Pigweed’s GN Python Build#

See also

Python GN Templates for detailed template usage.
pw_build for other GN templates available within Pigweed.
Build system for a high level guide and background information on Pigweed’s build system as a whole.

Pigweed uses a custom GN-based build system to manage its Python code. The Pigweed Python build supports packaging, installation and distribution of interdependent local Python packages. It also provides for fast, incremental static analysis and test running suitable for live use during development (e.g. with pw_watch) or in continuous integration.

Pigweed’s Python code is exclusively managed by GN, but the GN-based build may be used alongside CMake, Bazel, or any other build system. Pigweed’s environment setup uses GN to set up the initial Python environment, regardless of the final build system. As needed, non-GN projects can declare just their Python packages in GN.

How it Works#

In addition to compiler commands a Pigweed GN build will execute Python scripts for various reasons including running tests, linting code, generating protos and more. All these scripts are run as part of a pw_python_action GN template which will ultimately run python. Running Python on it’s own by default will make any Python packages installed on the users system available for importing. This is not good and can lead to flaky builds when different packages are installed on each developer workstation. To get around this the Python community uses virtual environments (venvs) that expose a specific set of Python packages separate from the host system.

When a Pigweed GN build starts a single venv is created for use by all pw_python_actions throughout the build graph. Once created, all required third-party Python packages needed for the project are installed. At that point no further modifications are made to the venv. Of course if a new third-party package dependency is added it will be installed too. Beyond that all venvs remain static. More venvs can be created with the pw_python_venv template if desired, but only one is used by default.

Every pw_python_action is run inside a venv

flowchart LR out[GN Build Dir<br/>fa:fa-folder out] out -->|ninja -C out| createvenvs createvenvs(Create venvs) createvenvs --> pyactions1 createvenvs --> pyactions2 subgraph pyactions1[Python venv 1] direction TB venv1(fa:fa-folder out/python-venv &nbsp) a1["pw_python_action('one')"] a2["pw_python_action('two')"] venv1 --> a1 venv1 --> a2 end subgraph pyactions2[Python venv 2] direction TB venv2(fa:fa-folder out/another-venv &nbsp) a3["pw_python_action('three')"] a4["pw_python_action('four')"] venv2 --> a3 venv2 --> a4 end

Note

Pigweed uses this venv target if a project does not specify it’s own build venv. See Build Time Python Virtualenv on how to define your own default venv.

Having a static venv containing only third-party dependencies opens the flood gates for python scripts to run. If the venv only contains third-party dependencies you may be wondering how you can import your own in-tree Python packages. Python code run in the build may still import any in-tree Python packages created with pw_python_package templates. However this only works if a correct python_deps arg is provided. Having that Python dependency defined in GN allows the pw_python_action to set PYTHONPATH so that given package can be imported. This has the benefit of the build failing if a dependency for any Python action or package is missing.

Benefits of Python venvs in GN

Using venvs to execute Python in GN provides reproducible builds with fixed third-party dependencies.
Using PYTHONPATH coupled with python_deps to import in-tree Python packages enforces dependency correctness.

Managing Python Requirements#

Build Time Python Virtualenv#

Pigweed’s GN Python build infrastructure relies on Python virtual environments for executing Python code. This provides a controlled isolated environment with a defined set of third party Python constraints where all Python tests, linting and pw_python_action targets are executed.

There must be at least one venv for Python defined in GN. There can be multiple venvs but one must be the designated default.

The default build venv is specified via a GN arg and is best set in the root .gn or BUILD.gn file. For example:

pw_build_PYTHON_BUILD_VENV = "//:project_build_venv"

Tip

Additional pw_python_venv targets can be created as needed. The pw_python_action template can take an optional venv argument to specify which Python venv it should run within. If not specified the target referred in the pw_build_PYTHON_BUILD_VENV is used.

Third-party Python Requirements and Constraints#

Your project may have third party Python dependencies you wish to install into the bootstrapped environment and in the GN build venv. There are two main ways to add Python package dependencies:

Adding Requirements Files

Add a install_requires entry to a setup.cfg file defined in a pw_python_package template. This is the best option if your in-tree Python package requires an external Python package.
Create a standard Python requirements.txt file in your project and add it to the pw_build_PIP_REQUIREMENTS GN arg list.

