Building and distribution

This chapter of the guide goes into detail on how to build and distribute projects using PyO3. The way to achieve this is very different depending on whether the project is a Python module implemented in Rust, or a Rust binary embedding Python. For both types of project there are also common problems such as the Python version to build for and the linker arguments to use.

The material in this chapter is intended for users who have already read the PyO3 README. It covers in turn the choices that can be made for Python modules and for Rust binaries. There is also a section at the end about cross-compiling projects using PyO3.

There is an additional sub-chapter dedicated to supporting multiple Python versions.

Configuring the Python version

PyO3 uses a build script (backed by the pyo3-build-config crate) to determine the Python version and set the correct linker arguments. By default it will attempt to use the following in order:

  • Any active Python virtualenv.
  • The python executable (if it's a Python 3 interpreter).
  • The python3 executable.

You can override the Python interpreter by setting the PYO3_PYTHON environment variable, e.g. PYO3_PYTHON=python3.7, PYO3_PYTHON=/usr/bin/python3.9, or even a PyPy interpreter PYO3_PYTHON=pypy3.

Once the Python interpreter is located, pyo3-build-config executes it to query the information in the sysconfig module which is needed to configure the rest of the compilation.

To validate the configuration which PyO3 will use, you can run a compilation with the environment variable PYO3_PRINT_CONFIG=1 set. An example output of doing this is shown below:

$ PYO3_PRINT_CONFIG=1 cargo build
   Compiling pyo3 v0.14.1 (/home/david/dev/pyo3)
error: failed to run custom build command for `pyo3 v0.14.1 (/home/david/dev/pyo3)`

Caused by:
  process didn't exit successfully: `/home/david/dev/pyo3/target/debug/build/pyo3-7a8cf4fe22e959b7/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-env-changed=PYO3_CROSS
  cargo:rerun-if-env-changed=PYO3_CROSS_LIB_DIR
  cargo:rerun-if-env-changed=PYO3_CROSS_PYTHON_VERSION
  cargo:rerun-if-env-changed=PYO3_PRINT_CONFIG

  -- PYO3_PRINT_CONFIG=1 is set, printing configuration and halting compile --
  implementation=CPython
  version=3.8
  shared=true
  abi3=false
  lib_name=python3.8
  lib_dir=/usr/lib
  executable=/usr/bin/python
  pointer_width=64
  build_flags=
  suppress_build_script_link_lines=false

The PYO3_ENVIRONMENT_SIGNATURE environment variable can be used to trigger rebuilds when its value changes, it has no other effect.

Advanced: config files

If you save the above output config from PYO3_PRINT_CONFIG to a file, it is possible to manually override the contents and feed it back into PyO3 using the PYO3_CONFIG_FILE env var.

If your build environment is unusual enough that PyO3's regular configuration detection doesn't work, using a config file like this will give you the flexibility to make PyO3 work for you. To see the full set of options supported, see the documentation for the InterpreterConfig struct.

Building Python extension modules

Python extension modules need to be compiled differently depending on the OS (and architecture) that they are being compiled for. As well as multiple OSes (and architectures), there are also many different Python versions which are actively supported. Packages uploaded to PyPI usually want to upload prebuilt "wheels" covering many OS/arch/version combinations so that users on all these different platforms don't have to compile the package themselves. Package vendors can opt-in to the "abi3" limited Python API which allows their wheels to be used on multiple Python versions, reducing the number of wheels they need to compile, but restricts the functionality they can use.

There are many ways to go about this: it is possible to use cargo to build the extension module (along with some manual work, which varies with OS). The PyO3 ecosystem has two packaging tools, maturin and setuptools-rust, which abstract over the OS difference and also support building wheels for PyPI upload.

PyO3 has some Cargo features to configure projects for building Python extension modules:

  • The extension-module feature, which must be enabled when building Python extension modules.
  • The abi3 feature and its version-specific abi3-pyXY companions, which are used to opt-in to the limited Python API in order to support multiple Python versions in a single wheel.

This section describes each of these packaging tools before describing how to build manually without them. It then proceeds with an explanation of the extension-module feature. Finally, there is a section describing PyO3's abi3 features.

Packaging tools

The PyO3 ecosystem has two main choices to abstract the process of developing Python extension modules:

  • maturin is a command-line tool to build, package and upload Python modules. It makes opinionated choices about project layout meaning it needs very little configuration. This makes it a great choice for users who are building a Python extension from scratch and don't need flexibility.
  • setuptools-rust is an add-on for setuptools which adds extra keyword arguments to the setup.py configuration file. It requires more configuration than maturin, however this gives additional flexibility for users adding Rust to an existing Python package that can't satisfy maturin's constraints.

