PEP: 662
Title: Editable installs via virtual wheels
Author: Bernát Gábor <gaborjbernat@gmail.com>
Sponsor: Brett Cannon <brett@python.org>
Discussions-To: https://discuss.python.org/t/discuss-tbd-editable-installs-by-gaborbernat/9071
Status: Rejected
Type: Standards Track
Topic: Packaging
Created: 28-May-2021
Post-History:
Resolution: https://discuss.python.org/t/pronouncement-on-peps-660-and-662-editable-installs/9450


Abstract
========

This document describes extensions to the build backend and frontend
communication (as introduced by :pep:`517`) to allow projects to be installed in
editable mode by introducing virtual wheels.

Motivation
==========

During development, many Python users prefer to install their libraries so that
changes to the underlying source code and resources are automatically reflected
in subsequent interpreter invocations without an additional installation step.
This mode is usually called "development mode" or "editable installs".
Currently, there is no standardized way to accomplish this, as it was explicitly
left out of :pep:`517` due to the complexity of the actual observed behaviors.

At the moment, users to get this behaviour perform one of the following:

-  For just Python code by adding the relevant source directories to
   ``sys.path`` (configurable from the command line interface via the
   ``PYTHONPATH`` environment variable). Note in this case, the users have to
   install the project dependencies themselves, and entry points or project
   metadata are not generated.

-  setuptools_ provides the `setup.py develop`_ mechanism: that installs a
   ``pth`` file that injects the project root onto the ``sys.path`` at
   interpreter startup time, generates the project metadata, and also installs
   project dependencies. pip_ exposes calling this mechanism via the
   `pip install -e <project_directory>`_ command-line interface.

-  flit_ provides the `flit install --symlink`_ command that symlinks the
   project files into the interpreters ``purelib`` folder, generates the
   project metadata, and also installs dependencies. Note, this allows
   supporting resource files too.

As these examples shows an editable install can be achieved in multiple ways
and at the moment there's no standard way of doing it. Furthermore, it's not
clear whose responsibility it is to achieve and define what an editable
installation is:

1. allow the build backend to define and materialize it,
2. allow the build frontend to define and materialize it,
3. explicitly define and standardize one method from the possible options.

The author of this PEP believes there's no one size fits all solution here,
each method of achieving editable effect has its pros and cons. Therefore
this PEP rejects option three as it's unlikely for the community to agree on a
single solution. Furthermore, question remains as to whether the frontend or the
build backend should own this responsibility. :pep:`660` proposes the build
backend to own this, while the current PEP proposes primarily the frontend,
but still allows the backend to take take control if it wants to do so.

Rationale
=========

:pep:`517` deferred "Editable installs" because this would have delayed further
its adoption, and there wasn't an agreement on how editable installs should be
achieved. Due to the popularity of the setuptools_ and pip_ projects, the status
quo prevailed, and the backend could achieve editable mode by providing a
``setup.py develop`` implementation, which the user could trigger via `pip
install -e <project_directory>`_. By defining an editable interface between the
build backend and frontend, we can eliminate the ``setup.py`` file and their
current communication method.

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

This PEP aims to delineate the frontend and the backend roles clearly and give
the developers of each the maximum ability to provide valuable features to
their users. In this proposal, the backend's role is to prepare the project for
an editable installation, and then provide enough information to the frontend
so that the frontend can manifest and enforce the editable installation.

The information the backend provides to the frontend is a wheel that follows
the existing specification within :pep:`427`. The wheel metadata about the
archive itself (``{distribution}-{version}.dist-info/WHEEL``) must also contain
the key ``Editable`` with value of ``true``.

However, instead of providing the project files within the wheel, it must
provide an ``editable.json`` file (at the root level of the wheel) that defines
the files to be exposed by the frontend. The content of this file is formulated
as a mapping of absolute source tree paths to relative target interpreter
destination paths within a scheme mapping.

A wheel that satisfies the previous two paragraphs is a virtual wheel. The
frontend's role is to take the virtual wheel and install the project in
editable mode. The way it achieves this is entirely up to the frontend and is
considered implementation detail.

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, some changes to the project code in the local
source tree will become effective without the need for a new installation step.
At a minimum, changes to the text of non-generated files that existed at the
installation time should be reflected upon the subsequent import of the
package.

