PEP: 633 Title: Dependency specification in pyproject.toml using an exploded TOML table Author: Laurie Opperman , Arun Babu Neelicattu Sponsor: Brett Cannon Discussions-To: https://discuss.python.org/t/dependency-specification-in-pyproject-toml-using-an-exploded-toml-table/5123/ Status: Rejected Type: Standards Track Topic: Packaging Content-Type: text/x-rst Created: 02-Sep-2020 Post-History: 02-Sep-2020 Resolution: https://discuss.python.org/t/how-to-specify-dependencies-pep-508-strings-or-a-table-in-toml/5243/38 Rejection Notice ================ This PEP has been rejected in favour of :pep:`631` due to its popularity, consistency with the existing usage of :pep:`508` strings, and compatibility with existing packaging tool suites. Abstract ======== This PEP specifies how to write a project's dependencies in a ``pyproject.toml`` file for packaging-related tools to consume using the fields defined in :pep:`621`, as an alternative to the :pep:`508`-based approach defined in :pep:`631`. Motivation ========== There are multiple benefits to using TOML tables and other data-types to represent requirements rather than :pep:`508` strings: - Easy initial validation via the TOML syntax. - Easy secondary validation using a schema, for example a `JSON Schema`_. - Potential for users to guess the keys of given features, rather than memorising a syntax. - Users of multiple other popular languages may already be familiar with the TOML syntax. - TOML directly represents the same data structures as in JSON, and therefore a sub-set of Python literals, so users can understand the hierarchy and type of value .. _JSON Schema: https://json-schema.org/ Rationale ========= Most of this is taken from discussions in the `PEP 621 dependencies topic`_. This has elements from `Pipfile`_, `Poetry`_, `Dart's dependencies`_ and `Rust's Cargo`_. A `comparison document`_ shows advantages and disadvantages between this format and :pep:`508`-style specifiers. In the specification of multiple requirements with the same distribution name (where environment markers choose the appropriate dependency), the chosen solution is similar to `Poetry`_'s, where an array of requirements is allowed. The direct-reference keys closely align with and utilise :pep:`610` and :pep:`440` as to reduce differences in the packaging ecosystem and rely on previous work in specification. .. _PEP 621 dependencies topic: https://discuss.python.org/t/pep-621-how-to-specify-dependencies/4599 .. _Pipfile: https://github.com/pypa/pipfile .. _Poetry: https://python-poetry.org/docs/dependency-specification/ .. _Dart's dependencies: https://dart.dev/tools/pub/dependencies .. _Rust's Cargo: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html .. _comparison document: https://github.com/uranusjr/packaging-metadata-comparisons/blob/master/topics/dependency-entries.md Specification ============= As in :pep:`621`, if metadata is improperly specified then tools MUST raise an error. The metadata MUST conform to the `TOML`_ specification. To reduce confusion with this document being a specification for specifying dependencies, the word "requirement" is used to mean a :pep:`508` dependency specification. The following tables are added to the ``project`` table specified in :pep:`621`. .. _TOML: https://toml.io/ ``dependencies`` ---------------- Format: table The keys inside this table are the names of the required distribution. The values can have one of the following types: - string: the requirement is defined only by a version requirement, with same specification as ``version`` in the requirement table, except allowing the empty string ``""`` to place no restriction on the version. - table: a requirement table. - array: an array of requirement tables. It is an error to specify an empty array ``[]`` as a value. Requirement table ^^^^^^^^^^^^^^^^^ The keys of the requirement table are as follows (all are optional): - ``version`` (string): a :pep:`440` version specifier, which is a comma-delimited list of version specifier clauses. The string MUST be non-empty. - ``extras`` (array of strings): a list of :pep:`508` extras declarations for the distribution. The list MUST be non-empty. - ``markers`` (string): a :pep:`508` environment marker expression. The string MUST be non-empty. - ``url`` (string): the URL of the artifact to install and satisfy the requirement. Note that ``file://`` is the prefix used for packages to be retrieved from the local filesystem. - ``git``, ``hg``, ``bzr`` or ``svn`` (string): the URL of a VCS repository (as specified in :pep:`440`) to clone, whose tree will be installed to satisfy the requirement. Further VCS keys will be added via amendments to :pep:`610`, however tools MAY opt to support other VCS's using their command-line