PEP: 711 Title: PyBI: a standard format for distributing Python Binaries Author: Nathaniel J. Smith PEP-Delegate: TODO Discussions-To: https://discuss.python.org/t/pep-711-pybi-a-standard-format-for-distributing-python-binaries/25547 Status: Draft Type: Standards Track Topic: Packaging Content-Type: text/x-rst Created: 06-Apr-2023 Post-History: `06-Apr-2023 `__ Abstract ======== “Like wheels, but instead of a pre-built python package, it’s a pre-built python interpreter” Motivation ========== End goal: Pypi.org has pre-built packages for all Python versions on all popular platforms, so automated tools can easily grab any of them and set it up. It becomes quick and easy to try Python prereleases, pin Python versions in CI, make a temporary environment to reproduce a bug report that only happens on a specific Python point release, etc. First step (this PEP): define a standard packaging file format to hold pre-built Python interpreters, that reuses existing Python packaging standards as much as possible. Examples ======== Example pybi builds are available at `pybi.vorpus.org `__. They're zip files, so you can unpack them and poke around inside if you want to get a feel for how they're laid out. You can also look at the `tooling I used to create them `__. Specification ============= Filename -------- Filename: ``{distribution}-{version}[-{build tag}]-{platform tag}.pybi`` This matches the wheel file format defined in :pep:`427`, except dropping the ``{python tag}`` and ``{abi tag}`` and changing the extension from ``.whl`` → ``.pybi``. For example: - ``cpython-3.9.3-manylinux_2014.pybi`` - ``cpython-3.10b2-win_amd64.pybi`` Just like for wheels, if a pybi supports multiple platforms, you can separate them by dots to make a “compressed tag set”: - ``cpython-3.9.5-macosx_11_0_x86_64.macosx_11_0_arm64.pybi`` (Though in practice this probably won’t be used much, e.g. the above filename is more idiomatically written as ``cpython-3.9.5-macosx_11_0_universal2.pybi``.) File contents ------------- A ``.pybi`` file is a zip file, that can be unpacked directly into an arbitrary location and then used as a self-contained Python environment. There’s no ``.data`` directory or install scheme keys, because the Python environment knows which install scheme it’s using, so it can just put things in the right places to start with. The “arbitrary location” part is important: the pybi can’t contain any hardcoded absolute paths. In particular, any preinstalled scripts MUST NOT embed absolute paths in their shebang lines. Similar to wheels’ ``-.dist-info`` directory, the pybi archive must contain a top-level directory named ``pybi-info/``. (Rationale: calling it ``pybi-info`` instead ``dist-info`` makes sure that tools don’t get confused about which kind of metadata they’re looking at; leaving off the ``{name}-{version}`` part is fine because only one pybi can be installed into a given directory.) The ``pybi-info/`` directory contains at least the following files: - ``.../PYBI``: metadata about the archive itself, in the same RFC822-ish format as ``METADATA`` and ``WHEEL`` files: :: Pybi-Version: 1.0 Generator: {name} {version} Tag: {platform tag} Tag: {another platform tag} Tag: {...and so on...} Build: 1 # optional - ``.../RECORD``: same as in wheels, except see the note about symlinks, below. - ``.../METADATA``: In the same format as described in the current core metadata spec, except that the following keys are forbidden because they don’t make sense: - ``Requires-Dist`` - ``Provides-Extra`` - ``Requires-Python`` And also there are some new, required keys described below. Pybi-specific core metadata ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here's an example of the new ``METADATA`` fields, before we give the full details:: Pybi-Environment-Marker-Variables: {"implementation_name": "cpython", "implementation_version": "3.10.8", "os_name": "posix", "platform_machine": "x86_64", "platform_system": "Linux", "python_full_version": "3.10.8", "platform_python_implementation": "CPython", "python_version": "3.10", "sys_platform": "linux"} Pybi-Paths: {"stdlib": "lib/python3.10", "platstdlib": "lib/python3.10", "purelib": "lib/python3.10/site-packages", "platlib": "lib/python3.10/site-packages", "include": "include/python3.10", "platinclude": "include/python3.10", "scripts": "bin", "data": "."