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PEP 662 -- Editable installs via virtual wheels

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

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 [7] provides the setup.py develop [6] 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 [5] exposes calling this mechanism via the pip install -e command-line interface.
  • flit [3] provides the flit install --symlink [4] 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 [7] and pip [5] 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. 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:

  1. The developer is using a tool, we'll call it here the frontend, to drive the project development (e.g., pip [5]). When the user wants to trigger a package build and installation of a project, they'll communicate with the frontend.
  2. The frontend uses a build frontend to trigger the build of a wheel (e.g., build [1]). The build frontend uses PEP 517 to communicate with the build backend (e.g. setuptools [7]) - with the build backend installed into a PEP 518 environment. Once invoked, the backend returns a wheel.
  3. 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

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

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:

{
  "$schema": "http://json-schema.org/draft-07/schema",
  "$id": "http://pypa.io/editables.json",
  "type": "object",
  "title": "Virtual wheel editable schema.",
  "required": ["version", "scheme"],
  "properties": {
    "version": {
      "$id": "#/properties/version",
      "type": "integer",
      "minimum": 1,
      "maximum": 1,
      "title": "The version of the schema."
    },
    "scheme": {
      "$id": "#/properties/scheme",
      "type": "object",
      "title": "Files to expose.",
      "required": ["purelib", "platlib", "data", "headers", "scripts"],
      "properties": {
        "purelib": { "$ref": "#/$defs/mapping" },
        "platlib": { "$ref": "#/$defs/mapping" },
        "data": { "$ref": "#/$defs/mapping" },
        "headers": { "$ref": "#/$defs/mapping" },
        "scripts": { "$ref": "#/$defs/mapping" }
      },
      "additionalProperties": true
    }
  },
  "additionalProperties": true,
  "$defs": {
    "mapping": {
      "type": "object",
      "description": "A mapping of source to target paths. The source is absolute path, the destination is relative path.",
      "additionalProperties": true
    }
  }
}

For example:

{
   "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:

{
  {"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 [7] does this today and is what pip install -e 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 [4]. 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 [2] 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.

{
   "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.

Source: https://github.com/python/peps/blob/master/pep-0662.rst