Some kinds of changes, such as adding or modifying entry points or new
dependencies, require a new installation step to become effective. These changes
are typically made in build backend configuration files (such as
``pyproject.toml``). This requirement is consistent with the general user
expectation that such modifications will only become effective after
re-installation.

While users expect editable installations to behave identically to standard
installations, this may not always be possible and may be in tension with other
user expectations. Depending on how a frontend implements the editable mode,
some differences may be visible, such as the presence of additional files
(compared to a typical installation), either in the source tree or the
interpreter's installation path.

Frontends should seek to minimize differences between the behavior of editable
and standard installations and document known differences.

For reference, a non-editable installation works as follows:

#. The **developer** is using a tool, we'll call it here the **frontend**, to
   drive the project development (e.g., pip_). When the user wants to trigger a
   package build and installation of a project, they'll communicate with the
   **frontend**.

#. The frontend uses a **build frontend** to trigger the build of a wheel (e.g.,
   build_). The build frontend uses :pep:`517` to communicate with the **build
   backend** (e.g. setuptools_) - with the build backend installed into a
   :pep:`518` environment. Once invoked, the backend returns a wheel.

#. The frontend takes the wheel and feeds it to an **installer**
   (e.g., installer_) to install the wheel into the target Python interpreter.

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

This PEP adds two optional hooks to the :pep:`517` backend interface. One of the
hooks is used to specify the build dependencies of an editable install. The
other hook returns the necessary information via the build frontend the frontend
needs to create an editable install.

``get_requires_for_build_editable``
-----------------------------------

.. code::

   def get_requires_for_build_editable(config_settings=None):
       ...

This hook MUST return an additional sequence of strings containing :pep:`508`
dependency specifications, above and beyond those specified in the
``pyproject.toml`` file. The frontend must ensure that these dependencies are
available in the build environment in which the ``build_editable`` hook is
called.

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

``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.


``build_editable``
------------------

.. code::

   def build_editable(self, 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. The wheel file must be of type virtual wheel as defined
under the terminology section.

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.

Backends which do not provide the ``prepare_metadata_for_build_editable`` hook may
either silently ignore the ``metadata_directory`` parameter to ``build_editable``,
or else raise an exception when it is set to anything other than ``None``.

The source directory may be read-only, in such cases the backend may raise an
error that the frontend can display to the user. The backend may store intermediate
artifacts in cache locations or temporary directories. The presence or absence of
any caches should not make a material difference to the final result of the build.

The content of the ``editable.json`` MUST pass against the following JSON schema:

.. include:: pep-0662/pep-0662-editable.json
   :code:

For example:

.. code::

   {
      "version": 1,
      "scheme": {
         "purelib": {"/src/tree/a.py": "tree/a.py"},
         "platlib": {},
         "data": {"/src/tree/py.typed": "tree/py.typed"},
         "headers": {},
         "scripts": {}
      }
   }

The scheme paths map from project source absolute paths to target directory
relative paths. We allow backends to change the project layout from the project
source directory to what the interpreter will see by using the mapping.

For example if the backend returns ``"purelib": {"/me/project/src": ""}`` this
would mean that expose all files and modules within ``/me/project/src`` at the
root of the ``purelib`` path within the target interpreter.

Build frontend requirements
---------------------------

The build frontend is responsible for setting up the environment for the build
backend to generate the virtual wheel. All recommendations from :pep:`517` for
the build wheel hook applies here too.

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

The frontend must install the virtual wheel exactly as defined within
:pep:`427`. Furthermore is responsible for also installing the files defined
within the ``editable.json`` file. The manner in which it does is left up to
the frontend, and is encouraged for the frontend to communicate with the user
exactly the method chosen, and what limitations that solution will have.

The frontend 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}``.

The frontend can rely on the ``prepare_metadata_for_build_editable`` hook when
installing in editable mode.

If the frontend concludes it cannot achieve an editable installation with the
information provided by the build backend it should fail and raise an error to
clarify to the user why not.

The frontend might implement one or more editable installation mechanisms and
can leave it up to the user the choose one that its optimal to the use case
of the user. For example, pip could add an editable mode flag, and allow the
user to choose between ``pth`` files or symlinks (
``pip install -e . --editable-mode=pth`` vs
``pip install -e . --editable-mode=symlink``).

Example editable implementations
--------------------------------

To show how this PEP might be used, we'll now present a few case studies. Note
the offered solutions are purely for illustration purpose and are not normative
for the frontend/backend.

