PEX

---

PEX files

When working with Python code, Pants makes frequent use of the PEX (Python EXecutable) format. So, you'll see PEX referenced frequently in this documentation.

A PEX is a self-contained Python environment, similar in spirit to a virtualenv. A Pex can contain combinations of Python source files, 3rd-party requirements (sdists or wheels), resource files, and metadata describing the contents.

Importantly, this metadata can include:

• Python interpreter constraints.
• Python platforms, like macosx_11_0_arm64-cp-39-cp39.
• An entry point or console script.

A PEX can be bundled into a single .pex file. This file, when executed, knows how to unpack itself, find an interpreter that matches its constraints, and run itself on that interpreter. Therefore deploying code packaged in a Pex file is as simple as copying the file to an environment that has a suitable Python interpreter.

Check out blog.pantsbuild.org/pants-pex-and-docker for how this workflow gets even better when combined with Pants's Docker support!

Building a Pex

You define a PEX using the pex_binary target type:

path/to/mybinary/BUILD

python_sources(name="lib")

pex_binary(
    name="bin",
    dependencies=[":lib"],
    execution_mode="venv",
)

You then use the package goal to build the PEX, which will be output under the [dist/] directory.

$ pants package path/to/mybinary:bin

There are several fields you can set on a pex_binary to configure the layout, entry point and behavior of the resulting PEX. See the documentation for details.

Setting the target platforms for a PEX

By default, the package goal builds a PEX that runs on the architecture and OS of the local machine (or local environment), and on a locally-found interpreter compatible with your code's interpreter constraints. However, you can also build a multiplatform PEX - one that will run on multiple architecture+OS+interpreter combinations.

To do so, you must configure the complete_platforms field on your pex_binary to point to file targets that provide detailed information about your target platforms. This is information that Pants can't determine itself because it's not running on those platforms:

BUILD

file(
    name="linux_x86_py39",
    source="linux_x86_py39.json",
)

file(
    name="linux_aarch64_py310",
    source="linux_aarch64_py310.json",
)

pex_binary(
    ...
    complete_platforms=[":linux_x86_py39", ":linux_aarch64_py310"]
    ...
)

You can generate the JSON content for these files by installing the Pex command-line tool on the target platform and running pex3 interpreter inspect --markers --tags against the appropriate interpreter. You can run pex3 interpreter inspect --help for more options, and in particular for how to select the desired target interpreter.

Platform-specific dependencies must be available as wheels

Some Python distributions include native code, and therefore require platform-specific artifacts. Often, such artifacts are pre-built and available on PyPI as platform-specific wheels. But in some cases they are only available as source distributions (sdists) and must be compiled into platform-specific wheels at build time. Pants can only build platform-specific sdists for the local machine or environment, and cannot cross-compile for other target platforms. Therefore, to build for platforms other than the local one, all the platform-specific third-party packages that your PEX transitively depends on must be available as prebuilt wheels for each platform you care about. If those wheels aren't available on PyPI you can always build them manually once and host them on a private package repository.

Setting the .pex file's shebang

You can invoke a .pex file either directly, as in path/to/mybinary.pex, or via an explicitly specified interpreter, as in path/to/python3.11 path/to/mybinary.pex. In the former case, PEX's bootstrapping logic will find an interpreter based on the .pex file's shebang. By default, this will be #!/usr/bin/env pythonX.Y where Python X.Y is some version compatible with your code's interpreter constraints. This default may not be appropriate in a few cases:

• If your PEX is compatible with multiple versions and the chosen one (X.Y) is not present on one of your target systems.
• If the interpreter on the target system is not present on the PATH.
• If /usr/bin/env is not available on the target system.

In these cases you can override the default shebang using the shebang field on the pex_binary target, or invoke the .pex via an explicit interpreter.

Setting Pex and Pip versions

Pants makes use of the Pex command-line tool internally for building PEXes. The Pex version that Pants uses is specified by the version option under the pex-cli subsystem. The known Pex versions are specified by the known_versions option under the pex-cli subsystem. You can see all Pex tool options and their current values by running pants help-advanced pex-cli. To upgrade the Pex version, update these option values accordingly. For instance, in pants.toml, to upgrade to Pex 2.1.143:

[pex-cli]
version = "v2.1.143"
known_versions = [
  "v2.1.143|macos_arm64|7dba8776000b4f75bc9af850cb65b2dc7720ea211733e8cb5243c0b210ef3c19|4194291",
  "v2.1.143|macos_x86_64|7dba8776000b4f75bc9af850cb65b2dc7720ea211733e8cb5243c0b210ef3c19|4194291",
  "v2.1.143|linux_x86_64|7dba8776000b4f75bc9af850cb65b2dc7720ea211733e8cb5243c0b210ef3c19|4194291",
  "v2.1.143|linux_arm64|7dba8776000b4f75bc9af850cb65b2dc7720ea211733e8cb5243c0b210ef3c19|4194291"
]

The Pex version determines which Pip versions are supported. To see the lists of Pip versions a certain version of Pex supports you can either install that version of Pex as a standalone CLI and run pex --help, or examine pex/pip/version.py in the sources of the relevant Pex version.

The Pip version that Pex uses is determined by the pip_version option in Pants. To upgrade the Pip version, update this option value accordingly. For instance, in pants.toml, to set the Pip version to be the latest supported by Pex:

[python]
pip_version = "latest"