Consult each project's documentation for full details on how to get started using them and how to upload wheels to PyPI. It should be noted that while maturin is able to build manylinux-compliant wheels out-of-the-box, setuptools-rust requires a bit more effort, relying on Docker for this purpose.

There are also maturin-starter and setuptools-rust-starter examples in the PyO3 repository.

Manual builds

To build a PyO3-based Python extension manually, start by running cargo build as normal in a library project which uses PyO3's extension-module feature and has the cdylib crate type.

Once built, symlink (or copy) and rename the shared library from Cargo's target/ directory to your desired output directory:

  • on macOS, rename libyour_module.dylib to your_module.so.
  • on Windows, rename libyour_module.dll to your_module.pyd.
  • on Linux, rename libyour_module.so to your_module.so.

You can then open a Python shell in the output directory and you'll be able to run import your_module.

If you're packaging your library for redistribution, you should indicated the Python interpreter your library is compiled for by including the platform tag in its name. This prevents incompatible interpreters from trying to import your library. If you're compiling for PyPy you must include the platform tag, or PyPy will ignore the module.

Bazel builds

To use PyO3 with bazel one needs to manually configure PyO3, PyO3-ffi and PyO3-macros. In particular, one needs to make sure that it is compiled with the right python flags for the version you intend to use. For example see:

  1. https://github.com/OliverFM/pytorch_with_gazelle -- for a minimal example of a repo that can use PyO3, PyTorch and Gazelle to generate python Build files.
  2. https://github.com/TheButlah/rules_pyo3 -- which has more extensive support, but is outdated.

Platform tags

Rather than using just the .so or .pyd extension suggested above (depending on OS), you can prefix the shared library extension with a platform tag to indicate the interpreter it is compatible with. You can query your interpreter's platform tag from the sysconfig module. Some example outputs of this are seen below:

# CPython 3.10 on macOS
.cpython-310-darwin.so

# PyPy 7.3 (Python 3.8) on Linux
$ python -c 'import sysconfig; print(sysconfig.get_config_var("EXT_SUFFIX"))'
.pypy38-pp73-x86_64-linux-gnu.so

So, for example, a valid module library name on CPython 3.10 for macOS is your_module.cpython-310-darwin.so, and its equivalent when compiled for PyPy 7.3 on Linux would be your_module.pypy38-pp73-x86_64-linux-gnu.so.

See PEP 3149 for more background on platform tags.

macOS

On macOS, because the extension-module feature disables linking to libpython (see the next section), some additional linker arguments need to be set. maturin and setuptools-rust both pass these arguments for PyO3 automatically, but projects using manual builds will need to set these directly in order to support macOS.

The easiest way to set the correct linker arguments is to add a build.rs with the following content:

fn main() {
    pyo3_build_config::add_extension_module_link_args();
}

Remember to also add pyo3-build-config to the build-dependencies section in Cargo.toml.

An alternative to using pyo3-build-config is add the following to a cargo configuration file (e.g. .cargo/config.toml):

[target.x86_64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

[target.aarch64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

Using the MacOS system python3 (/usr/bin/python3, as opposed to python installed via homebrew, pyenv, nix, etc.) may result in runtime errors such as Library not loaded: @rpath/Python3.framework/Versions/3.8/Python3. These can be resolved with another addition to .cargo/config.toml:

[build]
rustflags = [
  "-C", "link-args=-Wl,-rpath,/Library/Developer/CommandLineTools/Library/Frameworks",
]

Alternatively, on rust >= 1.56, one can include in build.rs:

fn main() {
    println!(
        "cargo:rustc-link-arg=-Wl,-rpath,/Library/Developer/CommandLineTools/Library/Frameworks"
    );
}

For more discussion on and workarounds for MacOS linking problems see this issue.

Finally, don't forget that on MacOS the extension-module feature will cause cargo test to fail without the --no-default-features flag (see the FAQ).

The extension-module feature

PyO3's extension-module feature is used to disable linking to libpython on Unix targets.

This is necessary because by default PyO3 links to libpython. This makes binaries, tests, and examples "just work". However, Python extensions on Unix must not link to libpython for manylinux compliance.

The downside of not linking to libpython is that binaries, tests, and examples (which usually embed Python) will fail to build. If you have an extension module as well as other outputs in a single project, you need to use optional Cargo features to disable the extension-module when you're not building the extension module. See the FAQ for an example workaround.

Py_LIMITED_API/abi3

By default, Python extension modules can only be used with the same Python version they were compiled against. For example, an extension module built for Python 3.5 can't be imported in 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.