Requirements files support a wide range of install locations including packages from pypi.org, the local file system and git repos. See pip’s Requirements File documentation for more info.

The GN arg can be set in your project’s root .gn or BUILD.gn file.
```
pw_build_PIP_REQUIREMENTS = [
  # Project specific requirements
  "//tools/requirements.txt",
]
```
See the GN File Structure for Python Code section below for a full code listing.

Adding Constraints Files

Every project should ideally inherit Pigweed’s third party Python package version. This is accomplished via Python constraints files. Constraints control which versions of packages get installed by pip if that package is installed. To inherit Pigweed’s Python constraints include constraint.list from the pw_env_setup module from in your top level .gn file. Additonal project specific constraints can be appended to this list.

pw_build_PIP_CONSTRAINTS = [
  "$dir_pw_env_setup/py/pw_env_setup/virtualenv_setup/constraint.list",
  "//tools/constraints.txt",
]

In-tree `pw_python_package` Requirements#

A given venv inherits a project’s requirements and constraint files by default via the pw_build_PIP_CONSTRAINTS and pw_build_PIP_REQUIREMENTS GN args as described above. This can be overridden if needed.

`generated_requirements.txt`#

To ensure the requirements of in-tree pw_python_package targets are installed pw_python_venv introduces the source_packages argument. This is a list of in-tree pw_python_package GN targets expected to be used within the venv. When the venv is created each pw_python_package’s setup.cfg file is read to pull the install_requires section for all third party dependencies. The full list of all in-tree packages and any in-tree transitive dependencies is then written to the out directory in a single generated_requirements.txt.

Take the //pw_build/py/gn_tests:downstream_tools_build_venv example below, its source package is a single pw_python_distribution package which bundles the pw_env_setup and pw_console pw_python_package``s. Those two packages each depend on a few other ``pw_python_package targets. The output generated_requirements.txt below merges all these package deps and adds -c lines for constraint files.

See also

The pip documentation on the Requirements File Format

pw_python_distribution("downstream_project_tools") {
  packages = [
    "$dir_pw_env_setup/py",
    "$dir_pw_console/py",
  ]
  generate_setup_cfg = {
    name = "downstream_project_tools"
    version = "0.0.1"
    include_default_pyproject_file = true
  }
}

pw_python_venv("downstream_tools_build_venv") {
  path = "$root_build_dir/python-venv-downstream-tools-test"
  requirements = []
  constraints =
      [ "$dir_pw_env_setup/py/pw_env_setup/virtualenv_setup/constraint.list" ]
  source_packages = [ ":downstream_project_tools" ]
}

out/python/gen/pw_build/py/gn_tests/downstream_tools_build_venv/generated_requirements.txt#

# Auto-generated requirements.txt from the following packages:
#
# //pw_arduino_build/py:py
# //pw_build/py/gn_tests:downstream_project_tools
# //pw_build/py:py
# //pw_cli/py:py
# //pw_console/py:py
# //pw_env_setup/py:py
# //pw_log_tokenized/py:py
# //pw_package/py:py
# //pw_presubmit/py:py
# //pw_stm32cube_build/py:py

# Constraint files:
-c ../../../../../../../pw_env_setup/py/pw_env_setup/virtualenv_setup/constraint.list

black>=23.1.0
build>=0.8.0
coloredlogs
coverage
ipython
jinja2
mypy>=0.971
parameterized
pip-tools>=6.12.3
prompt-toolkit>=3.0.26
psutil
ptpython>=3.0.20
pygments
pylint>=2.9.3
pyperclip
pyserial>=3.5,<4.0
pyyaml
setuptools
six
toml
types-pygments
types-pyserial>=3.5,<4.0
types-pyyaml
types-setuptools
types-six
websockets
wheel
yapf>=0.31.0

`compiled_requirements.txt`#

The above generated_requirements.txt file is then fed into the pip-compile command from the pip-tools package to fully expand and pin each package with hashes. The resulting compiled_requirements.txt can then be used as the single Python requirements file for replicating this pw_python_venv elsewhere. Each pw_python_venv will get this single file containing the exact versions of each required Python package.

