python-peps/pep-0517.txt

PEP: 517
Title: A build-system independent format for source trees
Version: $Revision$
Last-Modified: $Date$
Author: Nathaniel J. Smith <njs@pobox.com>,
        Thomas Kluyver <thomas@kluyver.me.uk>
BDFL-Delegate: Nick Coghlan <ncoghlan@gmail.com>
Discussions-To: <distutils-sig@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 30-Sep-2015
Post-History: 1 Oct 2015, 25 Oct 2015


==========
 Abstract
==========

While ``distutils`` / ``setuptools`` have taken us a long way, they
suffer from three serious problems: (a) they're missing important
features like usable build-time dependency declaration,
autoconfiguration, and even basic ergonomic niceties like `DRY
<https://en.wikipedia.org/wiki/Don%27t_repeat_yourself>`_-compliant
version number management, and (b) extending them is difficult, so
while there do exist various solutions to the above problems, they're
often quirky, fragile, and expensive to maintain, and yet (c) it's
very difficult to use anything else, because distutils/setuptools
provide the standard interface for installing packages expected by
both users and installation tools like ``pip``.

Previous efforts (e.g. distutils2 or setuptools itself) have attempted
to solve problems (a) and/or (b). This proposal aims to solve (c).

The goal of this PEP is get distutils-sig out of the business of being
a gatekeeper for Python build systems. If you want to use distutils,
great; if you want to use something else, then that should be easy to
do using standardized methods. The difficulty of interfacing with
distutils means that there aren't many such systems right now, but to
give a sense of what we're thinking about see `flit
<https://github.com/takluyver/flit>`_ or `bento
<https://cournape.github.io/Bento/>`_. Fortunately, wheels have now
solved many of the hard problems here -- e.g. it's no longer necessary
that a build system also know about every possible installation
configuration -- so pretty much all we really need from a build system
is that it have some way to spit out standard-compliant wheels and
sdists.

We therefore propose a new, relatively minimal interface for
installation tools like ``pip`` to interact with package source trees
and source distributions.

==========================
 Reversion to Draft Status
==========================

While this PEP was provisionally accepted for implementation in `pip` and other
tools, some additional concerns were subsequently raised around adequately
supporting out of tree builds. It has been reverted to Draft status while those
concerns are being resolved.

=======================
 Terminology and goals
=======================

A *source tree* is something like a VCS checkout. We need a standard
interface for installing from this format, to support usages like
``pip install some-directory/``.

A *source distribution* is a static snapshot representing a particular
release of some source code, like ``lxml-3.4.4.zip``. Source
distributions serve many purposes: they form an archival record of
releases, they provide a stupid-simple de facto standard for tools
that want to ingest and process large corpora of code, possibly
written in many languages (e.g. code search), they act as the input to
downstream packaging systems like Debian/Fedora/Conda/..., and so
forth. In the Python ecosystem they additionally have a particularly
important role to play, because packaging tools like ``pip`` are able
to use source distributions to fulfill binary dependencies, e.g. if
there is a distribution ``foo.whl`` which declares a dependency on
``bar``, then we need to support the case where ``pip install bar`` or
``pip install foo`` automatically locates the sdist for ``bar``,
downloads it, builds it, and installs the resulting package.

Source distributions are also known as *sdists* for short.

A *build frontend* is a tool that users might run that takes arbitrary
source trees or source distributions and builds wheels from them. The
actual building is done by each source tree's *build backend*. In a
command like ``pip wheel some-directory/``, pip is acting as a build
frontend.

An *integration frontend* is a tool that users might run that takes a
set of package requirements (e.g. a requirements.txt file) and
attempts to update a working environment to satisfy those
requirements. This may require locating, building, and installing a
combination of wheels and sdists. In a command like ``pip install
lxml==2.4.0``, pip is acting as an integration frontend.


==============
 Source trees
==============

There is an existing, legacy source tree format involving
``setup.py``. We don't try to specify it further; its de facto
specification is encoded in the source code and documentation of
``distutils``, ``setuptools``, ``pip``, and other tools. We'll refer
to it as the ``setup.py``\-style.

