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How to Build Python Apps with Paketo Buildpacks

This documentation explains how to use the Paketo Python Buildpack to build applications for several common use-cases. For more in-depth description of the buildpack’s behavior and configuration see the Paketo Python Buildpack Reference documentation.

Build a Sample App

To build a sample app locally with this buildpack using the pack CLI, run

git clone https://github.com/paketo-buildpacks/samples
cd samples/python/pip
pack build my-app --buildpack paketo-buildpacks/python \
  --builder paketobuildpacks/builder:base
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See samples for how to run the app.

The Paketo Python Buildpack supports several popular configurations for Python apps.

Install a Specific CPython Version

The Python Cloud Native Buildpack allows you to specify a version of CPython 3 (reference implementation of Python 3) to use during deployment. This version can be specified via the BP_CPYTHON_VERSION environment variable during build. When specifying a version of CPython, you must choose a version that is available within the buildpack. The supported versions can be found in the release notes.

You may set BP_CPYTHON_VERSION using a platfrom-specific option, or using a project.toml as shown in the following example:

[build]
  [[build.env]]
    name = "BP_CPYTHON_VERSION"
    value = "3.6.*" # any valid semver constraints (e.g. 3.6.7, 3.*) are acceptable
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Specifying a version of CPython is not required. In the case this is not specified, the buildpack will provide the default version, which can be seen in the buildpack.toml file.

Some tools (like poetry) are able to detect the version of python defined in configuration files (like pyproject.toml). If present, the buildpack will use that specific version as long as it is supported.

Use a Package Manager

With the Python CNB, there are four options available for package management depending on your application:

You can find specific information for each option below.

Pip

Pip is a popular option for managing third-party application dependencies for Python apps. Including a valid requirements.txt file at the root of your app source code triggers the pip installation process by the buildpack. The buildpack will install the application packages and make it available to the app.

The buildpack allows you to configure the version of Pip to be used in the installation process. You can set this using the $BP_PIP_VERSION variable during build. When specifying a version of Pip, you must choose a version that is available within the buildpack. The supported versions can be found in the release notes.

Pipenv

Pipenv is another common option for managing dependencies. Including a valid Pipfile file at the root of your app source code triggers the pipenv installation process by the buildpack. The buildpack will install the application packages and make it available to the app.

The buildpack allows you to configure the version of Pipenv to be used in the installation process. You can set this using the $BP_PIPENV_VERSION variable during build. When specifying a version of Pipenv, you must choose a version that is available within the buildpack. The supported versions can be found in the release notes.

The buildpack also takes into consideration the Python version requirement specified by Pipfile.lock, but BP_CPYTHON_VERSION takes precedence over this as discussed in this section above.

Miniconda

Miniconda is a package management and environment management system supported by the Python buildpack. The buildpack will create or update a conda environment from an environment.yml file or a package-list.txt file located at the root of the app source code.

Configuring a version of miniconda is not supported.

Poetry

Poetry is a tool to manage both third-party application dependencies and virtual environments. Including a pyproject.toml file at the root of your app source code triggers the poetry installation process. The buildpack will invoke poetry to install the application dependencies defined in pyproject.toml and set up a virtual environment.

The buildpack allows you to configure the version of Poetry to be used in the installation process. You can set this using the $BP_POETRY_VERSION variable during build. When specifying a version of Poetry, you must choose a version that is available within the buildpack. The supported versions can be found in the release notes.

Enable Process Reloading

watchexec is a tool that can watch files for changes and run a command whenever it detects modifications. The Python buildpack can install this tool in your app container so that you can restart your server process when files in the app’s working directory change. This may facilitate a shorter feedback loop for iterating on code changes. This feature may be used in conjunction with a dev orchestrator like Tilt.

Using BP_LIVE_RELOAD_ENABLED

To make watchexec available in the app container, set the $BP_LIVE_RELOAD_ENABLED environment variable at build time, either by passing a flag to the platform or by adding it to your project.toml. See the Cloud Native Buildpacks documentation to learn more about project.toml files.

With a pack build flag

pack build myapp --env BP_LIVE_RELOAD_ENABLED=true
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In a project.toml file

[[ build.env ]]
  name = 'BP_LIVE_RELOAD_ENABLED'
  value = 'true'
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In a Tiltfile with the pack resource

You can use the Paketo Python buildpack with Tilt. This example uses the pack extension for Tilt.

pack('my-app',
  buildpacks=["paketo-buildpacks/python"],
  env_vars=["BP_LIVE_RELOAD_ENABLED=true"],
  live_update=[
    sync('.', '/workspace'),
  ]
)
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Setting a reloadable start command

You can then use a Procfile to set a start command for the app that uses watchexec. For instance, for an app whose entrypoint is server.py, you could use a Procfile as follows:

web: watchexec --verbose --restart --watch /workspace 'server.py'
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This will cause the server process to restart whenever changes in /workspace are detected. See watchexec documentation for more about how to configure the tool.

Install a Custom CA Certificate

Python Buildpack users can provide their own CA certificates and have them included in the container root truststore at build-time and runtime by following the instructions outlined in the CA Certificates section of our configuration docs.

Override the Start Process Set by the Buildpack

Python Buildpack users can set custom start processes for their app image by following the instructions in the Procfiles section of our configuration docs.

Set Environment Variables for App Launch Time

Python Buildpack users can embed launch-time environment variables in their app image by following the documentation for the Environment Variables Buildpack.

Add Custom Labels to the App Image

Python Buildpack users can add labels to their app image by following the instructions in the Applying Custom Labels section of our configuration docs.

Access the software bill of materials

The Python buildpack includes support for the software bill of materials (SBOM). Check out the SBOM how-to documentation for details on how to access the SBOM supplied by the buildpacks.

SBOMs will be generated for applications which leverage Pip, Pipenv, or Poetry.

Currently the Python buildpack has limited support for generating an SBOM for applications which leverage Miniconda. Specifically - in order to generate an SBOM for a Miniconda application, applications must vendor their dependencies in addition to defining them via a package-list.txt file. Miniconda applications that declare their dependencies via a package-list.txt file but do not vendor them will result in an empty SBOM. This is due to a limitation in the upstream SBOM generation library (Syft).

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Last modified: May 12, 2022