command prior to the acceptance of the amendment. - ``revision`` (string): the identifier for a specific revision of the specified VCS repository to check-out before installation. Users MUST only provide this when one of ``git``, ``hg``, ``bzr``, ``svn``, or another VCS key is used to identify the distribution to install. Revision identifiers are suggested in :pep:`610`. At most one of the following keys can be specified simultaneously, as they logically conflict with each other in the requirement: ``version``, ``url``, ``git``, ``hg``, ``bzr``, ``svn``, and any other VCS key. An empty requirement table ``{}`` places no restriction on the requirement, in addition to the empty string ``""``. Any keys provided which are not specified in this document MUST cause an error in parsing. ``optional-dependencies`` -------------------------- Format: table The keys inside this table are the names of an extra's required distribution. The values can have one of the following types: - table: a requirement table. - array: an array of requirement tables. These requirement tables have `the same specification as above `_, with the addition of the following required key: - ``for-extra`` (string): the name of the :pep:`508` extra that this requirement is required for. Reference implementation ======================== Tools will need to convert this format to :pep:`508` requirement strings. Below is an example implementation of that conversion (assuming validation is already performed): .. code-block:: def convert_requirement_to_pep508(name, requirement): if isinstance(requirement, str): requirement = {"version": requirement} pep508 = name if "extras" in requirement: pep508 += " [" + ", ".join(requirement["extras"]) + "]" if "version" in requirement: pep508 += " " + requirement["version"] if "url" in requirement: pep508 += " @ " + requirement["url"] for vcs in ("git", "hg", "bzr", "svn"): if vcs in requirement: pep508 += " @ " + vcs + "+" + requirement[vcs] if "revision" in requirement: pep508 += "@" + requirement["revision"] extra = None if "for-extra" in requirement: extra = requirement["for-extra"] if "markers" in requirement: markers = requirement["markers"] if extra: markers = "extra = '" + extra + "' and (" + markers + ")" pep508 += "; " + markers return pep508, extra def convert_requirements_to_pep508(dependencies): pep508s = [] extras = set() for name, req in dependencies.items(): if isinstance(req, list): for sub_req in req: pep508, extra = convert_requirement_to_pep508(name, sub_req) pep508s.append(pep508) if extra: extras.add(extra) else: pep508, extra = convert_requirement_to_pep508(name, req) pep508s.append(pep508) if extra: extras.add(extra) return pep508s, extras def convert_project_requirements_to_pep508(project): reqs, _ = convert_requirements_to_pep508(project.get("dependencies", {})) optional_reqs, extras = convert_requirements_to_pep508( project.get("optional-dependencies", {}) ) reqs += optional_reqs return reqs, extras JSON schema ----------- For initial validation, a JSON-schema can be used. Not only does this help tools have a consistent validation, but it allows code editors to highlight validation errors as users are building the dependencies list. .. code-block:: { "$id": "spam", "$schema": "http://json-schema.org/draft-07/schema#", "title": "Project metadata", "type": "object", "definitions": { "requirementTable": { "title": "Full project dependency specification", "type": "object", "properties": { "extras": { "title": "Dependency extras", "type": "array", "items": { "title": "Dependency extra", "type": "string" } }, "markers": { "title": "Dependency environment markers", "type": "string" } }, "propertyNames": { "enum": [ "extras", "markers", "version", "url", "git", "hg", "bzr", "svn", "for-extra" ] }, "oneOf": [ { "title": "Version requirement", "properties": { "version": { "title": "Version", "type": "string" } } }, { "title": "URL requirement", "properties": { "url": { "title": "URL", "type": "string", "format": "uri" } }, "required": [ "url" ] }, { "title": "VCS requirement", "properties": { "revision": { "title": "VCS repository revision", "type": "string" } }, "oneOf": [ { "title": "Git repository", "properties": { "git": { "title": "Git URL", "type": "string", "format": "uri" } }, "required": [ "git" ] }, { "title": "Mercurial repository", "properties": { "hg": { "title": "Mercurial URL", "type": "string", "format": "uri" } }, "required": [ "hg" ] }, { "title": "Bazaar repository", "properties": { "bzr": { "title": "Bazaar URL", "type": "string", "format": "uri" } }, "required": [ "bzr" ] }, { "title": "Subversion repository", "properties": { "svn": { "title": "Subversion URL", "type": "string", "format": "uri" } }, "required": [ "svn" ] } ] } ] }, "requirementVersion": { "title": "Version