Here we define a new style of source tree based around the
``pyproject.toml`` file defined in PEP 518, extending the
``[build-system]`` table in that file with one additional key,
``build-backend``. Here's an example of how it would look::

    [build-system]
    # Defined by PEP 518:
    requires = ["flit"]
    # Defined by this PEP:
    build-backend = "flit.api:main"

``build-backend`` is a string naming a Python object that will be
used to perform the build (see below for details). This is formatted
following the same ``module:object`` syntax as a ``setuptools`` entry
point. For instance, if the string is ``"flit.api:main"`` as in the
example above, this object would be looked up by executing the
equivalent of::

    import flit.api
    backend = flit.api.main

It's also legal to leave out the ``:object`` part, e.g. ::

    build-backend = "flit.api"

which acts like::

    import flit.api
    backend = flit.api

Formally, the string should satisfy this grammar::

    identifier = (letter | '_') (letter | '_' | digit)*
    module_path = identifier ('.' identifier)*
    object_path = identifier ('.' identifier)*
    entry_point = module_path (':' object_path)?

And we import ``module_path`` and then lookup
``module_path.object_path`` (or just ``module_path`` if
``object_path`` is missing).

If the ``pyproject.toml`` file is absent, or the ``build-backend``
key is missing, the source tree is not using this specification, and
tools should fall back to running ``setup.py``.

Where the ``build-backend`` key exists, it takes precedence over
``setup.py``, and source trees need not include ``setup.py`` at all.
Projects may still wish to include a ``setup.py`` for compatibility
with tools that do not use this spec.

=========================
 Build backend interface
=========================

The build backend object is expected to have attributes which provide
some or all of the following hooks. The common ``config_settings``
argument is described after the individual hooks::

  def get_build_requires(config_settings):
      ...

This hook MUST return an additional list of strings containing PEP 508
dependency specifications, above and beyond those specified in the
``pyproject.toml`` file. Example::

  def get_build_requires(config_settings):
      return ["wheel >= 0.25", "setuptools"]

Optional. If not defined, the default implementation is equivalent to
``return []``.

::

  def prepare_wheel_metadata(metadata_directory, config_settings):
      ...

Must create a ``.dist-info`` directory containing wheel metadata
inside the specified ``metadata_directory`` (i.e., creates a directory
like ``{metadata_directory}/{package}-{version}.dist-info/``. This
directory MUST be a valid ``.dist-info`` directory as defined in the
wheel specification, except that it need not contain ``RECORD`` or
signatures. The hook MAY also create other files inside this
directory, and a build frontend MUST ignore such files; the intention
here is that in cases where the metadata depends on build-time
decisions, the build backend may need to record these decisions in
some convenient format for re-use by the actual wheel-building step.

This must return the basename (not the full path) of the ``.dist-info``
directory it creates, as a unicode string.

Optional. If a build frontend needs this information and the method is
not defined, it should call ``build_wheel`` and look at the resulting
metadata directly.

::

  def build_wheel(wheel_directory, config_settings, metadata_directory=None):
      ...

Must build a .whl file, and place it in the specified ``wheel_directory``.

If the build frontend has previously called ``prepare_wheel_metadata`` and
depends on the wheel resulting from this call to have metadata
matching this earlier call, then it should provide the path to the
previous ``metadata_directory`` as an argument. If this argument is
provided, then ``build_wheel`` MUST produce a wheel with identical
metadata. The directory passed in by the build frontend MUST be
identical to the directory created by ``prepare_wheel_metadata``,
including any unrecognized files it created.

This must return the basename (not the full path) of the ``.whl`` file it
creates, as a unicode string.

Mandatory.

::

  def build_sdist(sdist_directory, config_settings):
      ...

Must build a .tar.gz source distribution and place it in the specified
``sdist_directory``.

A .tar.gz source distribution (sdist) contains a single top-level directory called
``{name}-{version}`` (e.g. ``foo-1.0``), containing the source files of the
package. This directory must also contain the
``pyproject.toml`` from the build directory, and a PKG-INFO file containing
metadata in the format described in
`PEP 345 <https://www.python.org/dev/peps/pep-0345/>`_. Although historically
zip files have also been used as sdists, this hook should produce a gzipped
tarball. This is already the more common format for sdists, and having a
consistent format makes for simpler tooling.

The generated tarball should use the modern POSIX.1-2001 pax tar format, which
specifies UTF-8 based file names. This is not yet the default for the tarfile
module shipped with Python 3.6, so backends using the tarfile module need to
explicitly pass ``format=tarfile.PAX_FORMAT``.

This must return the basename (not the full path) of the ``.tar.gz`` file it
creates, as a unicode string.

Mandatory, but it may not succeed in all situations: for instance, some tools
can only build an sdist from a VCS checkout.

::

  def prepare_build_files(build_directory, config_settings):
      ...

Must copy or create any files needed to build a wheel of this package into
``build_directory``. For instance, ``pyproject.toml`` should
be copied unmodified into the root of this directory. For tools such
as `setuptools_scm <https://github.com/pypa/setuptools_scm>`_, this may include
extracting some information from a version control system.
The ``build_wheel`` hook will subsequently be run from the ``build_directory``
populated by this hook. The contents of the resulting wheel should be the same
whether ``build_wheel`` is invoked in an original source directory, the build
directory populated by this hook, or an unpacked sdist directory.

Because the wheel will be built from a temporary build directory, ``build_wheel``
may create intermediate files in the working directory, and does not need to
take care to clean them up.

The return value will be ignored.

Optional. If this hook is not defined, frontends may call ``build_sdist``
and unpack the archive to use as a build directory. Backends in which
building an sdist has additional requirements should define
``prepare_build_files``.

.. note:: Editable installs

   This PEP originally specified another hook, ``install_editable``, to do an
   editable install (as with ``pip install -e``). It was removed due to the
   complexity of the topic, but may be specified in a later PEP.

   Briefly, the questions to be answered include: what reasonable ways existing
   of implementing an 'editable install'? Should the backend or the frontend
   pick how to make an editable install? And if the frontend does, what does it
   need from the backend to do so.

::

  config_settings

This argument, which is passed to all hooks, is an arbitrary
dictionary provided as an "escape hatch" for users to pass ad-hoc
configuration into individual package builds. Build backends MAY
assign any semantics they like to this dictionary. Build frontends
SHOULD provide some mechanism for users to specify arbitrary
string-key/string-value pairs to be placed in this dictionary. For
example, they might support some syntax like ``--package-config
CC=gcc``. Build frontends MAY also provide arbitrary other mechanisms
for users to place entries in this dictionary. For example, ``pip``
might choose to map a mix of modern and legacy command line arguments
like::

  pip install                                           \
    --package-config CC=gcc                             \
    --global-option="--some-global-option"              \
    --build-option="--build-option1"                    \
    --build-option="--build-option2"

into a ``config_settings`` dictionary like::

  {
   "CC": "gcc",
   "--global-option": ["--some-global-option"],
   "--build-option": ["--build-option1", "--build-option2"],
  }

Of course, it's up to users to make sure that they pass options which
make sense for the particular build backend and package that they are
building.

The hooks may be called with positional or keyword arguments, so backends
implementing them should be careful to make sure that their signatures match
both the order and the names of the arguments above.

All hooks are run with working directory set to the root of the source
tree, and MAY print arbitrary informational text on stdout and
stderr. They MUST NOT read from stdin, and the build frontend MAY
close stdin before invoking the hooks.

The build frontend may capture stdout and/or stderr from the backend. If the
backend detects that an output stream is not a terminal/console (e.g.
``not sys.stdout.isatty()``), it SHOULD ensure that any output it writes to that
stream is UTF-8 encoded. The build frontend MUST NOT fail if captured output is
not valid UTF-8, but it MAY not preserve all the information in that case (e.g.
it may decode using the *replace* error handler in Python). If the output stream
is a terminal, the build backend is responsible for presenting its output
accurately, as for any program running in a terminal.

If a hook raises an exception, or causes the process to terminate,
then this indicates an error.


Build environment
=================

One of the responsibilities of a build frontend is to set up the
Python environment in which the build backend will run.

We do not require that any particular "virtual environment" mechanism
be used; a build frontend might use virtualenv, or venv, or no special
mechanism at all. But whatever mechanism is used MUST meet the
following criteria:

- All requirements specified by the project's build-requirements must
  be available for import from Python. In particular:

  - The ``get_build_requires`` hook is executed in an environment
    which contains the bootstrap requirements specified in the
    ``pyproject.toml`` file.

  - All other hooks are executed in an environment which contains both
    the bootstrap requirements specified in the ``pyproject.toml``
    hook and those specified by the ``get_build_requires`` hook.

- This must remain true even for new Python subprocesses spawned by
  the build environment, e.g. code like::

    import sys, subprocess
    subprocess.check_call([sys.executable, ...])

  must spawn a Python process which has access to all the project's
  build-requirements. This is necessary e.g. for build backends that
  want to run legacy ``setup.py`` scripts in a subprocess.

- All command-line scripts provided by the build-required packages
  must be present in the build environment's PATH. For example, if a
  project declares a build-requirement on `flit
  <https://flit.readthedocs.org/en/latest/>`__, then the following must
  work as a mechanism for running the flit command-line tool::

    import subprocess
    subprocess.check_call(["flit", ...])

A build backend MUST be prepared to function in any environment which
meets the above criteria. In particular, it MUST NOT assume that it
has access to any packages except those that are present in the
stdlib, or that are explicitly declared as build-requirements.


Recommendations for build frontends (non-normative)
---------------------------------------------------

A build frontend MAY use any mechanism for setting up a build
environment that meets the above criteria. For example, simply
installing all build-requirements into the global environment would be
sufficient to build any compliant package -- but this would be
sub-optimal for a number of reasons. This section contains
non-normative advice to frontend implementors.

A build frontend SHOULD, by default, create an isolated environment
for each build, containing only the standard library and any
explicitly requested build-dependencies. This has two benefits:

- It allows for a single installation run to build multiple packages
  that have contradictory build-requirements. E.g. if package1
  build-requires pbr==1.8.1, and package2 build-requires pbr==1.7.2,
  then these cannot both be installed simultaneously into the global
  environment -- which is a problem when the user requests ``pip
  install package1 package2``. Or if the user already has pbr==1.8.1
  installed in their global environment, and a package build-requires
  pbr==1.7.2, then downgrading the user's version would be rather
  rude.

- It acts as a kind of public health measure to maximize the number of
  packages that actually do declare accurate build-dependencies. We
  can write all the strongly worded admonitions to package authors we
  want, but if build frontends don't enforce isolation by default,
  then we'll inevitably end up with lots of packages on PyPI that
  build fine on the original author's machine and nowhere else, which
  is a headache that no-one needs.

However, there will also be situations where build-requirements are
problematic in various ways. For example, a package author might
accidentally leave off some crucial requirement despite our best
efforts; or, a package might declare a build-requirement on ``foo >=
1.0`` which worked great when 1.0 was the latest version, but now 1.1
is out and it has a showstopper bug; or, the user might decide to
build a package against numpy==1.7 -- overriding the package's
preferred numpy==1.8 -- to guarantee that the resulting build will be
compatible at the C ABI level with an older version of numpy (even if
this means the resulting build is unsupported upstream). Therefore,
build frontends SHOULD provide some mechanism for users to override
the above defaults. For example, a build frontend could have a
``--build-with-system-site-packages`` option that causes the
``--system-site-packages`` option to be passed to
virtualenv-or-equivalent when creating build environments, or a
``--build-requirements-override=my-requirements.txt`` option that
overrides the project's normal build-requirements.

The general principle here is that we want to enforce hygiene on
package *authors*, while still allowing *end-users* to open up the
hood and apply duct tape when necessary.


======================
 Source distributions
======================

For now, we continue with the legacy sdist format which is mostly
undefined, but basically comes down to: a file named
``{NAME}-{VERSION}.{EXT}``, which unpacks into a buildable source tree
called ``{NAME}-{VERSION}/``. Traditionally these have always
contained ``setup.py``\-style source trees; we now allow them to also
contain ``pyproject.toml``\-style source trees.

Integration frontends require that an sdist named
``{NAME}-{VERSION}.{EXT}`` will generate a wheel named
``{NAME}-{VERSION}-{COMPAT-INFO}.whl``.


===================================
 Comparison to competing proposals
===================================

The primary difference between this and competing proposals (in
particular, PEP 516) is
that our build backend is defined via a Python hook-based interface
rather than a command-line based interface.