Tip

The compiled_requirements.txt generated by a pw_python_venv is used by the pw_python_zip_with_setup template when producing a self contained zip of in-tree and third party Python packages.

Below is a snippet of the compiled_requirements.txt for this pw_python_venv target: //pw_build/py/gn_tests:downstream_tools_build_venv

out/python/gen/pw_build/py/gn_tests/downstream_tools_build_venv/compiled_requirements.txt#

#
# This file is autogenerated by pip-compile with Python 3.11
# by the following command:
#
#    pip-compile --allow-unsafe --generate-hashes
#      --output-file=python/gen/pw_build/py/gn_tests/downstream_tools_build_venv/compiled_requirements.txt
#      --resolver=backtracking
#      python/gen/pw_build/py/gn_tests/downstream_tools_build_venv/generated_requirements.txt
#
appdirs==1.4.4 \
    --hash=sha256:7d5d0167b2b1ba821647616af46a749d1c653740dd0d2415100fe26e27afdf41 \
    --hash=sha256:a841dacd6b99318a741b166adb07e19ee71a274450e68237b4650ca1055ab128
    # via
    #   -c python/gen/pw_build/py/gn_tests/downstream_tools_build_venv/../../../../../../../pw_env_setup/py/pw_env_setup/virtualenv_setup/constraint.list
    #   ptpython
astroid==2.14.2 \
    --hash=sha256:0e0e3709d64fbffd3037e4ff403580550f14471fd3eaae9fa11cc9a5c7901153 \
    --hash=sha256:a3cf9f02c53dd259144a7e8f3ccd75d67c9a8c716ef183e0c1f291bc5d7bb3cf
    # via
    #   -c python/gen/pw_build/py/gn_tests/downstream_tools_build_venv/../../../../../../../pw_env_setup/py/pw_env_setup/virtualenv_setup/constraint.list
    #   pylint
...

The presence of hashes in the above example can be controlled via the pip_generate_hashes arg to the pw_python_venv template.

Caching Python Packages for Offline Installation#

Downloading Packages#

The pw_python_venv target adds an optional sub target that will download all Python packages from remote servers into a local directory. The remote server is typically pypi.org.

Taking the //pw_build/py/gn_tests:downstream_tools_build_venv target as an example again let’s build a local cache. To run the download target append .vendor_wheels to the end of the pw_python_venv target name. In this example it would be //pw_build/py/gn_tests:downstream_tools_build_venv.vendor_wheels

To build that one gn target with ninja, pass the output name from gn as a target name for ninja:

gn gen out
ninja -C out \
  $(gn ls out --as=output \
    '//pw_build/py/gn_tests:downstream_tools_build_venv.vendor_wheels')

This creates a wheels folder with all downloaded packages and a pip_download_log.txt with verbose logs from running pip download.

Vendor wheels output directory#

out/python/gen/pw_build/py/gn_tests/downstream_tools_build_venv.vendor_wheels/
├── pip_download_log.txt
└── wheels
    ├── appdirs-1.4.4-py2.py3-none-any.whl
    ├── astroid-2.14.2-py3-none-any.whl
    ├── backcall-0.2.0-py2.py3-none-any.whl
    ├── black-23.1.0-cp311-cp311-manylinux_2_17_x86_64.manylinux2014_x86_64.whl
    ├ ...
    ├── websockets-10.4-cp311-cp311-manylinux_2_5_x86_64.manylinux1_x86_64.manylinux_2_17_x86_64.manylinux2014_x86_64.whl
    ├── wheel-0.40.0-py3-none-any.whl
    ├── wrapt-1.14.1.tar.gz
    └── yapf-0.31.0-py2.py3-none-any.whl

Note the above output has both Python wheel .whl and source distribution .tar.gz files. The .whl may contain Python packages with precompiled C extensions. This is denoted by this part of the filename: cp311-cp311-manylinux_2_17_x86_64.whl. These binary packages are selected by the pip download command based on the host machine python version, OS, and CPU architecture.

Warning

If you need to cache Python packages for multiple platforms the .vendor_wheels target will need to be run for each combination of Python version, host operating system and architecture. For example, look at the files available for numpy. Some combinations are:

cp311, manylinux_2_17_x86_64
cp311, manylinux2014_x86_64
cp311, macosx_11_0_arm64
cp311, macosx_10_9_x86_64
cp311, win_amd64
cp311, win32

Plus all of the above duplicated for Python 3.10 and 3.9 (cp310 and cp39).

