PEP: 621
Title: Storing project metadata in pyproject.toml
Author: Brett Cannon <brett@python.org>,
        Dustin Ingram <di@python.org>,
        Paul Ganssle <paul at ganssle.io>,
        Pradyun Gedam <pradyunsg@gmail.com>,
        Sébastien Eustace <sebastien@eustace.io>,
        Thomas Kluyver <thomas@kluyver.me.uk>,
        Tzu-Ping Chung <uranusjr@gmail.com>
Discussions-To: https://discuss.python.org/t/pep-621-round-3/5472
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 22-Jun-2020
Post-History: 22-Jun-2020,
              18-Oct-2020,
              24-Oct-2020,
              31-Oct-2020



Abstract
========

This PEP specifies how to write a project's `core metadata`_ in a
``pyproject.toml`` file for packaging-related tools to consume.


Motivation
==========

The key motivators of this PEP are:

- Encourage users to specify core metadata statically for speed,
  ease of specification, unambiguity, and deterministic consumption by
  build back-ends
- Provide a tool-agnostic way of specifying metadata for ease of
  learning and transitioning between build back-ends
- Allow for more code sharing between build back-ends for the
  "boring parts" of a project's metadata

To speak specifically to the motivation for static metadata, that has
been an overall goal of the packaging ecosystem for some time. As
such, making it easy to specify metadata statically is important. This
also means that raising the cost of specifying data as dynamic is
acceptable as users should skew towards wanting to provide static
metadata.

Requiring the distinction between static and dynamic metadata also
helps with disambiguation for when metadata isn't specified. When any
metadata *may* be dynamic, it means you never know if the absense of
metadata is on purpose or because it is to be provided later. By
requiring that dynamic metadata be specified, it disambiguates the
intent when metadata goes unspecified.

This PEP does **not** attempt to standardize all possible metadata
required by a build back-end, only the metadata covered by the
`core metadata`_ specification which are very common across projects
and would stand to benefit from being static and consistently
specified. This means build back-ends are still free and able to
innovate around patterns like how to specify the files to include in a
wheel. There is also an included escape hatch for users and build
back-ends to use when they choose to partially opt-out of this PEP
(compared to opting-out of this PEP entirely, which is also possible).

This PEP is also not trying to change the underlying `core metadata`_
in any way. Such considerations should be done in a separate PEP which
may lead to changes or additions to what this PEP specifies.


Rationale
=========

The design guidelines the authors of this PEP followed were:

- Define a representation of as much of the `core metadata`_ in
  ``pyproject.toml`` as is reasonable
- Define the metadata statically with an escape hatch for those who
  want to define it dynamically later via a build back-end
- Use familiar names where it makes sense, but be willing to use more
  modern terminology
- Try to be ergonomic within a TOML file instead of mirroring how
  build back-ends specify metadata at a low-level when it makes sense
- Learn from other build back-ends in the packaging ecosystem which
  have used TOML for their metadata
- Don't try to standardize things which lack a pre-existing standard
  at a lower-level
- *When* metadata is specified using this PEP, it is considered
  canonical


Specification
=============

When specifying project metadata, tools MUST adhere and honour the
metadata as specified in this PEP. If metadata is improperly specified
then tools MUST raise an error to notify the user about their mistake.

Data specified using this PEP is considered canonical. Tools CANNOT
remove, add or change data that has been statically specified. Only
when a field is marked as `dynamic` may a tool provide a "new" value.


Details
-------

Table name
''''''''''

Tools MUST specify fields defined by this PEP in a table named
``[project]``. No tools may add fields to this table which are not
defined by this PEP or subsequent PEPs. For tools wishing to store
their own settings in ``pyproject.toml``, they may use the ``[tool]``
table as defined in :pep:`518`. The lack of a ``[project]`` table
implicitly means the build back-end will dynamically provide all
fields.

``name``
''''''''
- Format: string
- `Core metadata`_: ``Name``
  (`link <https://packaging.python.org/specifications/core-metadata/#name>`__)
- Synonyms

  - Flit_: ``module``/``dist-name``
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``name``
    (`link <https://python-poetry.org/docs/pyproject/#name>`__)
  - Setuptools_: ``name``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The name of the project.

Tools MUST require users to statically define this field.

Tools SHOULD normalize this name, as specified by :pep:`503`, as soon
as it is read for internal consistency.