We do *not* expect that this will, by itself, intrinsically reduce the
complexity calling into the backend, because build frontends will
in any case want to run hooks inside a child -- this is important to
isolate the build frontend itself from the backend code and to better
control the build backends execution environment. So under both
proposals, there will need to be some code in ``pip`` to spawn a
subprocess and talk to some kind of command-line/IPC interface, and
there will need to be some code in the subprocess that knows how to
parse these command line arguments and call the actual build backend
implementation. So this diagram applies to all proposals equally::

  +-----------+          +---------------+           +----------------+
  | frontend  | -spawn-> | child cmdline | -Python-> |    backend     |
  |   (pip)   |          |   interface   |           | implementation |
  +-----------+          +---------------+           +----------------+



The key difference between the two approaches is how these interface
boundaries map onto project structure::

  .-= This PEP =-.

  +-----------+          +---------------+    |      +----------------+
  | frontend  | -spawn-> | child cmdline | -Python-> |    backend     |
  |   (pip)   |          |   interface   |    |      | implementation |
  +-----------+          +---------------+    |      +----------------+
                                              |
  |______________________________________|    |
     Owned by pip, updated in lockstep        |
                                              |
                                              |
                                   PEP-defined interface boundary
                                 Changes here require distutils-sig


  .-= Alternative =-.

  +-----------+    |     +---------------+           +----------------+
  | frontend  | -spawn-> | child cmdline | -Python-> |    backend     |
  |   (pip)   |    |     |   interface   |           | implementation |
  +-----------+    |     +---------------+           +----------------+
                   |
                   |     |____________________________________________|
                   |      Owned by build backend, updated in lockstep
                   |
      PEP-defined interface boundary
    Changes here require distutils-sig


By moving the PEP-defined interface boundary into Python code, we gain
three key advantages.

**First**, because there will likely be only a small number of build
frontends (``pip``, and... maybe a few others?), while there will
likely be a long tail of custom build backends (since these are chosen
separately by each package to match their particular build
requirements), the actual diagrams probably look more like::

  .-= This PEP =-.

  +-----------+          +---------------+           +----------------+
  | frontend  | -spawn-> | child cmdline | -Python+> |    backend     |
  |   (pip)   |          |   interface   |        |  | implementation |
  +-----------+          +---------------+        |  +----------------+
                                                  |
                                                  |  +----------------+
                                                  +> |    backend     |
                                                  |  | implementation |
                                                  |  +----------------+
                                                  :
                                                  :

  .-= Alternative =-.

  +-----------+          +---------------+           +----------------+
  | frontend  | -spawn+> | child cmdline | -Python-> |    backend     |
  |   (pip)   |       |  |   interface   |           | implementation |
  +-----------+       |  +---------------+           +----------------+
                      |
                      |  +---------------+           +----------------+
                      +> | child cmdline | -Python-> |    backend     |
                      |  |   interface   |           | implementation |
                      |  +---------------+           +----------------+
                      :
                      :

That is, this PEP leads to less total code in the overall
ecosystem. And in particular, it reduces the barrier to entry of
making a new build system. For example, this is a complete, working
build backend::

    # mypackage_custom_build_backend.py
    import os.path
    import pathlib

    def get_build_requires(config_settings, config_directory):
        return ["wheel"]

    def build_wheel(wheel_directory, config_settings, config_directory=None):
        from wheel.archive import archive_wheelfile
        filename = "mypackage-0.1-py2.py3-none-any"
        path = os.path.join(wheel_directory, filename)
        archive_wheelfile(path, "src/")
        return filename

    def _exclude_hidden_and_special_files(archive_entry):
        """Tarfile filter to exclude hidden and special files from the archive"""
        if entry.isfile() or entry.isdir():
            if not os.path.basename(archive_entry.name).startswith("."):
                return archive_entry
        return None

    def build_sdist(sdist_dir, config_settings):
        sdist_subdir = "mypackage-0.1"
        sdist_path = pathlib.Path(sdist_dir) / (sdist_subdir + ".tar.gz")
        sdist = tarfile.open(sdist_path, "w:gz", format=tarfile.PAX_FORMAT)
        # Tar up the whole directory, minus hidden and special files
        sdist.add(os.getcwd(), arcname=sdist_subdir,
                  filter=_exclude_hidden_and_special_files)
        return sdist_subdir + ".tar.gz"

Of course, this is a *terrible* build backend: it requires the user to
have manually set up the wheel metadata in
``src/mypackage-0.1.dist-info/``; when the version number changes it
must be manually updated in multiple places... but it works, and more features
could be added incrementally. Much experience suggests that large successful
projects often originate as quick hacks (e.g., Linux -- "just a hobby,
won't be big and professional"; `IPython/Jupyter
<https://en.wikipedia.org/wiki/IPython#Grants_and_awards>`_ -- `a grad
student's ``$PYTHONSTARTUP`` file
<http://blog.fperez.org/2012/01/ipython-notebook-historical.html>`_),
so if our goal is to encourage the growth of a vibrant ecosystem of
good build tools, it's important to minimize the barrier to entry.


**Second**, because Python provides a simpler yet richer structure for
describing interfaces, we remove unnecessary complexity from the
specification -- and specifications are the worst place for
complexity, because changing specifications requires painful
consensus-building across many stakeholders. In the command-line
interface approach, we have to come up with ad hoc ways to map
multiple different kinds of inputs into a single linear command line
(e.g. how do we avoid collisions between user-specified configuration
arguments and PEP-defined arguments? how do we specify optional
arguments? when working with a Python interface these questions have
simple, obvious answers). When spawning and managing subprocesses,
there are many fiddly details that must be gotten right, subtle
cross-platform differences, and some of the most obvious approaches --
e.g., using stdout to return data for the ``build_requires`` operation
-- can create unexpected pitfalls (e.g., what happens when computing
the build requirements requires spawning some child processes, and
these children occasionally print an error message to stdout?
obviously a careful build backend author can avoid this problem, but
the most obvious way of defining a Python interface removes this
possibility entirely, because the hook return value is clearly
demarcated).

In general, the need to isolate build backends into their own process
means that we can't remove IPC complexity entirely -- but by placing
both sides of the IPC channel under the control of a single project,
we make it much cheaper to fix bugs in the IPC interface than if
fixing bugs requires coordinated agreement and coordinated changes
across the ecosystem.

**Third**, and most crucially, the Python hook approach gives us much
more powerful options for evolving this specification in the future.

For concreteness, imagine that next year we add a new
``prepare_wheel_metadata2`` hook, which replaces the current
``prepare_wheel_metadata`` hook with something that produces more data, or a
different metadata format. In order to
manage the transition, we want it to be possible for build frontends
to transparently use ``prepare_wheel_metadata2`` when available and fall
back onto ``prepare_wheel_metadata`` otherwise; and we want it to be
possible for build backends to define both methods, for compatibility
with both old and new build frontends.

Furthermore, our mechanism should also fulfill two more goals: (a) If
new versions of e.g. ``pip`` and ``flit`` are both updated to support
the new interface, then this should be sufficient for it to be used;
in particular, it should *not* be necessary for every project that
*uses* ``flit`` to update its individual ``pyproject.toml`` file. (b)
We do not want to have to spawn extra processes just to perform this
negotiation, because process spawns can easily become a bottleneck when
deploying large multi-package stacks on some platforms (Windows).

In the interface described here, all of these goals are easy to
achieve. Because ``pip`` controls the code that runs inside the child
process, it can easily write it to do something like::

    command, backend, args = parse_command_line_args(...)
    if command == "prepare_wheel_metadata":
       if hasattr(backend, "prepare_wheel_metadata2"):
           backend.prepare_wheel_metadata2(...)
       elif hasattr(backend, "prepare_wheel_metadata"):
           backend.prepare_wheel_metadata(...)
       else:
           # error handling

In the alternative where the public interface boundary is placed at
the subprocess call, this is not possible -- either we need to spawn
an extra process just to query what interfaces are supported (as was
included in an earlier draft of PEP 516, an alternative to this), or
else we give up on autonegotiation entirely (as in the current version
of that PEP), meaning that any changes in the interface will require
N individual packages to update their ``pyproject.toml`` files before
any change can go live, and that any changes will necessarily be
restricted to new releases.