} Pybi-Wheel-Tag: cp310-cp310-PLATFORM Pybi-Wheel-Tag: cp310-abi3-PLATFORM Pybi-Wheel-Tag: cp310-none-PLATFORM Pybi-Wheel-Tag: cp39-abi3-PLATFORM Pybi-Wheel-Tag: cp38-abi3-PLATFORM Pybi-Wheel-Tag: cp37-abi3-PLATFORM Pybi-Wheel-Tag: cp36-abi3-PLATFORM Pybi-Wheel-Tag: cp35-abi3-PLATFORM Pybi-Wheel-Tag: cp34-abi3-PLATFORM Pybi-Wheel-Tag: cp33-abi3-PLATFORM Pybi-Wheel-Tag: cp32-abi3-PLATFORM Pybi-Wheel-Tag: py310-none-PLATFORM Pybi-Wheel-Tag: py3-none-PLATFORM Pybi-Wheel-Tag: py39-none-PLATFORM Pybi-Wheel-Tag: py38-none-PLATFORM Pybi-Wheel-Tag: py37-none-PLATFORM Pybi-Wheel-Tag: py36-none-PLATFORM Pybi-Wheel-Tag: py35-none-PLATFORM Pybi-Wheel-Tag: py34-none-PLATFORM Pybi-Wheel-Tag: py33-none-PLATFORM Pybi-Wheel-Tag: py32-none-PLATFORM Pybi-Wheel-Tag: py31-none-PLATFORM Pybi-Wheel-Tag: py30-none-PLATFORM Pybi-Wheel-Tag: py310-none-any Pybi-Wheel-Tag: py3-none-any Pybi-Wheel-Tag: py39-none-any Pybi-Wheel-Tag: py38-none-any Pybi-Wheel-Tag: py37-none-any Pybi-Wheel-Tag: py36-none-any Pybi-Wheel-Tag: py35-none-any Pybi-Wheel-Tag: py34-none-any Pybi-Wheel-Tag: py33-none-any Pybi-Wheel-Tag: py32-none-any Pybi-Wheel-Tag: py31-none-any Pybi-Wheel-Tag: py30-none-any Specification: - ``Pybi-Environment-Marker-Variables``: The value of all PEP 508 environment marker variables that are static across installs of this Pybi, as a JSON dict. So for example: - ``python_version`` will always be present, because a Python 3.10 package always has ``python_version == "3.10"``. - ``platform_version`` will generally not be present, because it gives detailed information about the OS where Python is running, for example:: #60-Ubuntu SMP Thu May 6 07:46:32 UTC 2021 ``platform_release`` has similar issues. - ``platform_machine`` will *usually* be present, except for macOS universal2 pybis: these can potentially be run in either x86-64 or arm64 mode, and we don't know which until the interpreter is actually invoked, so we can't record it in static metadata. **Rationale:** In many cases, this should allow a resolver running on Linux to compute package pins for a Python environment on Windows, or vice-versa, so long as the resolver has access to the target platform’s .pybi file. (Note that ``Requires-Python`` constraints can be checked by using the ``python_full_version`` value.) While we have to leave out a few keys sometimes, they're either fairly useless (``platform_version``, ``platform_release``) or can be reconstructed by the resolver (``platform_machine``). The markers are also just generally useful information to have accessible. For example, if you have a ``pypy3-7.3.2`` pybi, and you want to know what version of the Python language that supports, then that’s recorded in the ``python_version`` marker. (Note: we may want to deprecate/remove ``platform_version`` and ``platform_release``? They're problematic and I can't figure out any cases where they're useful. But that's out of scope of this particular PEP.) - ``Pybi-Paths``: The install paths needed to install wheels (same keys as ``sysconfig.get_paths()``), as relative paths starting at the root of the zip file, as a JSON dict. These paths MUST be written in Unix format, using forward slashes as a separator, not backslashes. It must be possible to invoke the Python interpreter by running ``{paths["scripts"]}/python``. If there are alternative interpreter entry points (e.g. ``pythonw`` for Windows GUI apps), then they should also be in that directory under their conventional names, with no version number attached. (You can *also* have a ``python3.11`` symlink if you want; there’s no rule against that. It’s just that ``python`` has to exist and work.) **Rationale:** ``Pybi-Paths`` and ``Pybi-Wheel-Tag``\ s (see below) are together enough to let an installer choose wheels and install them into an unpacked pybi environment, without invoking Python. Besides, we need to write down the interpreter location somewhere, so it’s two birds with one stone. - ``Pybi-Wheel-Tag``: The wheel tags supported by this interpreter, in preference order (most-preferred first, least-preferred last), except that the special platform tag ``PLATFORM`` should replace any platform tags that depend on the final installation system. **Discussion:** It would be nice™ if installers could compute a pybi’s corresponding wheel tags ahead of time, so that they could install wheels into the unpacked pybi without needing to actually invoke the python interpreter to query its tags – both for efficiency and to allow for more exotic use cases like setting up a Windows environment from a Linux host. But unfortunately, it’s impossible to compute the full set of platform tags supported by a Python installation ahead of time, because they can depend on the final system: - A pybi tagged ``manylinux_2_12_x86_64`` can always use wheels tagged as ``manylinux_2_12_x86_64``. It also *might* be able to use wheels tagged ``manylinux_2_17_x86_64``, but only if the final installation system has glibc 2.17+. - A pybi tagged ``macosx_11_0_universal2`` (= x86-64 + arm64 support in the same binary) might be able to use wheels tagged as ``macosx_11_0_arm64``, but only if it’s installed on an “Apple Silicon” machine and running in arm64 mode. In these two cases, an installation tool can still work out the appropriate set of wheel tags by computing the local platform tags, taking the wheel tag templates from ``Pybi-Wheel-Tag``, and swapping in the actual supported platforms in place of the magic ``PLATFORM`` string. However, there are other cases that are even more complicated: - You can (usually) run both 32- and 64-bit apps on 64-bit Windows. So a pybi installer might compute the set of allowable pybi tags on the current platform as [``win32``, ``win_amd64``]. But you can’t then just take that set and swap it into the pybi’s wheel tag template or you get nonsense: :: [ "cp39-cp39-win32", "cp39-cp39-win_amd64", "cp39-abi3-win32", "cp39-abi3-win_amd64", ... ] To handle this, the installer needs to somehow understand that a ``manylinux_2_12_x86_64`` pybi can use a ``manylinux_2_17_x86_64`` wheel as long as those are both valid tags on the current machine, but a ``win32`` pybi *can’t* use a ``win_amd64`` wheel, even if those are both valid tags on the current machine. - A pybi tagged ``macosx_11_0_universal2`` might be able to use wheels tagged as ``macosx_11_0_x86_64``, but only if it’s installed on an x86-64 machine *or* it’s installed on an ARM machine *and* the interpreter is invoked with the magic incantation that tells macOS to run a binary in x86-64 mode. So how the installer plans to invoke the pybi matters too! So actually using ``Pybi-Wheel-Tag`` values is less trivial than it might seem, and they’re probably only useful with fairly sophisticated tooling. But, smart pybi installers will already have to understand a lot of these platform compatibility issues in order to select a working pybi, and for the cross-platform pinning/environment building case, users can potentially provide whatever information is needed to disambiguate exactly what platform they’re targeting. So, it’s still useful enough to include in the PyBI metadata -- tools that don't find it useful can simply ignore it. You can probably generate these metadata values by running this script on the built interpreter: .. code:: python import packaging.markers import packaging.tags import sysconfig import os.path import json import sys marker_vars = packaging.markers.default_environment() # Delete any keys that depend on the final installation del marker_vars["platform_release"] del marker_vars["platform_version"] # Darwin binaries are often multi-arch, so play it safe and # delete the architecture marker. (Better would be to only # do this if the pybi actually is multi-arch.) if marker_vars["sys_platform"] == "darwin": del marker_vars["platform_machine"] # Copied and tweaked version of packaging.tags.sys_tags tags = [] interp_name = packaging.tags.interpreter_name() if interp_name == "cp": tags += list(packaging.tags.cpython_tags(platforms=["xyzzy"])) else: tags += list(packaging.tags.generic_tags(platforms=["xyzzy"])) tags += list(packaging.tags.compatible_tags(platforms=["xyzzy"])) # Gross hack: packaging.tags normalizes platforms by lowercasing them, # so we generate the tags with a unique string and then replace it # with our special uppercase placeholder. str_tags = [str(t).replace("xyzzy", "PLATFORM") for t in tags] (base_path,) = sysconfig.get_config_vars("installed_base") # For some reason, macOS framework builds report their # installed_base as a directory deep inside the framework. while "Python.framework" in base_path: base_path = os.path.dirname(base_path) paths = {key: os.path.relpath(path, base_path).replace("\\", "/") for (key, path) in sysconfig.get_paths().items()} json.dump({"marker_vars": marker_vars, "tags": str_tags, "paths": paths}, sys.stdout) This emits a JSON dict on stdout with separate entries for each set of pybi-specific tags. Symlinks -------- Currently, symlinks are used by default in all Unix Python installs (e.g., ``bin/python3 -> bin/python3.9``). And furthermore, symlinks are *required* to store macOS framework builds in ``.pybi`` files. So, unlike wheel files, we absolutely have to support symlinks in ``.pybi`` files for them to be useful at all. Representing symlinks in zip files ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The de-facto standard for