The advantage of building extension modules using the limited Python API is that package vendors only need to build and distribute a single copy (for each OS / architecture), and users can install it on all Python versions from the minimum version and up. The downside of this is that PyO3 can't use optimizations which rely on being compiled against a known exact Python version. It's up to you to decide whether this matters for your extension module. It's also possible to design your extension module such that you can distribute abi3 wheels but allow users compiling from source to benefit from additional optimizations - see the support for multiple python versions section of this guide, in particular the #[cfg(Py_LIMITED_API)] flag.

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 = "0.21.0-beta.0", 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. maturin >= 0.9.0 and setuptools-rust >= 0.11.4 support abi3 wheels. See the corresponding PRs for more.

  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-py37, abi3-py38, abi3-py39 etc. to set the minimum required Python version for your abi3 wheel. For example, if you set the abi3-py37 feature, your extension wheel can be used on all Python 3 versions from Python 3.7 and up. maturin and setuptools-rust will give the wheel a name like my-extension-1.0-cp37-abi3-manylinux2020_x86_64.whl.

As your extension module may be run with multiple different Python versions you may occasionally find you need to check the Python version at runtime to customize behavior. See the relevant section of this guide on supporting multiple Python versions at runtime.

PyO3 is only able to link your extension module to abi3 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 3.7, the build will fail.

Note: If you set more that one of these abi3 version feature flags the lowest version always wins. For example, with both abi3-py37 and abi3-py38 set, PyO3 would build a wheel which supports Python 3.7 and up.

Building abi3 extensions without a Python interpreter

As an advanced feature, you can build PyO3 wheel without calling Python interpreter with the environment variable PYO3_NO_PYTHON set. Also, if the build host Python interpreter is not found or is too old or otherwise unusable, PyO3 will still attempt to compile abi3 extension modules after displaying a warning message. On Unix-like systems this works unconditionally; on Windows you must also set the RUSTFLAGS environment variable to contain -L native=/path/to/python/libs so that the linker can find python3.lib.

If the python3.dll import library is not available, an experimental generate-import-lib crate feature may be enabled, and the required library will be created and used by PyO3 automatically.

Note: MSVC targets require LLVM binutils (llvm-dlltool) to be available in PATH for the automatic import library generation feature to work.

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:

  • #[pyo3(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 until Python 3.11 or greater.
  • Optimizations which rely on knowledge of the exact Python version compiled against.

Embedding Python in Rust

If you want to embed the Python interpreter inside a Rust program, there are two modes in which this can be done: dynamically and statically. We'll cover each of these modes in the following sections. Each of them affect how you must distribute your program. Instead of learning how to do this yourself, you might want to consider using a project like PyOxidizer to ship your application and all of its dependencies in a single file.

PyO3 automatically switches between the two linking modes depending on whether the Python distribution you have configured PyO3 to use (see above) contains a shared library or a static library. The static library is most often seen in Python distributions compiled from source without the --enable-shared configuration option. For example, this is the default for pyenv on macOS.

Dynamically embedding the Python interpreter

Embedding the Python interpreter dynamically is much easier than doing so statically. This is done by linking your program against a Python shared library (such as libpython.3.9.so on UNIX, or python39.dll on Windows). The implementation of the Python interpreter resides inside the shared library. This means that when the OS runs your Rust program it also needs to be able to find the Python shared library.

This mode of embedding works well for Rust tests which need access to the Python interpreter. It is also great for Rust software which is installed inside a Python virtualenv, because the virtualenv sets up appropriate environment variables to locate the correct Python shared library.

For distributing your program to non-technical users, you will have to consider including the Python shared library in your distribution as well as setting up wrapper scripts to set the right environment variables (such as LD_LIBRARY_PATH on UNIX, or PATH on Windows).

Note that PyPy cannot be embedded in Rust (or any other software). Support for this is tracked on the PyPy issue tracker.

Statically embedding the Python interpreter

Embedding the Python interpreter statically means including the contents of a Python static library directly inside your Rust binary. This means that to distribute your program you only need to ship your binary file: it contains the Python interpreter inside the binary!

On Windows static linking is almost never done, so Python distributions don't usually include a static library. The information below applies only to UNIX.

The Python static library is usually called libpython.a.

Static linking has a lot of complications, listed below. For these reasons PyO3 does not yet have first-class support for this embedding mode. See issue 416 on PyO3's GitHub for more information and to discuss any issues you encounter.

The auto-initialize feature is deliberately disabled when embedding the interpreter statically because this is often unintentionally done by new users to PyO3 running test programs. Trying out PyO3 is much easier using dynamic embedding.

