PEP: 660
Title: Editable installs for pyproject.toml based builds (wheel based)
Author: Daniel Holth <dholth@gmail.com>, Stéphane Bidoul <stephane.bidoul@gmail.com>
Sponsor: Paul Moore <p.f.moore@gmail.com>
Discussions-To: https://discuss.python.org/t/draft-pep-editable-installs-for-pep-517-style-build-backends/8510
Status: Final
Type: Standards Track
Topic: Packaging
Created: 30-Mar-2021
Post-History:
Resolution: https://discuss.python.org/t/pronouncement-on-peps-660-and-662-editable-installs/9450


Abstract
========

This document describes a :pep:`517` style method for the installation of packages
in editable mode.

Motivation
==========

Python programmers want to be able to develop packages without having to
install (i.e. copy) them into ``site-packages``, for example, by working in a
checkout of the source repository.

While this can be done by adding the relevant source directories to
``PYTHONPATH``, ``setuptools`` provides the ``setup.py develop`` mechanism that
makes the process easier, and also installs dependencies and entry points such
as console scripts. ``pip`` exposes this mechanism via its ``pip install
--editable`` option.

The installation of projects in such a way that the python code being
imported remains in the source directory is known as the *editable*
installation mode.

Now that :pep:`517` provides a mechanism to create alternatives to setuptools, and
decouple installation front ends from build backends, we need a new mechanism
to install packages in editable mode.

Rationale
=========

:pep:`517` deferred "Editable installs", meaning non-``setup.py``
distributions lacked that feature. The only way to retain ``editable`` installs
for these distributions was to provide a compatible ``setup.py develop``
implementation. By defining an editable hook other build frontends gain
parity with ``setup.py``.

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

The editable installation mode implies that the source code of the project
being installed is available in a local directory.

Once the project is installed in editable mode, users expect that changes to
the project *python* code in the local source tree become effective without the
need of a new installation step.

Some kind of changes, such as the addition or modification of entry points, or
the addition of new dependencies, require a new installation step to become
effective. These changes are typically made in build backend configuration
files (such as ``pyproject.toml``), so it is consistent with the general user
expectation that *python* source code is imported from the source tree.

The modification of non-python source code such a C extension modules obviously
require a compilation and/or installation step to become effective. The exact
steps to perform will remain specific to the build backend used.

When a project is installed in editable mode, users expect the installation to
behave identically as a regular installation. In particular the code must be
importable by other code, and metadata must be available to standard mechanisms
such as ``importlib.metadata``.

Depending on the way build backends implement this specification, some minor
differences may be visible such as the presence of additional files that are in
the source tree and would not be part of a regular install. Build backends are
encouraged to document such potential differences.

The Mechanism
=============

This PEP adds three optional hooks to the :pep:`517` backend interface. These hooks
are used to build a wheel that, when installed, allows that distribution to be
imported from its source folder.

build_editable
--------------

::

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

Must build a ``.whl`` file, and place it in the specified ``wheel_directory``.
It must return the basename (not the full path) of the .whl file it creates, as
a unicode string.

May do an in-place build of the distribution as a side effect so that any
extension modules or other built artifacts are ready to be used.

The .whl file must comply with the Wheel binary file format specification (PEP
427). In particular it must contain a compliant .dist-info directory.
Metadata must be identical as the one that would have been produced by
``build_wheel`` or ``prepare_metadata_for_build_wheel``, except for
``Requires-Dist`` which may differ slightly as explained below.

Build-backends must produce wheels that have the same dependencies
(``Requires-Dist`` metadata) as wheels produced by the ``build_wheel`` hook,
with the exception that they can add dependencies necessary for their editable
mechanism to function at runtime (such as `editables`_).

The filename for the "editable" wheel needs to be :pep:`427` compliant too. It
does not need to use the same tags as ``build_wheel`` but it must be tagged as
compatible with the system.

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

An "editable" wheel uses the wheel format not for distribution but as ephemeral
communication between the build system and the front end. This avoids having
the build backend install anything directly. This wheel must not be exposed
to end users, nor cached, nor distributed.

get_requires_for_build_editable
-------------------------------

::

  def get_requires_for_build_editable(config_settings=None):
      ...

This hook MUST return an additional list of strings containing :pep:`508`
dependency specifications, above and beyond those specified in the
``pyproject.toml`` file, to be installed when calling the
``build_editable`` hooks.

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

prepare_metadata_for_build_editable
-----------------------------------

::

  def prepare_metadata_for_build_editable(metadata_directory, config_settings=None):
      ...

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 preserve, but otherwise 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.

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

What to put in the wheel
------------------------

Build backends must populate the generated wheel with files that when installed will result in an editable install.
Build backends may use different techniques to achieve the goals of an editable
install. This section provides examples and is not normative.