``version``
'''''''''''
- Format: string
- `Core metadata`_: ``Version``
  (`link <https://packaging.python.org/specifications/core-metadata/#version>`__)
- Synonyms

  - Flit_: N/A (read from a ``__version__`` attribute)
    (`link <https://flit.readthedocs.io/en/latest/index.html#usage>`__)
  - Poetry_: ``version``
    (`link <https://python-poetry.org/docs/pyproject/#version>`__)
  - Setuptools_: ``version``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The version of the project as supported by :pep:`440`.

Users SHOULD prefer to specify already-normalized versions.

``description``
'''''''''''''''
- Format: string
- `Core metadata`_: ``Summary``
  (`link <https://packaging.python.org/specifications/core-metadata/#summary>`__)
- Synonyms

  - Flit_: N/A
  - Poetry_: ``description``
    (`link <https://python-poetry.org/docs/pyproject/#description>`__)
  - Setuptools_: ``description``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The summary description of the project.

``readme``
''''''''''
- Format: String or table
- `Core metadata`_: ``Description``
  (`link <https://packaging.python.org/specifications/core-metadata/#description>`__)
- Synonyms

  - Flit_: ``description-file``
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``readme``
    (`link <https://python-poetry.org/docs/pyproject/#readme>`__)
  - Setuptools_: ``long_description``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The full description of the project (i.e. the README).

The field accepts either a string or a table. If it is a string then
it is the relative path to a text file containing the full
description. Tools MUST assume the file's encoding is UTF-8. If the
file path ends in a case-insensitive ``.md`` suffix, then tools MUST
assume the content-type is ``text/markdown``. If the file path ends in
a case-insensitive ``.rst``, then tools MUST assume the content-type
is ``text/x-rst``. If a tool recognizes more extensions than this PEP,
they MAY infer the content-type for the user without specifying this
field as ``dynamic``. For all unrecognized suffixes when a
content-type is not provided, tools MUST raise an error.

The ``readme`` field may also take a table. The ``file`` key has a
string value representing a relative path to a file containing the
full description. The ``text`` key has a string value which is the
full description. These keys are mutually-exclusive, thus tools MUST
raise an error if the metadata specifies both keys.

A table specified in the ``readme`` field also has a ``content-type``
field which takes a string specifying the content-type of the full
description. A tool MUST raise an error if the metadata does not
specify this field in the table. If the metadata does not specify the
``charset`` parameter, then it is assumed to be UTF-8. Tools MAY
support other encodings if they choose to. Tools MAY support
alternative content-types which they can transform to a content-type
as supported by the `core metadata`_. Otherwise tools MUST raise an
error for unsupported content-types.

``requires-python``
'''''''''''''''''''
- Format: string
- `Core metadata`_: ``Requires-Python``
  (`link <https://packaging.python.org/specifications/core-metadata/#summary>`__)
- Synonyms

  - Flit_: ``requires-python``
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: As a ``python`` dependency in the
    ``[tool.poetry.dependencies]`` table
    (`link <https://python-poetry.org/docs/pyproject/#dependencies-and-dev-dependencies>`__)
  - Setuptools_: ``python_requires``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The Python version requirements of the project.

``license``
'''''''''''
- Format: Table
- `Core metadata`_: ``License``
  (`link <https://packaging.python.org/specifications/core-metadata/#license>`__)
- Synonyms

  - Flit_: ``license``
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``license``
    (`link <https://python-poetry.org/docs/pyproject/#license>`__)
  - Setuptools_: ``license``, ``license_file``, ``license_files``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The table may have one of two keys. The ``file`` key has a string
value that is a relative file path to the file which contains the
license for the project. Tools MUST assume the file's encoding is
UTF-8. The ``text`` key has a string value which is the license of the
project.  These keys are mutually exclusive, so a tool MUST raise an
error if the metadata specifies both keys.

A practical string value for the ``license`` key has been purposefully
left out to allow for a future PEP to specify support for SPDX_
expressions (the same logic applies to any sort of "type" field
specifying what license the ``file`` or ``text`` represents).

