PEP: 660 Title: Editable installs for pyproject.toml based builds (wheel based) Author: Daniel Holth , Stéphane Bidoul Sponsor: Paul Moore 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 Content-Type: text/x-rst 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 `__) Build backends: - enscons (`pull request 1 `__, `pull request 2 `__) - flit (`pull request `__) - hatchling (`sdist `__) - pdm (`pull request `__) - setuptools (`setuptools_pep660 repository `_) 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 `_ 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. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: