PEP 660 – Editable installs for pyproject.toml based builds (wheel based)
- Daniel Holth <dholth at gmail.com>, Stéphane Bidoul <stephane.bidoul at gmail.com>
- Paul Moore <p.f.moore at gmail.com>
- Discourse thread
- Standards Track
- Discourse thread
Table of Contents
- Terminology and goals
- The Mechanism
- Rejected ideas
This document describes a PEP 517 style method for the installation of packages in editable mode.
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
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
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.
PEP 517 deferred “Editable installs”, meaning non-
distributions lacked that feature. The only way to retain
for these distributions was to provide a compatible
implementation. By defining an editable hook other build frontends gain
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
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.
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.
def build_editable(wheel_directory, config_settings=None, metadata_directory=None): ...
Must build a
.whl file, and place it in the specified
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
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
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
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
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.
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
If not defined, the default implementation is equivalent to
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
directory MUST be a valid
.dist-info directory as defined in the
wheel specification, except that it need not contain
signatures. The hook MAY also create other files inside this
directory, and a build frontend MUST preserve, but otherwise ignore, such files;
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
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
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
.pthfile at the root of the
.whlfile, 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
srclayout) and is similar to what
setup.py developcurrently 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.pyand 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
wheelspecification 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
builddirectory of the source tree, and add that directory to the python path via a
.pthfile 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.
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
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
Frontends must execute
get_requires_for_build_editable hooks in
an environment which contains the bootstrap requirements specified in the
Frontends must execute the
build_editable hooks in an environment which contains the bootstrap
pyproject.toml and those specified by the
Frontends must not expose the wheel obtained from
to end users. The wheel must be discarded after installation and must not be
cached nor distributed.
With regard to the wheel
.data directory, this PEP focuses on making the
platlib categories (installed into site-packages)
“editable”. It does not make special provision for the other categories such as
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.
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.
- pip (pull request)
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
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.
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.
This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.
Last modified: 2022-04-20 09:53:08 GMT