One specific consequence of this is that in this PEP, we're able to
make the ``prepare_wheel_metadata`` command optional. In our design, this
can easily be worked around by a tool like ``pip``, which can put code
in its subprocess runner like::

    def prepare_wheel_metadata(output_dir, config_settings):
         if hasattr(backend, "prepare_wheel_metadata"):
             backend.prepare_wheel_metadata(output_dir, config_settings)
         else:
             backend.build_wheel(output_dir, config_settings)
             touch(output_dir / "PIP_ALREADY_BUILT_WHEELS")
             unzip_metadata(output_dir/*.whl)

    def build_wheel(output_dir, config_settings, metadata_dir):
         if os.path.exists(metadata_dir / "PIP_ALREADY_BUILT_WHEELS"):
             copy(metadata_dir / *.whl, output_dir)
         else:
             backend.build_wheel(output_dir, config_settings, metadata_dir)

and thus expose a totally uniform interface to the rest of ``pip``,
with no extra subprocess calls, no duplicated builds, etc. But
obviously this is the kind of code that you only want to write as part
of a private, within-project interface.

(And, of course, making the ``metadata`` command optional is one piece
of lowering the barrier to entry, as discussed above.)


Other differences
=================

Besides the key command line versus Python hook difference described
above, there are a few other differences in this proposal:

* Metadata command is optional (as described above).

* We return metadata as a directory, rather than a single METADATA
  file. This aligns better with the way that in practice wheel metadata
  is distributed across multiple files (e.g. entry points), and gives us
  more options in the future. (For example, instead of following the PEP
  426 proposal of switching the format of METADATA to JSON, we might
  decide to keep the existing METADATA the way it is for backcompat,
  while adding new extensions as JSON "sidecar" files inside the same
  directory. Or maybe not; the point is it keeps our options more open.)

* We provide a mechanism for passing information between the metadata
  step and the wheel building step. I guess everyone probably will
  agree this is a good idea?

* We provide more detailed recommendations about the build environment,
  but these aren't normative anyway.


====================
 Evolutionary notes
====================

A goal here is to make it as simple as possible to convert old-style
sdists to new-style sdists. (E.g., this is one motivation for
supporting dynamic build requirements.) The ideal would be that there
would be a single static ``pyproject.toml`` that could be dropped into any
"version 0" VCS checkout to convert it to the new shiny. This is
probably not 100% possible, but we can get close, and it's important
to keep track of how close we are... hence this section.

A rough plan would be: Create a build system package
(``setuptools_pypackage`` or whatever) that knows how to speak
whatever hook language we come up with, and convert them into calls to
``setup.py``. This will probably require some sort of hooking or
monkeypatching to setuptools to provide a way to extract the
``setup_requires=`` argument when needed, and to provide a new version
of the sdist command that generates the new-style format. This all
seems doable and sufficient for a large proportion of packages (though
obviously we'll want to prototype such a system before we finalize
anything here). (Alternatively, these changes could be made to
setuptools itself rather than going into a separate package.)

But there remain two obstacles that mean we probably won't be able to
automatically upgrade packages to the new format:

1) There currently exist packages which insist on particular packages
   being available in their environment before setup.py is
   executed. This means that if we decide to execute build scripts in
   an isolated virtualenv-like environment, then projects will need to
   check whether they do this, and if so then when upgrading to the
   new system they will have to start explicitly declaring these
   dependencies (either via ``setup_requires=`` or via static
   declaration in ``pyproject.toml``).

2) There currently exist packages which do not declare consistent
   metadata (e.g. ``egg_info`` and ``bdist_wheel`` might get different
   ``install_requires=``). When upgrading to the new system, projects
   will have to evaluate whether this applies to them, and if so they
   will need to stop doing that.


==================
 Rejected options
==================

* We discussed making the wheel and sdist hooks build unpacked directories
  containing the same contents as their respective archives. In some cases this
  could avoid the need to pack and unpack an archive, but this seems like
  premature optimisation. It's advantageous for tools to work with archives
  as the canonical interchange formats (especially for wheels, where the archive
  format is already standardised). Close control of archive creation is
  important for reproducible builds. And it's not clear that tasks requiring an
  unpacked distribution will be more common than those requiring an archive.

===========
 Copyright
===========

This document has been placed in the public domain.


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