Add the source tree as is to the interpreter
''''''''''''''''''''''''''''''''''''''''''''

This is one of the simplest implementations, it will add the source tree as is
into the interpreters scheme paths, the ``editable.json`` within the virtual wheel
might look like:

.. code::

   {
     {"version": 1, "scheme": {"purelib": {"<project dir>": "<project dir>"}}}
   }

The frontend then could either:

-  Add the source directory onto the target interpreters ``sys.path`` during
   startup of it. This is done by creating a ``pth`` file into the target
   interpreters ``purelib`` folder. setuptools_ does this today and is what `pip
   install -e <project_directory>`_ translate too. This solution is fast and
   cross-platform compatible. However, this puts the entire source tree onto the
   system, potentially exposing modules that would not be available in a
   standard installation case.

-  Symlink the folder, or the individual files within it. This method is what
   flit does via its `flit install --symlink`_. This solution requires the
   current platform to support symlinks. Still, it allows potentially to symlink
   individual files, which could solve the problem of including files that
   should be excluded from the source tree.

Using custom importers
''''''''''''''''''''''

For a more robust and more dynamic collaboration between the build backend and
the target interpreter, we can take advantage of the import system allowing the
registration of custom importers. See :pep:`302` for more details and editables_
as an example of this. The backend can generate a new importer during the
editable build (or install it as an additional dependency) and register it at
interpreter startup by adding a ``pth`` file.

.. code::

   {
      "version": 1,
      "scheme": {
         "purelib": {
           "<project dir>/.editable/_register_importer.pth": "<project dir>/_register_importer.pth".
           "<project dir>/.editable/_editable_importer.py": "<project dir>/_editable_importer.py"
         }
       }
     }
   }

The backend here registered a hook that is called whenever a new module is
imported, allowing dynamic and on-demand functionality. Potential use cases
where this is useful:

-  Expose a source folder, but honor module excludes: the backend may generate
   an import hook that consults the exclusion table before allowing a source
   file loader to discover a file in the source directory or not.

-  For a project, let there be two modules, ``A.py`` and ``B.py``. These are two
   separate files in the source directory; however, while building a wheel, they
   are merged into one mega file ``project.py``. In this case, with this PEP,
   the backend could generate an import hook that reads the source files at
   import time and merges them in memory before materializing it as a module.

-  Automatically update out-of-date C-extensions: the backend may generate an
   import hook that checks the last modified timestamp for a C-extension source
   file. If it is greater than the current C-extension binary, trigger an update
   by calling the compiler before import.

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

This PEP competes with :pep:`660` and rejects that proposal because we think
the mechanism of achieving an editable installation should be within the
frontend rather than the build backend. Furthermore, this approach allows the
ecosystem to use alternative means to accomplish the editable installation
effect (e.g., insert path on ``sys.path`` or symlinks instead of just implying
the loose wheel mode from the backend described by that PEP).

Prominently, :pep:`660` does not allow using symlinks to expose code and data
files without also extending the wheel file standard with symlink support. It's
not clear how the wheel format could be extended to support symlinks that refer
not to files within the wheel itself, but files only available on the local
disk. It's important to note that the backend itself (or backend generated
code) must not generate these symlinks (e.g., at interpreter startup time) as
that would conflict with the frontends book keeping of what files need to be
uninstalled.

Finally, :pep:`660` adds support only for ``purelib`` and ``platlib`` files. It
purposefully avoids supporting other types of information that the wheel format
supports: ``include``, ``data`` and ``scripts``. With this path the frontend
can support these on a best effort basis via the symlinks mechanism (though
this feature is not universally available - on Windows require enablement). We
believe its beneficial to add best effort support for these file types, rather
than exclude the possibility of supporting them at all.

References
==========

.. _build: https://pypa-build.readthedocs.io

.. _editables: https://pypi.org/project/editables

.. _flit: https://flit.readthedocs.io/en/latest/index.html

.. _flit install --symlink: https://flit.readthedocs.io/en/latest/cmdline.html#cmdoption-flit-install-s

.. _installer: https://pypi.org/project/installer

.. _pip: https://pip.pypa.io

.. _pip install -e <project_directory>: https://pip.pypa.io/en/stable/cli/pip_install/#install-editable

.. _setup.py develop: https://setuptools.readthedocs.io/en/latest/userguide/commands.html#develop-deploy-the-project-source-in-development-mode

.. _setuptools: https://setuptools.readthedocs.io/en/latest/

Copyright
=========

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