``authors``/``maintainers``
'''''''''''''''''''''''''''
- Format: Array of inline tables with string keys and values
- `Core metadata`_: ``Author``/``Author-email``/``Maintainer``/``Maintainer-email``
  (`link <https://packaging.python.org/specifications/core-metadata/#author>`__)
- Synonyms

  - Flit_: ``author``/``author-email``/``maintainer``/``maintainer-email``
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``authors``/``maintainers``
    (`link <https://python-poetry.org/docs/pyproject/#authors>`__)
  - Setuptools_: ``author``/``author_email``/``maintainer``/``maintainer_email``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The people or organizations considered to be the "authors" of the
project. The exact meaning is open to interpretation — it may list the
original or primary authors, current maintainers, or owners of the
package.

The "maintainers" field is similar to "authors" in that its exact
meaning is open to interpretation.

These fields accept an array of tables with 2 keys: ``name`` and
``email``. Both values must be strings. The ``name`` value MUST be a
valid email name (i.e. whatever can be put as a name, before an email,
in `RFC #822`_) and not contain commas. The ``email`` value MUST be a
valid email address. Both keys are optional.

Using the data to fill in `core metadata`_ is as follows:

1. If only ``name`` is provided, the value goes in
   ``Author``/``Maintainer`` as appropriate.
2. If only ``email`` is provided, the value goes in
   ``Author-email``/``Maintainer-email`` as appropriate.
3. If both ``email`` and ``name`` are provided, the value goes in
   ``Author-email``/``Maintainer-email`` as appropriate, with the
   format ``{name} <{email}>``.
4. Multiple values should be separated by commas.

``keywords``
''''''''''''
- Format: array of strings
- `Core metadata`_: ``Keywords``
  (`link <https://packaging.python.org/specifications/core-metadata/#keywords>`__)
- Synonyms

  - Flit_: ``keywords``
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``keywords``
    (`link <https://python-poetry.org/docs/pyproject/#keywords>`_)
  - Setuptools_: ``keywords``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The keywords for the project.

``classifiers``
'''''''''''''''
- Format: array of strings
- `Core metadata`_: ``Classifier``
  (`link <https://packaging.python.org/specifications/core-metadata/#classifier-multiple-use>`__)
- Synonyms

  - Flit_: ``classifiers``
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``classifiers``
    (`link <https://python-poetry.org/docs/pyproject/#classifiers>`__)
  - Setuptools_: ``classifiers``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

`Trove classifiers`_ which apply to the project.

``urls``
''''''''
- Format: Table, with keys and values of strings
- `Core metadata`_: ``Project-URL``
  (`link <https://packaging.python.org/specifications/core-metadata/#project-url-multiple-use>`__)
- Synonyms

  - Flit_: ``[tool.flit.metadata.urls]`` table
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``[tool.poetry.urls]`` table
    (`link <https://python-poetry.org/docs/pyproject/#urls>`__)
  - Setuptools_: ``project_urls``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

A table of URLs where the key is the URL label and the value is the
URL itself.

Entry points
''''''''''''
- Format: Table (``[project.scripts]``, ``[project.gui-scripts]``, and
  ``[project.entry-points]``)
- `Core metadata`_: N/A;
  `Entry point specification <https://packaging.python.org/specifications/entry-points/>`_
- Synonyms

  - Flit_: ``[tool.flit.scripts]`` table for console scripts,
    ``[tool.flit.entrypoints]`` for the rest
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#scripts-section>`__)
  - Poetry_: ``[tool.poetry.scripts]`` table for console scripts
    (`link <https://python-poetry.org/docs/pyproject/#scripts>`__)
  - Setuptools_: ``entry_points``
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

There are three tables related to entry points. The
``[project.scripts]`` table corresponds to the ``console_scripts``
group in the `core metadata`_. The key of the table is the name of the
entry point and the value is the object reference.

The ``[project.gui-scripts]`` table corresponds to the ``gui_scripts``
group in the `core metadata`_. Its format is the same as
``[project.scripts]``.

The ``[project.entry-points]`` table is a collection of tables. Each
sub-table's name is an entry point group. The key and value semantics
are the same as ``[project.scripts]``. Users MUST NOT create
nested sub-tables but instead keep the entry point groups to only one
level deep.

Build back-ends MUST raise an error if the metadata defines a
``[project.entry-points.console_scripts]`` or
``[project.entry-points.gui_scripts]`` table, as they would
be ambiguous in the face of ``[project.scripts]`` and
``[project.gui-scripts]``, respectively.