The output of multiple .vendor_wheels runs on different host systems can all be merged into the same output directory.

.vendor_wheels can attempt to download binary packages for multiple platforms all at once by setting a GN arg:

pw_build_PYTHON_PIP_DOWNLOAD_ALL_PLATFORMS = true

This will invoke pip download for each combination of platform, architecture and Python version. This can take a significant amount of time to complete. The current set of combinations is shown below:

    # These platform args are derived from the cffi pypi package:
    #   https://pypi.org/project/cffi/#files
    # See also these pages on Python wheel filename format:
    #   https://peps.python.org/pep-0491/#file-name-convention
    # and Platform compatibility tags:
    #   https://packaging.python.org/en/latest/specifications/
    #      platform-compatibility-tags/
    platform_args = [
        '--platform=any',
        '--platform=macosx_10_9_universal2',
        '--platform=macosx_10_9_x86_64',
        '--platform=macosx_11_0_arm64',
        '--platform=manylinux2010_x86_64',
        '--platform=manylinux2014_aarch64',
        '--platform=manylinux2014_x86_64',
        '--platform=manylinux_2_17_aarch64',
        '--platform=manylinux_2_17_x86_64',
        '--platform=musllinux_1_1_x86_64',
        '--platform=win_amd64',
        # Note: These 32bit platforms are omitted
        # '--platform=manylinux2010_i686',
        # '--platform=manylinux2014_i686',
        # '--platform=manylinux_2_12_i686'
        # '--platform=musllinux_1_1_i686',
        # '--platform=win32',
    ]

    # Pigweed supports Python 3.8 and up.
    python_version_args = [
        [
            '--implementation=py3',
            '--abi=none',
        ],
        [
            '--implementation=cp',
            '--python-version=3.8',
            '--abi=cp38',
        ],
        [
            '--implementation=cp',
            '--python-version=3.9',
            '--abi=cp39',
        ],
        [
            '--implementation=cp',
            '--python-version=3.10',
            '--abi=cp310',
        ],
        [
            '--implementation=cp',
            '--python-version=3.11',
            '--abi=cp311',
        ],
    ]

Warning

The set of Python packages that will be downloaded is determined by the compiled_requirements.txt file. This file can only be generated for the current host OS and Python version. pip-tools does not expand requirements for platform specific dependencies. For example ipython defines these two requirements:

appnope; sys_platform == "darwin"
colorama; sys_platform == "win32"

If pip-tools is run on Linux then the above packages will not appear in compiled_requirements.txt and not downloaded by the .vendor_wheels target.

Installing Offline#

Once the vendor wheel output is saved to a directory in your project you can use this as the default pip install location in two different ways.

GN Args#

Setting these args in the //.gn file will add the relevant pip command line args to perform offline installations.

# Adds --no-index forcing pip to not reach out to the internet (pypi.org) to
# download packages. Using this option requires setting
# pw_build_PYTHON_PIP_INSTALL_FIND_LINKS as well.
pw_build_PYTHON_PIP_INSTALL_OFFLINE = true

# List of paths to folders containing Python wheels (*.whl) or source tar
# files (*.tar.gz). Pip will check each of these directories when looking for
# potential install candidates.
pw_build_PYTHON_PIP_INSTALL_FIND_LINKS = [
  "//environment/cipd/packages/python_packages/universal",
  "//environment/cipd/packages/python_packages/linux/cp311",
]

# Optional: Adds '--no-cache-dir' forcing pip to ignore any previously cached
# Python packages. On most systems this is located in ~/.cache/pip/
pw_build_PYTHON_PIP_INSTALL_DISABLE_CACHE = false

Using a `.pip.conf` File#

Create a //pip.conf file containing:
//pip.conf#
```
[global]
# Disable searching pypi.org for packages
no-index = True
# Find packages in these directories:
find-links =
    file://third_party/python_packages/universal
    file://third_party/python_packages/linux/cp311
```
This tells pip to not search pypi.org for packages and only look in third_party/python_packages/universal and third_party/python_packages/linux/cp311. These paths can be absolute or are relative to the pip.conf file.
In the project bootstrap.sh set PIP_CONFIG_FILE to the location of this file.
```
export PIP_CONFIG_FILE="${PW_PROJECT_ROOT}/pip.conf"
```
With that environment var set all invocations of pip will apply the config file settings above.