project dependency specification", "type": "string" }, "requirement": { "title": "Project dependency specification", "oneOf": [ { "$ref": "#/definitions/requirementVersion" }, { "$ref": "#/definitions/requirementTable" }, { "title": "Multiple specifications", "type": "array", "items": { "$ref": "#/definitions/requirementTable" }, "minLength": 1 } ] }, "optionalRequirementTable": { "title": "Project optional dependency specification table", "allOf": [ { "$ref": "#/definitions/requirementTable" }, { "properties": { "for-extra": { "title": "Dependency's extra", "type": "string" } }, "required": [ "for-extra" ] } ] }, "optionalRequirement": { "title": "Project optional dependency specification", "oneOf": [ { "$ref": "#/definitions/optionalRequirementTable" }, { "title": "Multiple specifications", "type": "array", "items": { "$ref": "#/definitions/optionalRequirementTable" }, "minLength": 1 } ] } }, "properties": { "dependencies": { "title": "Project dependencies", "type": "object", "additionalProperties": { "$ref": "#/definitions/requirement" } }, "optional-dependencies": { "title": "Project dependencies", "type": "object", "additionalProperties": { "$ref": "#/definitions/optionalRequirement" } } } } Examples ======== Full artificial example: .. code-block:: [project.dependencies] flask = { } django = { } requests = { version = ">= 2.8.1, == 2.8.*", extras = ["security", "tests"], markers = "python_version < '2.7'" } pip = { url = "https://github.com/pypa/pip/archive/1.3.1.zip" } sphinx = { git = "ssh://git@github.com/sphinx-doc/sphinx.git" } numpy = "~=1.18" pytest = [ { version = "<6", markers = "python_version < '3.5'" }, { version = ">=6", markers = "python_version >= '3.5'" }, ] [project.optional-dependencies] pytest-timout = { for-extra = "dev" } pytest-mock = [ { version = "<6", markers = "python_version < '3.5'", for-extra = "dev" }, { version = ">=6", markers = "python_version >= '3.5'", for-extra = "dev" }, ] In homage to :pep:`631`, the following is an equivalent dependencies specification for `docker-compose`_: .. code-block:: [project.dependencies] cached-property = ">= 1.2.0, < 2" distro = ">= 1.2.0, < 2" docker = { extras = ["ssh"], version = ">= 4.2.2, < 5" } docopt = ">= 0.6.1, < 1" jsonschema = ">= 2.5.1, < 4" PyYAML = ">= 3.10, < 6" python-dotenv = ">= 0.13.0, < 1" requests = ">= 2.20.0, < 3" texttable = ">= 0.9.0, < 2" websocket-client = ">= 0.32.0, < 1" # Conditional "backports.shutil_get_terminal_size" = { version = "== 1.0.0", markers = "python_version < '3.3'" } "backports.ssl_match_hostname" = { version = ">= 3.5, < 4", markers = "python_version < '3.5'" } colorama = { version = ">= 0.4, < 1", markers = "sys_platform == 'win32'" } enum34 = { version = ">= 1.0.4, < 2", markers = "python_version < '3.4'" } ipaddress = { version = ">= 1.0.16, < 2", markers = "python_version < '3.3'" } subprocess32 = { version = ">= 3.5.4, < 4", markers = "python_version < '3.2'" } [project.optional-dependencies] PySocks = { version = ">= 1.5.6, != 1.5.7, < 2", for-extra = "socks" } ddt = { version = ">= 1.2.2, < 2", for-extra = "tests" } pytest = { version = "< 6", for-extra = "tests" } mock = { version = ">= 1.0.1, < 4", markers = "python_version < '3.4'", for-extra = "tests" } .. _docker-compose: https://github.com/docker/compose/blob/789bfb0e8b2e61f15f423d371508b698c64b057f/setup.py#L28-L61 Compatibility Examples ====================== The authors of this PEP recognise that various tools need to both read from and write to this format for dependency specification. This section aims to provide direct comparison with and examples for translating to/from the currently used standard, :pep:`508`. .. note:: For simplicity and clarity, various ways in which TOML allows you to specify each specification is not represented. These examples use the standard inline representation. For example, while following are considered equivalent in TOML, we choose the second form for the examples in this section. .. code-block:: aiohttp.version = "== 3.6.2" aiohttp = { version = "== 3.6.2" } Version Constrained Dependencies -------------------------------- **No Version Constraint** .. code-block:: aiohttp .. code-block:: aiohttp = {} **Simple Version Constraint** .. code-block:: aiohttp >= 3.6.2, < 4.0.0 .. code-block:: aiohttp = { version = ">= 3.6.2, < 4.0.0" } .. note:: This can, for conciseness, be also represented as a string. .. code-block:: aiohttp = ">= 3.6.2, < 4.0.0" Direct Reference Dependencies ----------------------------- **URL Dependency** .. code-block:: aiohttp @ https://files.pythonhosted.org/packages/97/d1/1cc7a1f84097d7abdc6c09ee8d2260366f081f8e82da36ebb22a25cdda9f/aiohttp-3.6.2-cp35-cp35m-macosx_10_13_x86_64.whl .. code-block:: aiohttp = { url = "https://files.pythonhosted.org/packages/97/d1/1cc7a1f84097d7abdc6c09ee8d2260366f081f8e82da36ebb22a25cdda9f/aiohttp-3.6.2-cp35-cp35m-macosx_10_13_x86_64.whl" } **VCS Dependency** .. code-block:: aiohttp @ git+ssh://git@github.com/aio-libs/aiohttp.git@master .. code-block:: aiohttp = { git = "ssh://git@github.com/aio-libs/aiohttp.git", revision = "master" } Environment Markers ------------------- .. code-block:: aiohttp >= 3.6.1; python_version >= '3.8' .. code-block:: aiohttp = { version = ">= 3.6.1", markers = "python_version >= '3.8'" } A slightly extended example of the above, where a particular version of ``aiohttp`` is required based on the interpreter version. .. code-block:: aiohttp >= 3.6.1; python_version >= '3.8' aiohttp >= 3.0.0, < 3.6.1; python_version < '3.8' .. code-block:: aiohttp = [ { version = ">= 3.6.1", markers = "python_version >= '3.8'" }, { version = ">= 3.0.0, < 3.6.1", markers = "python_version < '3.8'" } ] Package Extras -------------- **Specifying dependency for a package extra** .. code-block:: aiohttp >= 3.6.2; extra == 'http' .. code-block:: aiohttp = { version = ">= 3.6.2", for-extra = "http" } **Using extras from a dependency** .. code-block:: aiohttp [speedups] >= 3.6.2 .. code-block:: aiohttp = { version = ">= 3.6.2", extras = ["speedups"] } Complex Examples ---------------- **Version Constraint** .. code-block:: aiohttp [speedups] >= 3.6.2; python_version >= '3.8' and extra == 'http' .. code-block:: aiohttp = { version = ">= 3.6.2", extras = ["speedups"], markers = "python_version >= '3.8'", for-extra = "http" } **Direct Reference (VCS)** .. code-block:: aiohttp [speedups] @ git+ssh://git@github.com/aio-libs/aiohttp.git@master ; python_version >= '3.8' and extra == 'http' .. code-block:: aiohttp = { git = "ssh://git@github.com/aio-libs/aiohttp.git", revision = "master", extras = ["speedups"], markers = "python_version >= '3.8'", for-extra = "http" } Rejected Ideas ============== Switch to an array for ``dependencies`` --------------------------------------- Use an array instead of a table in order to have each element only be a table (with a ``name`` key) and no arrays of requirement tables. This was very verbose and restrictive in the TOML format, and having multiple requirements for a given distribution isn't very common. Replace ``optional-dependencies`` with ``extras`` ------------------------------------------------- Remove the ``optional-dependencies`` table in favour of both including an ``optional`` key in the requirement and an ``extras`` table which specifies which (optional) requirements are needed for a project's extra. This reduces the number of table with the same specification (to 1) and allows for requirements to be specified once but used in multiple extras, but distances some of the requirement's properties (which extra(s) it belongs to), groups required and optional dependencies together (possibly mixed), and there may not be a simple way to choose a requirement when a distribution has multiple requirements. This was rejected as ``optional-dependencies`` has already been used in the :pep:`621` draft. ``direct`` table in requirement ------------------------------- Include the direct-reference keys in a ``direct`` table, have the VCS specified as the value of a ``vcs`` key. This was more explicit and easier to include in a JSON-schema validation, but was decided to be too verbose and not as readable. Include hash ------------ Include hash in direct-reference requirements. This was only for package lock-files, and didn't really have a place in the project's metadata. Dependency tables for each extra -------------------------------- Have the ``optional-dependencies`` be a table of dependency tables for each extra, with the table name being the extra's name. This made ``optional-dependencies`` a different type (table of tables of requirements) from ``dependencies`` (table of requirements), which could be jarring for users and harder to parse. Environment marker keys ----------------------- Make each :pep:`508` environment marker as a key (or child-table key) in the requirement. This arguably increases readability and ease of parsing. The ``markers`` key would still be allowed for more advanced specification, with which the key-specified environment markers are ``and``'d with the result of. This was deferred as more design needs to be undertaken. Multiple extras which one requirement can satisfy ------------------------------------------------- Replace the ``for-extra`` key with ``for-extras``, with the value being an array of extras which the requirement satisfies. This reduces some duplication, but in this case that duplication makes explicit which extras have which dependencies. Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.