Bazel build system can be used to manage Python code. It is convenient to have one build system for many different languages which is common for large projects. In addition Bazel provides at least two more advantages: it can run and cache tests and it does not pollute the code with pyc files.

This simple example demonstrates a possible way to organize python code + unittest + bazel in a convenient folder structure.

Bazel BUILD targets

We normally use Bazel rules py_binary and py_library for Python targets.

Example for our case:

py_binary(
    name = "sample",
    srcs = ["sample.py"],
)

Bazel test targets for python are defined using py_test rule.

py_test(
    name = "test_one",
    srcs = ["test/test_one.py"],
    deps = ["sample"],
)

py_test(
    name = "test_two",
    srcs = ["test/test_two.py"],
    deps = ["sample"],
)

py_test(
    name = "test_three",
    srcs = ["test/test_three.py"],
    deps = ["sample"],
)

Tests can be also grouped in suites by test_suite rule.

test_suite(
    name = "all_tests",
    tests = [
        "test_one",
        "test_two",
        "test_three",
    ],
)

With targets defined like above we can use bazel run and bazel test commands to execute python code and unit tests.

Advantages:

• Source code is separated from tests
• Folder is “clean” from *.pyc files (when using Bazel)
• Can run both Bazel test and Python unittest

Files:

├── sample.py              - Sample python module
├── BUILD                  - Bazel build file
├── WORKSPACE              - Bazel workspace file
└── test                   - Folder for unittests
     ├── test/run.py       - Test launcher
     └── test/test_*.py    - Unit tests

How to build?

Build all using Bazel

bazel build :*

How to run?

Run sample module using Bazel:

bazel run :sample

How to test?

Run all Bazel tests:

bazel test :*

or run Bazel test suite:

bazel test :all_tests

How to debugging tests?

You can also run all or particular tests using run_tests (test/run.py) launcher:

bazel run :run_tests

See options:

bazel run :run_tests -- -h

Increase verbosity:

bazel run :run_tests -- -vv

Run particular test case:

bazel run :run_tests -- test_one.TestOne

or particular test function:

bazel run :run_tests -- test_one.TestOne.test_one

Run test function with debug logs:

bazel run :run_tests -- test_one.TestOne.test_one -vvv

You can also run unit tests directly without Bazel with the same options:

python test/run.py

Note on test launcher

You have probably noticed python file test/run.py which is in fact optional and provides the possibility to run unittests in normal Python environment. It is just a wrapper script which parses arguments (using argparse), load tests (unittest.TestLoader) and finally calls unittest.TextTestRunner.

Note that the following command can be used instead of that script:

python -B -m unittest discover -s test