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pyo3/guide/src/building_and_distribution.md
David Hewitt a8d8003b31 guide: document PYO3_NO_PYTHON on Windows
2021-01-10 10:16:37 +00:00

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Building and Distribution

Python version

PyO3 uses a build script to determine the Python version and set the correct linker arguments. By default it uses the python3 executable. You can override the Python interpreter by setting PYO3_PYTHON, e.g., PYO3_PYTHON=python3.6.

Linking

Different linker arguments must be set for libraries/extension modules and binaries, which includes both standalone binaries and tests. (More specifically, binaries must be told where to find libpython and libraries must not link to libpython for manylinux compliance).

Since PyO3's build script can't know whether you're building a binary or a library, you have to activate the extension-module feature to get the build options for a library, or it'll default to binary.

If you have e.g. a library crate and a profiling crate alongside, you need to use optional features. E.g. you put the following in the library crate:

[dependencies]
pyo3 = "0.6"

[lib]
name = "hyperjson"
crate-type = ["rlib", "cdylib"]

[features]
default = ["pyo3/extension-module"]

And this in the profiling crate:

[dependencies]
my_main_crate = { path = "..", default-features = false }
pyo3 = "0.6"

On Linux/macOS you might have to change LD_LIBRARY_PATH to include libpython, while on windows you might need to set LIB to include pythonxy.lib (where x and y are major and minor version), which is normally either in the libs or Lib folder of a Python installation.

Distribution

There are two ways to distribute your module as a Python package: The old, setuptools-rust, and the new, maturin. setuptools-rust needs several configuration files (setup.py, MANIFEST.in, build-wheels.sh, etc.). maturin doesn't need any configuration files, however it does not support some functionality of setuptools such as package data (pyo3/maturin#258) and requires a rigid project structure, while setuptools-rust allows (and sometimes requires) configuration with python code.

Py_LIMITED_API/abi3

By default, Python extension modules can only be used with the same Python version they were compiled against -- if you build an extension module with Python 3.5, you can't import it using Python 3.8. PEP 384 introduced the idea of the limited Python API, which would have a stable ABI enabling extension modules built with it to be used against multiple Python versions. This is also known as abi3.

Note that maturin >= 0.9.0 or setuptools-rust >= 0.11.4 support abi3 wheels. See the corresponding PRs for more.

There are three steps involved in making use of abi3 when building Python packages as wheels:

  1. Enable the abi3 feature in pyo3. This ensures pyo3 only calls Python C-API functions which are part of the stable API, and on Windows also ensures that the project links against the correct shared object (no special behavior is required on other platforms):
[dependencies]
pyo3 = { version = "...", features = ["abi3"]}

  1. Ensure that the built shared objects are correctly marked as abi3. This is accomplished by telling your build system that you're using the limited API.

  2. Ensure that the .whl is correctly marked as abi3. For projects using setuptools, this is accomplished by passing --py-limited-api=cp3x (where x is the minimum Python version supported by the wheel, e.g. --py-limited-api=cp35 for Python 3.5) to setup.py bdist_wheel.

Minimum Python version for abi3

Because a single abi3 wheel can be used with many different Python versions, PyO3 has feature flags abi3-py36, abi3-py37, abi-py38 etc. to set the minimum required Python version for your abi3 wheel. For example, if you set the abi3-py36 feature, your extension wheel can be used on all Python 3 versions from Python 3.6 and up. maturin and setuptools-rust will give the wheel a name like my-extension-1.0-cp36-abi3-manylinux2020_x86_64.whl. If you set more that one of these api version feature flags the highest version always wins. For example, with both abi3-py36 and abi3-py38 set, PyO3 would build a wheel which supports Python 3.8 and up. PyO3 is only able to link your extension module to api3 version up to and including your host Python version. E.g., if you set abi3-py38 and try to compile the crate with a host of Python 3.6, the build will fail.

As an advanced feature, you can build PyO3 wheel without calling Python interpreter with the environment variable PYO3_NO_PYTHON set. On unix systems this works unconditionally; on Windows you must also set the RUSTFLAGS evironment variable to contain -L native=/path/to/python/libs so that the linker can find python3.lib.

Missing features

Due to limitations in the Python API, there are a few pyo3 features that do not work when compiling for abi3. These are:

  • #[text_signature] does not work on classes until Python 3.10 or greater.
  • The dict and weakref options on classes are not supported until Python 3.9 or greater.
  • The buffer API is not supported.

Cross Compiling

Cross compiling PyO3 modules is relatively straightforward and requires a few pieces of software:

  • A toolchain for your target.
  • The appropriate options in your Cargo .config for the platform you're targeting and the toolchain you are using.
  • A Python interpreter that's already been compiled for your target.
  • A Python interpreter that is built for your host and available through the PATH or setting the PYO3_PYTHON variable.
  • The headers that match the above interpreter.

See https://github.com/japaric/rust-cross for a primer on cross compiling Rust in general.

After you've obtained the above, you can build a cross compiled PyO3 module by setting a few extra environment variables:

  • PYO3_CROSS_LIB_DIR: This variable must be set to the directory containing the target's libpython DSO and the associated _sysconfigdata*.py file.
  • PYO3_CROSS_PYTHON_VERSION: Major and minor version (e.g. 3.9) of the target Python installation. This variable is only needed if pyo3 cannot determine the version to target by other means:
    • From PYO3_CROSS_INCLUDE_DIR or abi3-py3* features when targeting Windows, or
    • if there are multiple versions of python present in PYO3_CROSS_LIB_DIR when targeting unix.
  • PYO3_CROSS_INCLUDE_DIR: This variable can optionally be set to the directory containing the headers for the target's Python interpreter when targeting Windows.

An example might look like the following (assuming your target's sysroot is at /home/pyo3/cross/sysroot and that your target is armv7):

export PYO3_CROSS_LIB_DIR="/home/pyo3/cross/sysroot/usr/lib"

cargo build --target armv7-unknown-linux-gnueabihf

If there are multiple python versions at the cross lib directory and you cannot set a more precise location to include both the libpython DSO and _sysconfigdata*.py files, you can set the required version:

export PYO3_CROSS_PYTHON_VERSION=3.8
export PYO3_CROSS_LIB_DIR="/home/pyo3/cross/sysroot/usr/lib"

cargo build --target armv7-unknown-linux-gnueabihf

Or another example with the same sys root but building for windows:

export PYO3_CROSS_INCLUDE_DIR="/home/pyo3/cross/sysroot/usr/include"
export PYO3_CROSS_LIB_DIR="/home/pyo3/cross/sysroot/usr/lib"

cargo build --target x86_64-pc-windows-gnu

Bazel

For an example of how to build python extensions using Bazel, see https://github.com/TheButlah/rules_pyo3