representing symlinks in zip files is the Info-Zip symlink extension, which works as follows: - The symlink’s target path is stored as if it were the file contents - The top 4 bits of the Unix permissions field are set to ``0xa``, i.e.: ``permissions & 0xf000 == 0xa000`` - The Unix permissions field, in turn, is stored as the top 16 bits of the “external attributes” field. So if using Python’s ``zipfile`` module, you can check whether a ``ZipInfo`` represents a symlink by doing: .. code:: python (zip_info.external_attr >> 16) & 0xf000 == 0xa000 Or if using Rust’s ``zip`` crate, the equivalent check is: .. code:: rust fn is_symlink(zip_file: &zip::ZipFile) -> bool { match zip_file.unix_mode() { Some(mode) => mode & 0xf000 == 0xa000, None => false, } } If you’re on Unix, your ``zip`` and ``unzip`` commands probably understands this format already. Representing symlinks in RECORD files ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Normally, a ``RECORD`` file lists each file + its hash + its length: .. code:: text my/favorite/file,sha256=...,12345 For symlinks, we instead write: .. code:: text name/of/symlink,symlink=path/to/symlink/target, That is: we use a special “hash function” called ``symlink``, and then store the actual symlink target as the “hash value”. And the length is left empty. **Rationale:** we’re already committed to the ``RECORD`` file containing a redundant check on everything in the main archive, so for symlinks we at least need to store some kind of hash, plus some kind of flag to indicate that this is a symlink. Given that symlink target strings are roughly the same size as a hash, we might as well store them directly. This also makes the symlink information easier to access for tools that don’t understand the Info-Zip symlink extension, and makes it possible to losslessly unpack and repack a Unix pybi on a Windows system, which someone might find handy at some point. Storing symlinks in ``pybi`` files ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When a pybi creator stores a symlink, they MUST use both of the mechanisms defined above: storing it in the zip archive directly using the Info-Zip representation, and also recording it in the ``RECORD`` file. Pybi consumers SHOULD validate that the symlinks in the archive and ``RECORD`` file are consistent with each other. We also considered using *only* the ``RECORD`` file to store symlinks, but then the vanilla ``unzip`` tool wouldn’t be able to unpack them, and that would make it hard to install a pybi from a shell script. Limitations ~~~~~~~~~~~ Symlinks enable a lot of potential messiness. To keep things under control, we impose the following restrictions: - Symlinks MUST NOT be used in ``.pybi``\ s targeting Windows, or other platforms that are missing first-class symlink support. - Symlinks MUST NOT be used inside the ``pybi-info`` directory. (Rationale: there’s no need, and it makes things simpler for resolvers that need to extract info from ``pybi-info`` without unpacking the whole archive.) - Symlink targets MUST be relative paths, and MUST be inside the pybi directory. - If ``A/B/...`` is recorded as a symlink in the archive, then there MUST NOT be any other entries in the archive named like ``A/B/.../C``. For example, if an archive has a symlink ``foo -> bar``, and then later in the archive there’s a regular file named ``foo/blah.py``, then a naive unpacker could potentially end up writing a file called ``bar/blah.py``. Don’t be naive. Unpackers MUST verify that these rules are followed, because without them attackers could create evil symlinks like ``foo -> /etc/passwd`` or ``foo -> ../../../../../etc`` + ``foo/passwd -> ...`` and cause havoc. Non-normative comments ====================== Why not just use conda? ----------------------- This isn't really in the scope of this PEP, but since conda is a popular way to distribute binary Python interpreters, it's a natural question. The simple answer is: conda is great! But, there are lots of python users who aren't conda users, and they deserve nice things too. This PEP just gives them another option. The deeper answer is: the maintainers who upload packages to PyPI are the backbone of the Python ecosystem. They're the first audience for Python packaging tools. And one thing they want is to upload a package once, and have it be accessible across all the different ways Python is deployed: in Debian and Fedora and Homebrew and FreeBSD, in Conda environments, in big companies' monorepos, in Nix, in Blender plugins, in RenPy games, ..... you get the idea. All of these environments