* Build backends may choose to place a ``.pth`` file at the root of the ``.whl`` file,
  containing the root directory of the source tree. This approach is simple but
  not very precise, although it may be considered good enough (especially when
  using the ``src`` layout) and is similar to what ``setup.py develop``
  currently does.
* The `editables`_ library shows how to build proxy modules that
  provide a high quality editable installation. It accepts a list of modules
  to include, and hide. When imported, these proxy modules replace themselves
  with the code from the source tree. Path-based methods make all scripts under
  a path importable, often including the project's own ``setup.py`` and other
  scripts that would not be part of a normal installation. The proxy strategy
  can achieve a higher level of fidelity than path-based methods.
* Symbolic links are another useful mechanism to realize editable installs.
  Since, at the time this writing, the ``wheel`` specification does not support
  symbolic links, they are not directly usable to set-up symbolic links in the
  target environment. It is however possible for the backend to create a
  symlink structure in some ``build`` directory of the source tree, and add
  that directory to the python path via a ``.pth`` file in the "editable"
  wheel. If some files linked in this manner depend on python implementation or
  version, ABI or platform, care must be taken to generate the link structure
  in different directories depending on compatibility tags, so the same project
  tree can be installed in editable mode in multiple environments.

Frontend requirements
---------------------

Frontends must install "editable" wheels in the same way as regular wheels.
This also means uninstallation of editables does not require any special treatment.

Frontends must create a ``direct_url.json`` file in the ``.dist-info``
directory of the installed distribution, in compliance with :pep:`610`. The
``url`` value must be a ``file://`` url pointing to the project directory
(i.e. the directory containing ``pyproject.toml``), and the ``dir_info`` value
must be ``{'editable': true}``.

Frontends must execute ``get_requires_for_build_editable`` hooks in
an environment which contains the bootstrap requirements specified in the
``pyproject.toml`` file.

Frontends must execute the ``prepare_metadata_for_build_editable`` and
``build_editable`` hooks in an environment which contains the bootstrap
requirements from ``pyproject.toml`` and those specified by the
``get_requires_for_build_editable`` hook.

Frontends must not expose the wheel obtained from ``build_editable``
to end users. The wheel must be discarded after installation and must not be
cached nor distributed.

Limitations
===========

With regard to the wheel ``.data`` directory, this PEP focuses on making the
``purelib`` and ``platlib`` categories (installed into site-packages)
"editable". It does not make special provision for the other categories such as
``headers``, ``data`` and ``scripts``. Package authors are encouraged to use
``console_scripts``, make their ``scripts`` tiny wrappers around library
functionality, or manage these from the source checkout during development.

Prototypes
==========

At the time of writing this PEP, several prototype implementations are
available in various frontends and backends. We provide links below to
illustrate possible approaches.

Frontends:

- pip (`pull request <https://github.com/pypa/pip/pull/8212>`__)

Build backends:

- enscons (`pull request 1 <https://github.com/dholth/enscons/pull/9>`__,
  `pull request 2 <https://github.com/dholth/enscons/pull/21>`__)
- flit (`pull request <https://github.com/takluyver/flit/pull/400>`__)
- hatchling (`sdist <https://pypi.org/project/hatchling/#files>`__)
- pdm (`pull request <https://github.com/pdm-project/pdm-pep517/pull/36>`__)
- setuptools (`setuptools_pep660 repository <https://github.com/dholth/setuptools_pep660>`_)

Rejected ideas
==============

``editable`` local version identifier
-------------------------------------

The ideas of having build backends append or modify the local version
identifier to include the ``editable`` string has been rejected because it
would not satisfy ``==`` version speicifier that include the local version
identifier. In other words ``pkg==1.0+local`` is not satisfied by version
``1.0+local.editable``.

Virtual wheel
-------------

Another approach was proposed in :pep:`662`, where
the build backend returns a mapping from source files and directories to the
installed layout. It is then up to the installer frontend to realize the
editable installation by whatever means it deems adequate for its users.

In terms of capabilities, both proposals provide the core "editable" feature.

The key difference is that :pep:`662` leaves it to the frontend to decide how the
editable installation will be realized, while with this PEP, the choice must be
made by the backend. Both approaches can in principle provide several editable
installation methods for a given project, and let the developer choose one at
install time.

At the time of writing this PEP, it is clear that the community has a wide
range of theoretical and practical expectations about editable installs. The
reality is that the only one there is wide experience with is path insertion
via .pth (i.e. what setup.py develop does).

We believe that :pep:`660` better addresses these "unknown unknowns" today in the
most reliable way, by letting project authors select the backend or implement
the method that provides the editable mechanism that best suit their
requirements, and test it works correctly. Since the frontend has no latitude
in *how* to install the "editable" wheel, in case of issue, there is only one
place to investigate: the build backend.

With :pep:`662`, issues need to be investigated in the frontend,
the backend and possiblty the specification. There is also a high probability
that different frontends, implementing the specification in different ways,
will produce installations that behave differently than project authors
intended, creating confusion, or worse, projects that only work with specific
frontends or IDEs.

Unpacked wheel
--------------

A `prototype <https://github.com/pypa/pip/pull/8154/files>`_ was made that
created an unpacked wheel in a temporary directory, to be copied to the target
environment by the frontend. This approach was not pursued because a wheel
archive is easy to create for the backend, and using a wheel as communication
mechanism is a better fit with the :pep:`517` philosophy, and therefore keeps
things simpler for the frontend.

References
==========

.. _`editables`: https://pypi.org/project/editables/

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.