``dependencies``/``optional-dependencies``
''''''''''''''''''''''''''''''''''''''''''
- Format: Array of :pep:`508` strings (``dependencies``) and a table
  with values of arrays of :pep:`508` strings
  (``optional-dependencies``)
- `Core metadata`_: ``Requires-Dist`` and ``Provides-Extra``
  (`link <https://packaging.python.org/specifications/core-metadata/#requires-dist-multiple-use>`__,
  `link <https://packaging.python.org/specifications/core-metadata/#provides-extra-multiple-use>`__)
- Synonyms

  - Flit_: ``requires`` for required dependencies, ``requires-extra``
    for optional dependencies
    (`link <https://flit.readthedocs.io/en/latest/pyproject_toml.html#metadata-section>`__)
  - Poetry_: ``[tool.poetry.dependencies]`` for dependencies (both
    required and for development),
    ``[tool.poetry.extras]`` for optional dependencies
    (`link <https://python-poetry.org/docs/pyproject/#dependencies-and-dev-dependencies>`__)
  - Setuptools_: ``install_requires`` for required dependencies,
    ``extras_require`` for optional dependencies
    (`link <https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata>`__)

The (optional) dependencies of the project.

For ``dependencies``, it is a key whose value is an array of strings.
Each string represents a dependency of the project and MUST be
formatted as a valid :pep:`508` string. Each string maps directly to
a ``Requires-Dist`` entry in the `core metadata`_.

For ``optional-dependencies``, it is a table where each key specifies
an extra and whose value is an array of strings. The strings of the
arrays must be valid :pep:`508` strings. The keys MUST be valid values
for the ``Provides-Extra`` `core metadata`_. Each value in the array
thus becomes a corresponding ``Requires-Dist`` entry for the matching
``Provides-Extra`` metadata.

``dynamic``
'''''''''''
- Format: Array of strings
- `Core metadata`_: N/A
- No synonyms

Specifies which fields listed by this PEP were intentionally
unspecified so another tool can/will provide such metadata
dynamically. This clearly delineates which metadata is purposefully
unspecified and expected to stay unspecified compared to being
provided via tooling later on.

- A build back-end MUST honour statically-specified metadata (which
  means the metadata did not list the field in ``dynamic``).
- A build back-end MUST raise an error if the metadata specifies the
  ``name`` in ``dynamic``.
- If the `core metadata`_ specification lists a field as "Required",
  then the metadata MUST specify the field statically or list it in
  ``dynamic`` (build back-ends MUST raise an error otherwise, i.e. it
  should not be possible for a required field to not be listed somehow
  in the ``[project]`` table).
- If the `core metadata`_ specification lists a field as "Optional",
  the metadata MAY list it in ``dynamic`` if the expectation is a
  build back-end will provide the data for the field later.
- Build back-ends MUST raise an error if the metadata specifies a
  field statically as well as being listed in ``dynamic``.
- If the metadata does not list a field in ``dynamic``, then a build
  back-end CANNOT fill in the requisite metadata on behalf of the user
  (i.e. ``dynamic`` is the only way to allow a tool to fill in
  metadata and the user must opt into the filling in).
- Build back-ends MUST raise an error if the metadata specifies a
  field in ``dynamic`` but is still unspecified in the final artifact
  (i.e. the build back-end was unable to provide the data for a field
  listed in ``dynamic``).

Example
-------
::

  [project]
  name = "spam"
  version = "2020.0.0"
  description = "Lovely Spam! Wonderful Spam!"
  readme = "README.rst"
  requires-python = ">=3.8"
  license = {file = "LICENSE.txt"}
  keywords = ["egg", "bacon", "sausage", "tomatoes", "Lobster Thermidor"]
  authors = [
    {email = "hi@pradyunsg.me"},
    {name = "Tzu-Ping Chung"}
  ]
  maintainers = [
    {name = "Brett Cannon", email = "brett@python.org"}
  ]
  classifiers = [
    "Development Status :: 4 - Beta",
    "Programming Language :: Python"
  ]

  dependencies = [
    "httpx",
    "gidgethub[httpx]>4.0.0",
    "django>2.1; os_name != 'nt'",
    "django>2.0; os_name == 'nt'"
  ]

  [project.optional-dependencies]
  test = [
    "pytest < 5.0.0",
    "pytest-cov[all]"
  ]