have their own tooling and strategies for managing packages and dependencies. So what's special about PyPI and wheels is that they're designed to describe dependencies in a *standard, abstract way*, that all these downstream systems can consume and convert into their local conventions. That's why package maintainers use Python-specific metadata and upload to PyPI: because it lets them address all of those systems simultaneously. Every time you build a Python package for conda, there's an intermediate wheel that's generated, because wheels are the common language that Python package build systems and conda can use to talk to each other. But then, if you're a maintainer releasing an sdist+wheels, then you naturally want to test what you're releasing, which may depend on arbitrary PyPI packages and versions. So you need tools that build Python environments directly from PyPI, and conda is fundamentally not designed to do that. So conda and pip are both necessary for different cases, and this proposal happens to be targeting the pip side of that equation. Sdists (or not) --------------- It might be cool to have an “sdist” equivalent for pybis, i.e., some kind of format for a Python source release that’s structured-enough to let tools automatically fetch and build it into a pybi, for platforms where prebuilt pybis aren’t available. But, this isn’t necessary for the MVP and opens a can of worms, so let’s worry about it later. What packages should be bundled inside a pybi? ---------------------------------------------- Pybi builders have the power to pick and choose what exactly goes inside. For example, you could include some preinstalled packages in the pybi’s ``site-packages`` directory, or prune out bits of the stdlib that you don’t want. We can’t stop you! Though if you do preinstall packages, then it's strongly recommended to also include the correct metadata (``.dist-info`` etc.), so that it’s possible for Pip or other tools to understand out what’s going on. For my prototype “general purpose” pybi’s, what I chose is: - Make sure ``site-packages`` is *empty*. **Rationale:** for traditional standalone python installers that are targeted at end-users, you probably want to include at least ``pip``, to avoid bootstrapping issues (:pep:`453`). But pybis are different: they’re designed to be installed by “smart” tooling, that consume the pybi as part of some kind of larger automated deployment process. It’s easier for these installers to start from a blank slate and then add whatever they need, than for them to start with some preinstalled packages that they may or may not want. (And besides, you can still run ``python -m ensurepip``.) - Include the full stdlib, *except* for ``test``. **Rationale:** the top-level ``test`` module contains CPython’s own test suite. It’s huge (CPython without ``test`` is ~37 MB, then ``test`` adds another ~25 MB on top of that!), and essentially never used by regular user code. Also, as precedent, the official nuget packages, the official manylinux images, and multiple Linux distributions all leave it out, and this hasn’t caused any major problems. So this seems like the best way to balance broad compatibility with reasonable download/install sizes. - I’m not shipping any ``.pyc`` files. They take up space in the download, can be generated on the final system at minimal cost, and dropping them removes a source of location-dependence. (``.pyc`` files store the absolute path of the corresponding ``.py`` file and include it in tracebacks; but, pybis are relocatable, so the correct path isn’t known until after install.) Backwards Compatibility ======================= No backwards compatibility considerations. Security Implications ===================== No security implications, beyond the fact that anyone who takes it upon themselves to distribute binaries has to come up with a plan to manage their security (e.g., whether they roll a new build after an OpenSSL CVE drops). But collectively, we core Python folks are already maintaining binary builds for all major platforms (macOS + Windows through python.org, and Linux builds through the official manylinux image), so even if we do start releasing official CPython builds on PyPI it doesn't really raise any new security issues. How to Teach This ================= This isn't targeted at end-users; their experience will simply be that e.g. their pyenv or tox invocation magically gets faster and more reliable (if those projects' maintainers decide to take advantage of this PEP). Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.