GN File Structure for Python Code#

Here is a full example of what is required to build Python packages using Pigweed’s GN build system. A brief file hierarchy is shown here with file content following. See also Pigweed Module Structure for Python Code below for details on the structure of Python packages.

Top level GN file hierarchy#

project_root/
├── .gn
├── BUILDCONFIG.gn
├── build_overrides/
│   └── pigweed.gni
├── BUILD.gn
│
├── python_package1/
│   ├── BUILD.gn
│   ├── setup.cfg
│   ├── setup.py
│   ├── pyproject.toml
│   │
│   ├── package_name/
│   │   ├── module_a.py
│   │   ├── module_b.py
│   │   ├── py.typed
│   │   └── nested_package/
│   │       ├── py.typed
│   │       └── module_c.py
│   │
│   ├── module_a_test.py
│   └── module_c_test.py
│
├── third_party/
│   └── pigweed/
│
└── ...

project_root/

.gn

buildconfig = "//BUILDCONFIG.gn"
import("//build_overrides/pigweed.gni")

default_args = {
  pw_build_PIP_CONSTRAINTS = [
    # Inherit Pigweed Python constraints
    "$dir_pw_env_setup/py/pw_env_setup/virtualenv_setup/constraint.list",

    # Project specific constraints file
    "//tools/constraint.txt",
  ]

  pw_build_PIP_REQUIREMENTS = [
    # Project specific requirements
    "//tools/requirements.txt",
  ]

  # Default gn build virtualenv target.
  pw_build_PYTHON_BUILD_VENV = "//:project_build_venv"
}

Tip

There are some additional gn args to control how pip installations are performed during the build.

  # Set pw_python_venv.vendor_wheel targets to download Python packages for all
  # platform combinations. This takes a significant amount of time.
  pw_build_PYTHON_PIP_DOWNLOAD_ALL_PLATFORMS = false

  # Adds '--require-hashes'. This option enforces hash checking on Python
  # package files.
  pw_build_PYTHON_PIP_INSTALL_REQUIRE_HASHES = false

  # Adds --no-index forcing pip to not reach out to the internet (pypi.org) to
  # download packages. Using this option requires setting
  # pw_build_PYTHON_PIP_INSTALL_FIND_LINKS as well.
  pw_build_PYTHON_PIP_INSTALL_OFFLINE = false

  # Adds '--no-cache-dir' forcing pip to ignore any previously cached Python
  # packages. On most systems this is located in ~/.cache/pip/
  pw_build_PYTHON_PIP_INSTALL_DISABLE_CACHE = false

  # List of paths to folders containing Python wheels (*.whl) or source tar
  # files (*.tar.gz). Pip will check each of these directories when looking for
  # potential install candidates. Each path will be passed to all 'pip install'
  # commands as '--find-links PATH'.
  pw_build_PYTHON_PIP_INSTALL_FIND_LINKS = []

  # General options passed to pip commands
  # https://pip.pypa.io/en/stable/cli/pip/#general-options
  pw_build_PYTHON_PIP_DEFAULT_OPTIONS = [ "--disable-pip-version-check" ]

BUILDCONFIG.gn

_pigweed_directory = {
  import("//build_overrides/pigweed.gni")
}

set_default_toolchain("${_pigweed_directory.dir_pw_toolchain}/default")

build_overrides / pigweed.gni

declare_args() {
  # Location of the Pigweed repository.
  dir_pigweed = "//third_party/pigweed/"
}

# Upstream Pigweed modules.
import("$dir_pigweed/modules.gni")

BUILD.gn

import("//build_overrides/pigweed.gni")

import("$dir_pw_build/python.gni")
import("$dir_pw_build/python_dist.gni")
import("$dir_pw_build/python_venv.gni")
import("$dir_pw_unit_test/test.gni")

# Lists all the targets build by default with e.g. `ninja -C out`.
group("default") {
  deps = [
    ":python.lint",
    ":python.tests",
  ]
}