  [project.urls]
  homepage = "example.com"
  documentation = "readthedocs.org"
  repository = "github.com"
  changelog = "github.com/me/spam/blob/master/CHANGELOG.md"

  [project.scripts]
  spam-cli = "spam:main_cli"

  [project.gui-scripts]
  spam-gui = "spam:main_gui"

  [project.entry-points."spam.magical"]
  tomatoes = "spam:main_tomatoes"


Backwards Compatibility
=======================

As this provides a new way to specify a project's `core metadata`_ and
is using a new table name which falls under the reserved namespace as
outlined in :pep:`518`, there are no backwards-compatibility concerns.


Security Implications
=====================

There are no direct security concerns as this PEP covers how to
statically define project metadata. Any security issues would stem
from how tools consume the metadata and choose to act upon it.


How to Teach This
=================

[How to teach users, new and experienced, how to apply the PEP to their work.]


Reference Implementation
========================

There are currently no proofs-of-concept from any build back-end
implementing this PEP.


Rejected Ideas
==============

Other table names
-----------------

Anything under ``[build-system]``
'''''''''''''''''''''''''''''''''
There was worry that using this table name would exacerbate confusion
between build metadata and project metadata, e.g. by using
``[build-system.metadata]`` as a table.

``[package]``
'''''''''''''
Garnered no strong support.

``[metadata]``
''''''''''''''
The strongest contender after ``[project]``, but in the end it was
agreed that ``[project]`` read better for certain sub-tables, e.g.
``[project.urls]``.

Support for a metadata provider
-------------------------------
Initially there was a proposal to add a middle layer between the
static metadata specified by this PEP and
``prepare_metadata_for_build_wheel()`` as specified by :pep:`517`. The
idea was that if a project wanted to insert itself between a build
back-end and the metadata there would be a hook to do so.

In the end the authors considered this idea unnecessarily complicated
and would move the PEP away from its design goal to push people to
define core metadata statically as much as possible.

Require a normalized project name
---------------------------------
While it would make things easier for tools to only work with the
normalized name as specified in :pep:`503`, the idea was ultimately
rejected as it would hurt projects transitioning to using this PEP.

Specify files to include when building
--------------------------------------
The authors decided fairly quickly during design discussions that
this PEP should focus exclusively on project metadata and not build
metadata. As such, specifying what files should end up in a source
distribution or wheel file is out of scope for this PEP.

Name the ``[project.urls]`` table ``[project.project-urls]``
------------------------------------------------------------
This suggestion came thanks to the corresponding `core metadata`_
being ``Project-Url``. But once the overall table name of ``[project]``
was chosen, the redundant use of the word "project" suggested the
current, shorter name was a better fit.

Have a separate ``url``/``home-page`` field
-------------------------------------------
While the `core metadata`_ supports it, having a single field for a
project's URL while also supporting a full table seemed redundant and
confusing.

Recommend that tools put development-related dependencies into a "dev" extra
----------------------------------------------------------------------------
As various tools have grown the concept of required dependencies
versus development dependencies, the idea of suggesting to tools that
they put such development tool into a "dev" grouping came up. In the
end, though, the authors deemed it out-of-scope for this specification
to suggest such a workflow.

Have the ``dynamic`` field only require specifying missing required fields
--------------------------------------------------------------------------
The authors considered the idea that the ``dynamic`` field would only
require the listing of missing required fields and make listing
optional fields optional. In the end, though, this went against the
design goal of promoting specifying as much information statically as
possible.

Different structures for the ``readme`` field
---------------------------------------------
The ``readme`` field had a proposed ``readme_content_type`` field, but
the authors considered the string/table hybrid more practical for the
common case while still accommodating the more complex case. Same goes
for using ``long_description`` and a corresponding
``long_description_content_type`` field.

The ``file`` key in the table format was originally proposed as
``path``, but ``file`` corresponds to setuptools' ``file`` key and
there is no strong reason otherwise to choose one over the other.

Allowing the ``readme`` field to imply ``text/plain``
-----------------------------------------------------
The authors considered allowing for unspecified content-types which
would default to ``text/plain``, but decided that it would be best to
be explicit in this case to prevent accidental incorrect renderings on
PyPI and to force users to be clear in their intent.