The known complications are:

  • To import compiled extension modules (such as other Rust extension modules, or those written in C), your binary must have the correct linker flags set during compilation to export the original contents of libpython.a so that extensions can use them (e.g. -Wl,--export-dynamic).

  • The C compiler and flags which were used to create libpython.a must be compatible with your Rust compiler and flags, else you will experience compilation failures.

    Significantly different compiler versions may see errors like this:

    lto1: fatal error: bytecode stream in file 'rust-numpy/target/release/deps/libpyo3-6a7fb2ed970dbf26.rlib' generated with LTO version 6.0 instead of the expected 6.2

    Mismatching flags may lead to errors like this:

    /usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/libpython3.9.a(zlibmodule.o): relocation R_X86_64_32 against `.data' can not be used when making a PIE object; recompile with -fPIE

If you encounter these or other complications when linking the interpreter statically, discuss them on issue 416 on PyO3's GitHub. It is hoped that eventually that discussion will contain enough information and solutions that PyO3 can offer first-class support for static embedding.

Import your module when embedding the Python interpreter

When you run your Rust binary with an embedded interpreter, any #[pymodule] created modules won't be accessible to import unless added to a table called PyImport_Inittab before the embedded interpreter is initialized. This will cause Python statements in your embedded interpreter such as import your_new_module to fail. You can call the macro append_to_inittab with your module before initializing the Python interpreter to add the module function into that table. (The Python interpreter will be initialized by calling prepare_freethreaded_python, with_embedded_python_interpreter, or Python::with_gil with the auto-initialize feature enabled.)

Cross Compiling

Thanks to Rust's great cross-compilation support, cross-compiling using PyO3 is relatively straightforward. To get started, you'll need 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 (optional when building "abi3" extension modules).
  • A Python interpreter that is built for your host and available through the PATH or setting the PYO3_PYTHON variable (optional when building "abi3" extension modules).

After you've obtained the above, you can build a cross-compiled PyO3 module by using Cargo's --target flag. PyO3's build script will detect that you are attempting a cross-compile based on your host machine and the desired target.

When cross-compiling, PyO3's build script cannot execute the target Python interpreter to query the configuration, so there are a few additional environment variables you may need to set:

  • PYO3_CROSS: If present this variable forces PyO3 to configure as a cross-compilation.
  • PYO3_CROSS_LIB_DIR: This variable can be set to the directory containing the target's libpython DSO and the associated _sysconfigdata*.py file for Unix-like targets, or the Python DLL import libraries for the Windows target. This variable is only needed when the output binary must link to libpython explicitly (e.g. when targeting Windows and Android or embedding a Python interpreter), or when it is absolutely required to get the interpreter configuration from _sysconfigdata*.py.
  • 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 from abi3-py3* features, or if PYO3_CROSS_LIB_DIR is not set, or if there are multiple versions of Python present in PYO3_CROSS_LIB_DIR.
  • PYO3_CROSS_PYTHON_IMPLEMENTATION: Python implementation name ("CPython" or "PyPy") of the target Python installation. CPython is assumed by default when this variable is not set, unless PYO3_CROSS_LIB_DIR is set for a Unix-like target and PyO3 can get the interpreter configuration from _sysconfigdata*.py.

An experimental pyo3 crate feature generate-import-lib enables the user to cross-compile extension modules for Windows targets without setting the PYO3_CROSS_LIB_DIR environment variable or providing any Windows Python library files. It uses an external python3-dll-a crate to generate import libraries for the Python DLL for MinGW-w64 and MSVC compile targets. python3-dll-a uses the binutils dlltool program to generate DLL import libraries for MinGW-w64 targets. It is possible to override the default dlltool command name for the cross target by setting PYO3_MINGW_DLLTOOL environment variable. Note: MSVC targets require LLVM binutils or MSVC build tools to be available on the host system. More specifically, python3-dll-a requires llvm-dlltool or lib.exe executable to be present in PATH when targeting *-pc-windows-msvc. The Zig compiler executable can be used in place of llvm-dlltool when the ZIG_COMMAND environment variable is set to the installed Zig program name ("zig" or "python -m ziglang").

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_PYTHON_VERSION=3.9
export PYO3_CROSS_LIB_DIR="/home/pyo3/cross/sysroot/usr/lib"

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

Any of the abi3-py3* features can be enabled instead of setting PYO3_CROSS_PYTHON_VERSION in the above examples.

PYO3_CROSS_LIB_DIR can often be omitted when cross compiling extension modules for Unix and macOS targets, or when cross compiling extension modules for Windows and the experimental generate-import-lib crate feature is enabled.

The following resources may also be useful for cross-compiling: