diff --git a/pep-0650.rst b/pep-0650.rst index ad20e2ca0..c55955571 100644 --- a/pep-0650.rst +++ b/pep-0650.rst @@ -18,14 +18,17 @@ Python package installers are not completely interoperable with each other. While pip is the most widely used installer and a de-facto standard, other installers such as Poetry_ or Pipenv_ are popular as well due to offering unique features which are optimal for certain -workflows. +workflows and not directly in line with how pip operates. While the abundance of installer options is good for end-users with -specific needs, the lack of interoperability between them makes them -hard to support uniformly. Specifically, the lack of a standard -requirements file for declaring dependencies means that each tool must -be explicitly used in order to install dependencies specified with -their respective format. +specific needs, the lack of interoperability between them makes it +hard to support all potential installers. Specifically, the lack of a +standard requirements file for declaring dependencies means that each +tool must be explicitly used in order to install dependencies +specified with their respective format. Otherwise tools must emit a +requirements file which leads to potential information loss for the +installer as well as an added export step as part of a developer's +workflow. By providing a standardized API that can be used to invoke a compatible installer, we can solve this problem without needing to @@ -45,27 +48,34 @@ Installer interface Universal installer An installer that can invoke an *installer backend* by calling the - optional invocation methods of the *installer interface*. + optional invocation methods of the *installer interface*. This can + also be thought of as the installer frontend, ala the build_ + project for :pep:`517`. Installer backend An installer that implements the *installer interface*, allowing it to be invoked by a *universal installer*. An *installer backend* may also be a *universal installer* as well, - but it is not required. + but it is not required. In comparison to :pep:`517`, this would + be Flit_. *Installer backends* may be wrapper packages around + a backing installer, e.g. Poetry could choose to not support this + API, but a package could act as a wrapper to invoke Poetry as + appropriate to use Poetry to perform an installation. Dependency group - A set of dependencies that are related. For example, a development - environment dependency group might include linting and formatting - modules while a production dependency group contains dependencies - required for deployment. + A set of dependencies that are related and required to be + installed simultaneously for some purpose. For example, a + "test" dependency group could include the dependencies required to + run the test suite. How dependency groups are specified is up to + the *installer backend*. Motivation ========== This specification allows anyone to invoke and interact with -installers that implement the specified interface, allowing for a -universally supported layer on top of existing tool-specific +*installer backends* that implement the specified interface, allowing +for a universally supported layer on top of existing tool-specific installation processes. This in turn would enable the use of all installers that implement the @@ -73,7 +83,7 @@ specified interface to be used in environments that support a single *universal installer*, as long as that installer implements this specification as well. -Below, we identify various use cases applicable to stakeholders in the +Below, we identify various use-cases applicable to stakeholders in the Python community and anyone who interacts with Python package installers. For developers or companies, this PEP would allow for increased functionality and flexibility with Python package @@ -92,16 +102,19 @@ Platform/Infrastructure Providers Platform providers (cloud environments, application hosting, etc.) and infrastructure service providers need to support package installers -for their users to install Python dependencies. Most support only pip, +for their users to install Python dependencies. Most only support pip, however there is user demand for other Python installers. Most providers do not want to maintain support for more than one installer because of the complexity it adds to their software or service and the resources it takes to do so. -Via this specification, we can enable the provider-supported +Via this specification, we can enable a provider-supported *universal installer* to invoke the user-desired *installer backend* without the provider’s platform needing to have specific knowledge of -said backend. +said backend. What this means is if Poetry implemented the installer +backend API proposed by this PEP (or some other package wrapped Poetry +to provide the API), then platform providers would support Poetry +implicitly. IDE Providers ^^^^^^^^^^^^^ @@ -131,7 +144,7 @@ which supports pip and Pipenv_). This dictates the installers that developers can use while working with these providers, which might not be optimal for their application or workflow. -Installers adopting this PEP to become installer backends would allow +Installers adopting this PEP to become *installer backends* would allow users to use third party platforms/infrastructure without having to worry about which Python package installer they are required to use as long as the provider uses a *universal installer*. @@ -144,7 +157,7 @@ Consequently, developers must use workarounds or hacky methods to install their dependencies if they use an unsupported package installer. -If the IDE uses/provides a *universal installer* it would allow for +If the IDE uses/provides a *universal installer* it would allow for any *installer backend* that the developer wanted to be used to install dependencies, freeing them of any extra work to install their dependencies in order to integrate into the IDE's workflow more @@ -172,14 +185,14 @@ hashes. Similar to Platform and IDE providers, most of these providers do not want to support N different Python package installers as that would require supporting N different file types. -The current system relies on these services/bots to keep up support as -new file formats and types are created and existing ones are changed. -By implementing this specification we can allow these services/bots to -interface through the spec and parse/write changes to dependencies -consistently, regardless of which installer is being used. Additionally -it would allow for more innovation in the space as it becomes easier -to support different installers and gives developers a standardized -way of interacting with them. +Currently, these services/bots have to implement support for each +package installer individually. Inevitably, the most popular +installers are supported first, and less popular tools are often never +supported. By implementing this specification, these services/bots can +support any (compliant) installer, allowing users to select the tool +of their choice. This will allow for more innovation in the space, as +platforms and IDEs are no longer forced to prematurely select a +"winner". Open Source Community --------------------- @@ -208,17 +221,18 @@ information will live in the ``pyproject.toml`` file under the ``install-system`` table. [install-system] ---------------------- +---------------- The install-system table is used to store install-system relevant data and information. There are multiple required keys for this table: ``requires`` and ``install-backend``. The ``requires`` key holds the -minimum requirements for the install system to execute. The -``install-backend`` key holds the name of the install backend’s entry -point. This will allow the *universal installer* to install the -requirements for the *installer backend* itself to execute (not the -requirements that the *installer backend* itself will install) as well -as invoke the *installer backend*. +minimum requirements for the *installer backend* to execute and which +will be installed by the *universal installer*. The ``install-backend`` +key holds the name of the install backend’s entry point. This will +allow the *universal installer* to install the requirements for the +*installer backend* itself to execute (not the requirements that the +*installer backend* itself will install) as well as invoke the +*installer backend*. If either of the required keys are missing or empty then the *universal installer* SHOULD raise an error. @@ -237,7 +251,7 @@ An example ``install-system`` table:: Installer Requirements: -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +^^^^^^^^^^^^^^^^^^^^^^^ The requirements specified by the ``requires`` key must be within the constraints specified by :pep:`517`. Specifically, that dependency cycles are not permitted and the *universal installer* SHOULD refuse @@ -259,9 +273,12 @@ dependency groups, the tool.poetry tables could look like this: [tool.poetry.deploy] dependencies = "deploy" +Data may also be stored in other ways as the installer backend sees +fit (e.g. separate configuration file). + Installer interface: ----------------------------------- +-------------------- The *installer interface* contains mandatory and optional hooks. Compliant *installer backends* MUST implement the mandatory hooks and MAY implement the optional hooks. A *universal installer* MAY @@ -301,9 +318,9 @@ then this indicates an error. Mandatory hooks: ------------------------------------- +---------------- invoke_install -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +^^^^^^^^^^^^^^ Installs the dependencies:: def invoke_install( @@ -321,7 +338,7 @@ Installs the dependencies:: group that the *installer backend* should install. The install will error if the dependency group doesn't exist. A user can find all dependency groups by calling - ``get_dependencies_to_install().keys()`` if dependency groups are + ``get_dependencies_groups()`` if dependency groups are supported by the *installer backend*. * ``**kwargs`` : Arbitrary parameters that a *installer backend* may require that are not already specified, allows for backwards @@ -334,10 +351,10 @@ The *universal installer* will use the exit code to determine if the installation is successful and SHOULD return the exit code itself. Optional hooks: ---------------------------------- +--------------- invoke_uninstall -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +^^^^^^^^^^^^^^^^ Uninstall the specified dependencies:: def invoke_uninstall( @@ -367,7 +384,7 @@ The *universal installer* will use the exit code to determine if the uninstall is successful and SHOULD return the exit code itself. get_dependencies_to_install -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +^^^^^^^^^^^^^^^^^^^^^^^^^^^ Returns the dependencies that would be installed by ``invoke_install(...)``. This allows package upgraders (e.g., Dependabot) to retrieve the dependencies attempting to be @@ -378,7 +395,7 @@ installed without parsing the dependency file:: *, dependency_group: str = None, **kwargs - ) -> List[str]: + ) -> Sequence[str]: ... * ``path`` : An absolute path where the *installer backend* should be @@ -405,7 +422,7 @@ MAY return the dependencies for the default/unspecified group, but otherwise MUST raise an error. get_dependency_groups -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +^^^^^^^^^^^^^^^^^^^^^ Returns the dependency groups available to be installed. This allows *universal installers* to enumerate all dependency groups the *installer backend* is aware of:: @@ -413,7 +430,7 @@ Returns the dependency groups available to be installed. This allows def get_dependency_groups( path: Union[str, bytes, PathLike[str]], **kwargs - ) -> FrozenSet[str]: + ) -> AbstractSet[str]: ... * ``path`` : An absolute path where the *installer backend* should be @@ -427,7 +444,7 @@ Returns the dependency groups available to be installed. This allows represents no dependency groups. update_dependencies -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +^^^^^^^^^^^^^^^^^^^ Outputs a dependency file based off of inputted package list:: def update_dependencies( @@ -456,6 +473,37 @@ Outputs a dependency file based off of inputted package list:: if unsuccessful. +Example +======= + +Let's consider implementing an *installer backend* that uses pip and +its requirements files for *dependency groups*. An implementation may +(very roughly) look like the following:: + + import os + import pathlib + import subprocess + import sys + + + def invoke_install(path, *, dependency_group=None, **kwargs): + file_name = "requirements.txt" + if dependency_group: + file_name = f"{dependency_group}-{file_name}" + requirements_path = pathlib.Path(path) / file_name + return subprocess.call( + [sys.executable, "-m", "pip", "install", "-r", os.fspath(requirements_path)] + ) + +If we named this package ``pep650pip``, then we could specify in +``pyproject.toml``:: + + [install-system] + #Eg : pipenv + requires = ["pep650pip", "pip"] + install-backend = "pep650pip:main" + + Rationale ========= @@ -469,6 +517,17 @@ invoked is an implementation detail of that tool. For example, an *installer backend* could act as a wrapper for a platform package manager (e.g., ``apt``). +The interface does not in any way try to specify *how* +*installer backends* should function. This is on purpose so that +*installer backends* can be allowed to innovate and solve problem in +their own way. This also means this PEP takes no stance on OS +packaging as that would be an *installer backend*'s domain. + +Defining the API in Python does mean that *some* Python code will +eventually need to be executed. That does not preclude non-Python +*installer backends* from being used, though (e.g. mamba_), as they +could be executed as a subprocess from Python code. + Backwards Compatibility ======================= @@ -514,7 +573,7 @@ Needs and information stored in dependency files between installers differ significantly and are dependent on installer functionality. For example, a Python package installer such as Poetry requires information for all Python versions and platforms and calculates -appropriate hashes while pip wouldn't. Additionally, pip would not be +appropriate hashes while pip doesn't. Additionally, pip would not be able to guarantee recreating the same environment (install the exact same dependencies) as it is outside the scope of its functionality. This makes a standardized lock file harder to implement and makes it @@ -524,16 +583,66 @@ seem more appropriate to make lock files tool specific. Have installer backends support creating virtual environments ------------------------------------------------------------- -Because installer backends will very likely have a concept of virtual +Because *installer backends* will very likely have a concept of virtual environments and how to install into them, it was briefly considered to have them also support creating virtual environments. In the end, though, it was considered an orthogonal idea. + +Open Issues +=========== + +Should the `dependency_group` argument take an iterable? +-------------------------------------------------------- + +This would allow for specifying non-overlapping dependency groups in +a single call, e.g. "docs" and "test" groups which have independent +dependencies but which a developer may want to install simultaneously +while doing development. + +Is the installer backend executed in-process? +--------------------------------------------- + +If the *installer backend* is executed in-process then it greatly +simplifies knowing what environment to install for/into, as the live +Python environment can be queried for appropriate information. + +Executing out-of-process allows for minimizing potential issues of +clashes between the environment being installed into and the +*installer backend* (and potentially *universal installer*). + +Enforce that results from the proposed interface feed into other parts? +----------------------------------------------------------------------- + +E.g. the results from ``get_dependencies_to_install()`` and +``get_dependency_groups()`` can be passed into ``invoke_install()``. +This would prevent drift between the results of various parts of the +proposed interface, but it makes more of the interface required +instead of optional. + +Raising exceptions instead of exit codes for failure conditions +--------------------------------------------------------------- + +It has been suggested that instead of returning an exit code the API +should raise exceptions. If you view this PEP as helping to translate +current installers into *installer backends*, then relying on exit +codes makes sense. There's is also the point that the APIs have no +specific return value, so passing along an exit code does not +interfere with what the functions return. + +Compare that to raising exceptions in case of an error. That could +potentially provide a more structured approach to error raising, +although to be able to capture errors it would require specifying +exception types as part of the interface. + References ========== +.. _build: https://github.com/pypa/build .. _Buildpack: https://elements.heroku.com/buildpacks/heroku/heroku-buildpack-python .. _Dependabot: https://dependabot.com/ +.. _Flit: https://flit.readthedocs.io +.. _mamba: https://github.com/mamba-org/mamba .. _pip: https://pip.pypa.io .. _Pipenv: https://pipenv-fork.readthedocs.io/en/latest/ .. _Poetry: https://python-poetry.org/