# This group is built during bootstrap to setup the interactive Python
# environment.
pw_python_group("python") {
  python_deps = [
    # Generate and pip install _all_python_packages
    ":pip_install_project_tools",
  ]
}

# In-tree Python packages
_project_python_packages = [
  "//python_package1",
]

# Pigweed Python packages to include
_pigweed_python_packages = [
  "$dir_pw_env_setup:core_pigweed_python_packages",
  "$dir_pigweed/targets/lm3s6965evb_qemu/py",
  "$dir_pigweed/targets/stm32f429i_disc1/py",
]

_all_python_packages =
    _project_python_packages + _pigweed_python_packages

# The default venv for Python actions in GN
# Set this gn arg in a declare_args block in this file 'BUILD.gn' or in '.gn' to
# use this venv.
#
#   pw_build_PYTHON_BUILD_VENV = "//:project_build_venv"
#
pw_python_venv("project_build_venv") {
  path = "$root_build_dir/python-venv"
  constraints = pw_build_PIP_CONSTRAINTS
  requirements = pw_build_PIP_REQUIREMENTS

  # Ensure all third party Python dependencies are installed into this venv.
  # This works by checking the setup.cfg files for all packages listed here and
  # installing the packages listed in the [options].install_requires field.
  source_packages = _all_python_packages
}

# This template collects all python packages and their dependencies into a
# single super Python package for installation into the bootstrapped virtual
# environment.
pw_python_distribution("generate_project_python_distribution") {
  packages = _all_python_packages
  generate_setup_cfg = {
    name = "project-tools"
    version = "0.0.1"
    append_date_to_version = true
    include_default_pyproject_file = true
  }
}

# Install the project-tools super Python package into the bootstrapped
# Python venv.
pw_python_pip_install("pip_install_project_tools") {
  packages = [ ":generate_project_python_distribution" ]
}

Pigweed Module Structure for Python Code#

Pigweed Python code is structured into standard Python packages. This makes it simple to package and distribute Pigweed Python packages with common Python tools.

Like all Pigweed source code, Python packages are organized into Pigweed modules. A module’s Python package is nested under a py/ directory (see Pigweed Module Stucture).

Example layout of a Pigweed Python package.#

module_name/
├── py/
│   ├── BUILD.gn
│   ├── setup.cfg
│   ├── setup.py
│   ├── pyproject.toml
│   ├── package_name/
│   │   ├── module_a.py
│   │   ├── module_b.py
│   │   ├── py.typed
│   │   └── nested_package/
│   │       ├── py.typed
│   │       └── module_c.py
│   ├── module_a_test.py
│   └── module_c_test.py
└── ...

The BUILD.gn declares this package in GN. For upstream Pigweed, a presubmit check in ensures that all Python files are listed in a BUILD.gn.

Pigweed prefers to define Python packages using setup.cfg files. In the above file tree setup.py and pyproject.toml files are stubs with the following content:

setup.py#

import setuptools  # type: ignore
setuptools.setup()  # Package definition in setup.cfg

pyproject.toml#

[build-system]
requires = ['setuptools', 'wheel']
build-backend = 'setuptools.build_meta'

The stub setup.py file is there to support running pip install --editable.

Each pyproject.toml file is required to specify which build system should be used for the given Python package. In Pigweed’s case it always specifies using setuptools.

See also

setup.cfg examples at Configuring setup() using setup.cfg files
pyproject.toml background at Build System Support - How to use it?

pw_python_package targets#

The key abstraction in the Python build is the pw_python_package. A pw_python_package represents a Python package as a GN target. It is implemented with a GN template. The pw_python_package template is documented in Python GN Templates.

The key attributes of a pw_python_package are

a setup.py file,
source files,
test files,
dependencies on other pw_python_package targets.

A pw_python_package target is composed of several GN subtargets. Each subtarget represents different functionality in the Python build.

<name> - Represents the Python files in the build, but does not take any actions. All subtargets depend on this target.
<name>.tests - Runs all tests for this package.
- <name>.tests.<test_file> - Runs the specified test.
<name>.lint - Runs static analysis tools on the Python code. This is a group of two subtargets:
- <name>.lint.mypy - Runs Mypy on all Python files, if enabled.
- <name>.lint.pylint - Runs Pylint on all Python files, if enabled.
<name>.install - Installs the package in a Python virtual environment.
<name>.wheel - Builds a Python wheel for this package.

To avoid unnecessary duplication, all Python actions are executed in the default toolchain, even if they are referred to from other toolchains.

Testing#

Tests for a Python package are listed in its pw_python_package target. Adding a new test is simple: write the test file and list it in its accompanying Python package. The build will run it when the test, the package, or one of its dependencies is updated.

Static analysis#

pw_python_package targets are preconfigured to run Pylint and Mypy on their source and test files. Users may specify which pylintrc and mypy.ini files to use on a per-package basis. The configuration files may also be provided in the directory structure; the tools will locate them using their standard means. Like tests, static analysis is only run when files or their dependencies change.

Packages may opt out of static analysis as necessary.

In addition to user specified mypy.ini files some arguments are always passed to mypy by default. They can be seen in this excerpt of //pw_build/python.gni below:

    args = [
      "--pretty",
      "--show-error-codes",

      # Use a mypy cache dir for this target only to avoid cache conflicts in
      # parallel mypy invocations.
      "--cache-dir",
      rebase_path(target_out_dir, root_build_dir) + "/.mypy_cache",
    ]

    # Use this environment variable to force mypy to colorize output.
    # See https://github.com/python/mypy/issues/7771
    environment = [ "MYPY_FORCE_COLOR=1" ]

Building Python wheels#

Wheels are the standard format for distributing Python packages. The Pigweed Python build supports creating wheels for individual packages and groups of packages. Building the .wheel subtarget creates a .whl file for the package using the PyPA’s build tool.

The .wheel subtarget of any pw_python_package or pw_python_distribution records the location of the generated wheel with GN metadata. Wheels for a Python package and its transitive dependencies can be collected from the pw_python_package_wheels key. See Python Distributable Templates.

Protocol buffers#

The Pigweed GN build supports protocol buffers with the pw_proto_library target (see pw_protobuf_compiler). Python protobuf modules are generated as standalone Python packages by default. Protocol buffers may also be nested within existing Python packages. In this case, the Python package in the source tree is incomplete; the final Python package, including protobufs, is generated in the output directory.

Generating setup.py#

The pw_python_package target in the BUILD.gn duplicates much of the information in the setup.py or setup.cfg file. In many cases, it would be possible to generate a setup.py file rather than including it in the source tree. However, removing the setup.py would preclude using a direct, editable installation from the source tree.

Pigweed packages containing protobufs are generated in full or in part. These packages may use generated setup files, since they are always packaged or installed from the build output directory.

Rationale#

Background#

Developing software involves much more than writing source code. Software needs to be compiled, executed, tested, analyzed, packaged, and deployed. As projects grow beyond a few files, these tasks become impractical to manage manually. Build systems automate these auxiliary tasks of software development, making it possible to build larger, more complex systems quickly and robustly.

Python is an interpreted language, but it shares most build automation concerns with other languages. Pigweed uses Python extensively and must address these needs for itself and its users.

Existing solutions#

The Python programming langauge does not have an official build automation system. However, there are numerous Python-focused build automation tools with varying degrees of adoption. See the Python Wiki for examples.

A few Python tools have become defacto standards, including setuptools, wheel, and pip. These essential tools address key aspects of Python packaging and distribution, but are not intended for general build automation. Tools like PyBuilder and tox provide more general build automation for Python.

The Bazel build system has first class support for Python and other languages used by Pigweed, including protocol buffers.

Challenges#

Pigweed’s use of Python is different from many other projects. Pigweed is a multi-language, modular project. It serves both as a library or middleware and as a development environment.

This section describes Python build automation challenges encountered by Pigweed.

Dependencies#

Pigweed is organized into distinct modules. In Python, each module is a separate package, potentially with dependencies on other local or PyPI packages.

The basic Python packaging tools lack dependency tracking for local packages. For example, a package’s setup.py or setup.cfg lists all of its dependencies, but pip is not aware of local packages until they are installed. Packages must be installed with their dependencies taken into account, in topological sorted order.

To work around this, one could set up a private PyPI server instance, but this is too cumbersome for daily development and incompatible with editable package installation.