Other names for ``dependencies``/``optional-dependencies``
----------------------------------------------------------
The authors originally proposed ``requires``/``extra-requires`` as
names, but decided to go with the current names after a survey of
other packaging ecosystems showed Python was an outlier:

1. `npm <https://docs.npmjs.com/files/package.json#optionaldependencies>`__
2. `Rust <https://doc.rust-lang.org/cargo/guide/dependencies.html>`__
3. `Dart <https://dart.dev/guides/packages>`__
4. `Swift <https://swift.org/package-manager/>`__
5. `Ruby <https://guides.rubygems.org/specification-reference/#add_runtime_dependency>`__

Normalizing on the current names helps minimize confusion for people coming from
other ecosystems without using terminology that is necessarily foreign to new
programmers. It also prevents potential confusion with ``requires`` in the
``[build-system]`` table as specified in :pep:`518`.

Drop ``maintainers`` to unify with ``authors``
----------------------------------------------
As the difference between ``Authors`` and ``Maintainers`` fields in
the `core metadata`_ is unspecified and ambiguous, this PEP originally
proposed unifying them as a single ``authors`` field. Other ecosystems
have selected "author" as the term to use, so the thinking was to
standardize on ``Author`` in the core metadata as the place to list
people maintaining a project.

In the end, though, the decision to adhere to the core metadata was
deemed more important to help with the the acceptance of this PEP,
rather than trying to introduce a new interpretation for some of the
core metadata.

Support an arbitrary depth of tables for ``project.entry-points``
-----------------------------------------------------------------
There was a worry that keeping ``project.entry-points`` to a depth of 1 for sub-tables
would cause confusion to users if they use a dotted name and are not used to table
names using quotation marks (e.g. ``project.entry-points."spam.magical"``). But
supporting an arbitrary depth -- e.g. ``project.entry-points.spam.magical`` -- would
preclude any form of an exploded table format in the future. It would also complicate
things for build back-ends as they would have to make sure to traverse the full
table structure rather than a single level and raising errors as appropriate on
value types.

Backfilling trove classifiers SHOULD occur instead of MAY happen
----------------------------------------------------------------
Originally this PEP said that tools SHOULD backfill appropriate trove classifiers.
This was changed to say it MAY occur to emphasize it was entirely optional for
build back-ends to implement.

Using structured TOML dictionaries to specify dependencies
----------------------------------------------------------
The format for specifying the dependencies of a project was the most
hotly contested topic in terms of data format. It led to the creation
of both :pep:`631` and :pep:`633` which represent what is in this PEP
and using TOML dictionaries more extensively, respectively. The
decision on those PEPs can be found at
https://discuss.python.org/t/how-to-specify-dependencies-pep-508-strings-or-a-table-in-toml/5243/38.

The authors briefly considered supporting both formats, but decided
that it would lead to confusion as people would need to be familiar
with two formats instead of just one.

Require build backe-ends to update ``pyproject.toml`` when generating an sdist
------------------------------------------------------------------------------
When this PEP was written, sdists did not require having static,
canonical metadata like this PEP does. The idea was then considered to
use this PEP as a way to get such metadata into sdists. In the end,
though, the idea of updating ``pyproject.toml`` was not generally
liked, and so the idea was rejected in favour of separately pursuing
standardizing metadata in sdists.

Allow tools to add/extend data
------------------------------
In an earlier version of this PEP, tools were allowed to extend data
for fields. For instance, build back-ends could take the version
number and add a local version for when they built the wheel. Tools
could also add more trove classifiers for things like the license or
supported Python versions.

In the end, though, it was thought better to start out stricter and
contemplate loosening how static the data could be considered based
on real-world usage.


Open Issues
===========
None at the moment.

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.


.. _PyPI: https://pypi.org
.. _core metadata: https://packaging.python.org/specifications/core-metadata/
.. _flit: https://flit.readthedocs.io/
.. _poetry: https://python-poetry.org/
.. _setuptools: https://setuptools.readthedocs.io/
.. _setuptools metadata: https://setuptools.readthedocs.io/en/latest/setuptools.html#metadata
.. _survey of tools: https://github.com/uranusjr/packaging-metadata-comparisons
.. _trove classifiers: https://pypi.org/classifiers/
.. _SPDX: https://spdx.dev/
.. _RFC #822: https://tools.ietf.org/html/rfc822

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