mentioned pytest options can be combined with the coverage options. As an example, the -k option allows you to pass a boolean string using file names, test class/test function names, and marks. Using -k in the following command we can get the report of a specific file while also filtering out certain tests:

pytest tests/backend/ --cov --cov-report=term-missing -k 'not TestBaseGaussianMethods'

Passing the --cov option without any modules specified will generate a coverage report for all modules of Strawberry Fields.


Additional packages are required to build the documentation, as specified in doc/requirements.txt. These packages can be installed using:

pip install -r doc/requirements.txt

from within the top-level directory. To then build the HTML documentation, run

make docs

The documentation can be found in the doc/_build/html/ directory.

Adding a new module to the docs

There are several steps to adding a new module to the documentation:

  1. Make sure your module has a one-to-two line module docstring, that summarizes what the module purpose is, and what it contains.

  2. Add a file doc/code/sf_module_name.rst, that contains the following:

    .. currentmodule:: strawberryfields.module_name
    .. automodapi:: strawberryfields.module_name
        :skip: <Place objects that shouldn't be documented here>
  3. Add code/sf_module_name to the table of contents at the bottom of doc/index.rst.

Adding a new package to the docs

Adding a new subpackage to the documentation requires a slightly different process than a module:

  1. Make sure your package file has a one-to-two line module docstring, that summarizes what the package purpose is, and what it contains.

  2. At the bottom of the docstring, add an autosummary table that contains all modules in your package:

    .. currentmodule:: strawberryfields.package_name
    .. autosummary::
       :toctree: api

    All modules should also contain a module docstring that summarizes the module.

  3. Add a file doc/code/sf_package_name.rst, that contains the following:

    .. rubric:: Modules
    .. automodule:: strawberryfields.package_name
  4. Add code/sf_package_name to the table of contents at the bottom of doc/index.rst.

Submitting a pull request

Before submitting a pull request, please make sure the following is done:

  • All new features must include a unit test. If you’ve fixed a bug or added code that should be tested, add a test to the tests directory.

    Strawberry Fields uses pytest for testing; common fixtures can be found in the tests/ file.

  • All new functions and code must be clearly commented and documented.

    Have a look through the source code at some of the existing function docstrings— the easiest approach is to simply copy an existing docstring and modify it as appropriate.

    If you do make documentation changes, make sure that the docs build and render correctly by running make docs.

  • Ensure that the test suite passes, by running make test.

  • Make sure the modified code in the pull request conforms to the PEP8 coding standard.

    The Strawberry Fields source code conforms to PEP8 standards. Before submitting the PR, you can autoformat your code changes using the Black Python autoformatter, with max-line length set to 100:

    black -l 100 strawberryfields/path/to/modified/

    We check all of our code against Pylint for errors. To lint modified files, simply pip install pylint, and then from the source code directory, run

    pylint strawberryfields/path/to/modified/

When ready, submit your fork as a pull request to the Strawberry Fields repository, filling out the pull request template. This template is added automatically to the comment box when you create a new issue.

  • When describing the pull request, please include as much detail as possible regarding the changes made/new features added/performance improvements. If including any bug fixes, mention the issue numbers associated with the bugs.

  • Once you have submitted the pull request, three things will automatically occur:

    • The test suite will automatically run on Travis CI to ensure that all tests continue to pass.

    • Once the test suite is finished, a code coverage report will be generated on Codecov. This will calculate the percentage of Strawberry Fields covered by the test suite, to ensure that all new code additions are adequately tested.

    • Finally, the code quality is calculated by Codefactor, to ensure all new code additions adhere to our code quality standards.

Based on these reports, we may ask you to make small changes to your branch before merging the pull request into the master branch. Alternatively, you can also grant us permission to make changes to your pull request branch.