Testing#

Tests are crucial to having a healthy, maintainable codebase. While they take some initial work to write, the time investment pays for itself many times over by contributing to the long-term resilience of a codebase. Despite their benefit, developers don’t always take the time to write tests. Any barriers to writing and running tests result in fewer tests and consequently more fragile, bug-prone codebases.

There are lots of great Python libraries for testing, such as unittest and pytest. These tools make it easy to write and execute individual Python tests, but are not well suited for managing suites of interdependent tests in a large project. Writing a test with these utilities does not automatically run them or keep running them as the codebase changes.

Static analysis#

See also

Automated analysis for info on other static analysis tools used in Pigweed.

Various static analysis tools exist for Python. Two widely used, powerful tools are Pylint and Mypy. Using these tools improves code quality, as they catch bugs, encourage good design practices, and enforce a consistent coding style. As with testing, barriers to running static analysis tools cause many developers to skip them. Some developers may not even be aware of these tools.

Deploying static analysis tools to a codebase like Pigweed has some challenges. Mypy and Pylint are simple to run, but they are extremely slow. Ideally, these tools would be run constantly during development, but only on files that change. These tools do not have built-in support for incremental runs or dependency tracking.

Another challenge is configuration. Mypy and Pylint support using configuration files to select which checks to run and how to apply them. Both tools only support using a single configuration file for an entire run, which poses a challenge to modular middleware systems where different parts of a project may require different configurations.

Protocol buffers#

Protocol buffers are an efficient system for serializing structured data. They are widely used by Google and other companies.

The protobuf compiler protoc generates Python modules from .proto files. protoc strictly generates protobuf modules according to their directory structure. This works well in a monorepo, but poses a challenge to a middleware system like Pigweed. Generating protobufs by path also makes integrating protobufs with existing packages awkward.

Requirements#

Pigweed aims to provide high quality software components and a fast, effective, flexible development experience for its customers. Pigweed’s high-level goals and the challenges described above inform these requirements for the Pigweed Python build.

Integrate seamlessly with the other Pigweed build tools.
Easy to use independently, even if primarily using a different build system.
Support standard packaging and distribution with setuptools, wheel, and pip.
Correctly manage interdependent local Python packages.
Out-of-the-box support for writing and running tests.
Preconfigured, trivial-to-run static analysis integration for Pylint and Mypy.
Fast, dependency-aware incremental rebuilds and test execution, suitable for use with pw_watch.
Seamless protocol buffer support.

Design Decision#

Existing Python tools may be effective for Python codebases, but their utility is more limited in a multi-language project like Pigweed. The cost of bringing up and maintaining an additional build automation system for a single language is high.

Pigweed uses GN as its primary build system for all languages. While GN does not natively support Python, adding support is straightforward with GN templates.

GN has strong multi-toolchain and multi-language capabilities. In GN, it is straightforward to share targets and artifacts between different languages. For example, C++, Go, and Python targets can depend on the same protobuf declaration. When using GN for multiple languages, Ninja schedules build steps for all languages together, resulting in faster total build times.

Not all Pigweed users build with GN. Of Pigweed’s three supported build systems, GN is the fastest, lightest weight, and easiest to run. It also has simple, clean syntax. This makes it feasible to use GN only for Python while building primarily with a different system.

Given these considerations, GN is an ideal choice for Pigweed’s Python build.

Pigweed’s GN Python Build#

How it Works#

Managing Python Requirements#

Build Time Python Virtualenv#

Third-party Python Requirements and Constraints#

In-tree pw_python_package Requirements#

generated_requirements.txt#

compiled_requirements.txt#

Caching Python Packages for Offline Installation#

Downloading Packages#

Installing Offline#

GN Args#

Using a .pip.conf File#

GN File Structure for Python Code#

Pigweed Module Structure for Python Code#

pw_python_package targets#

Testing#

Static analysis#

Building Python wheels#

Protocol buffers#

Generating setup.py#

Rationale#

Background#

Existing solutions#

Challenges#

Dependencies#

Testing#

Static analysis#

Protocol buffers#

Requirements#

Design Decision#

In-tree `pw_python_package` Requirements#

`generated_requirements.txt`#

`compiled_requirements.txt`#

Using a `.pip.conf` File#