Write a simple application

This course assumes that you selected the “Python” option when you ran aspect init to create a new repository and also enabled Code Generation, Formatting and Linting, Version Stamping, and OCI Containers.

Requests example code

Add the “getting started” code for the Python “requests” module. It gives a good example of installing an external package from PyPI.

Create a folder named app, then create a file __main__.py (double underscores before and after main). This is the Python convention for an executable entry-point.

Copy the first code block from https://www.w3schools.com/python/module_requests.asp:

import requests

x = requests.get('https://w3schools.com/python/demopage.htm')

print(x.text)

Declare and pin requirements

Since the requests package is a new dependency of the repository, you first need to declare that. This example uses a pyproject.toml. Bazel’s rules_python supports the simpler requirements.txt format as well.

Add the “requests” string to the dependencies section, which should now look like this:

dependencies = [
    "requests",
]

Next you need to “compile” or “pin” the dependencies. Bazel can only provide reproducible builds if you give fully-locked description of the dependencies. There are a few options for pip compile tools. This exercise uses uv pip compile which comes from rules_uv, and it provides executable targets to update the pinned dependencies, which you can use with bazel run //requirements:runtime.update.

Take a look in the requirements folder. There are files for different purposes; for example test.in lists the pytest dependency needed for testing. Since there are multiple pinned dependencies, it would require multiple bazel run commands to update everything. The repository has a convenience script to make this easy, so you should only need to run:

% ./tools/repin

This results in updates to the requirements/all.in file, which has the complete, constraint-solved set of dependencies for our repo. It includes hashes of each package for better supply-chain security.

This file is used by rules_python to install dependencies. Note in MODULE.bazel that it’s referenced by a pip.parse module extension:

pip.parse(
    hub_name = "pip",
    python_version = "3.12",
    requirements_lock = "//requirements:all.txt",
)

Now you should be ready to use the dependency from Bazel-managed code.

Generate BUILD files

For Bazel to understand the Python code, it expects BUILD files containing Python rules. Since you’ve written a program, you need to add a py_binary rule to make it runnable.

As you learned in the Bazel 101 course, you only need to run aspect configure.

Aspect compiled the Gazelle extension from rules_python, which is much easier than building it yourself. If you don’t use the Aspect CLI, see the 203 course for a guide to installing Gazelle.

Now look in app/BUILD.bazel to see the generated content.

At this point the py_binary should be functional, so you can run it with the following command:

% bazel run app:app_bin

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