I tried to lay out what a PEP-621 compliant could look like. We will take the following sample project as a basis for discussion:

[tool.poetry]
name = "my-package"
version = "1.2.3"
description = "Some description."
authors = [
    "Sébastien Eustace <sebastien@eustace.io>"
]
maintainers = [
    "Sébastien Eustace <sebastien@eustace.io>"
]
license = "MIT"

readme = "README.rst"

homepage = "https://python-poetry.org"
repository = "https://github.com/python-poetry/poetry"
documentation = "https://python-poetry.org/docs"

keywords = ["packaging", "dependency", "poetry"]

classifiers = [
    "Topic :: Software Development :: Build Tools",
    "Topic :: Software Development :: Libraries :: Python Modules"
]

# Requirements
[tool.poetry.dependencies]
python = "~2.7 || ^3.6"
cleo = "^0.6"
pendulum = { git = "https://github.com/sdispater/pendulum.git", branch = "2.0" }
tomlkit = { git = "https://github.com/sdispater/tomlkit.git", rev = "3bff550", develop = false, source = "foo" }
requests = { version = "^2.18", optional = true, extras=[ "security" ] }
pathlib2 = { version = "^2.2", python = "~2.7" }

orator = { version = "^0.9", optional = true }

# File dependency
demo = { path = "../distributions/demo-0.1.0-py2.py3-none-any.whl" }

# Dir dependency with setup.py
my-package = { path = "../project_with_setup/" }

# Dir dependency with pyproject.toml
simple-project = { path = "../simple_project/" }

# Dependency with markers
functools32 = { version = "^3.2.3", markers = "python_version ~= '2.7' and sys_platform == 'win32' or python_version in '3.4 3.5'" }

# Dependency with python constraint
dataclasses = {version = "^0.7", python = ">=3.6.1,<3.7"}


[tool.poetry.extras]
db = [ "orator" ]
network = ["requests"]

[tool.poetry.group.dev.dependencies]
pytest = "~3.4"


[tool.poetry.scripts]
my-script = "my_package:main"


[tool.poetry.plugins."blogtool.parsers"]
".rst" = "some_module::SomeClass"

[[tool.poetry.source]]
name = "foo"
url = "https://foo.bar/simple/"
priority = "supplemental"

Migrating it to PEP 621 could look like this:

[project]
name = "my-package"
version = "1.2.3"
description = "Some description."
readme = "README.md"
requires-python = ">=3.6"
license = { file = "LICENSE" }
keywords = ["packaging", "dependency", "poetry"]
authors = [
    { name="Sébastien Eustace", email="<sebastien@eustace.io>" }
]
maintainers = [
    { name="Sébastien Eustace", email="<sebastien@eustace.io>" }
]

classifiers = [
    "Topic :: Software Development :: Build Tools",
    "Topic :: Software Development :: Libraries :: Python Modules"
]

# Requirements
dependencies = [
    "cleo ~=0.6",
    "pendulum @ git+https://github.com/sdispater/pendulum.git@2.0",
    "tomlkit @ git+https://github.com/sdispater/tomlkit.git@3bff550",
    "pathlib2 ~=2.2 ; python_version = '2.7'" ,
    # File dependency
    "demo @ ../distributions/demo-0.1.0-py2.py3-none-any.whl",
    # Dir dependency with setup.py
    "my-package @ ../project_with_setup/",
    # Dir dependency with pyproject.toml
    "simple-project @ ../simple_project/",
    # Dependency with markers
    "functools32 ~=3.2.3 ; python_version ~= '2.7' and sys_platform == 'win32' or python_version in '3.4 3.5'",
    # Dependency with python constraint
    "dataclasses ~=0.7 ; python_full_version >= '3.6.1' and python_full_version < '3.7'"
]

[project.optional-dependencies]
db = [
    "orator ~=0.9"
]
network = [
    "requests[security] ~=2.18"
]

[project.urls]
homepage = "https://python-poetry.org"
repository = "https://github.com/python-poetry/poetry"
documentation = "https://python-poetry.org/docs"

[project.scripts]
my-script = "my_package:main"

[project.entry-points."blogtool.parsers"]
".rst" = "some_module::SomeClass"

[tool.poetry.dependency-options]
tomlkit = { develop = true, source = "foo" }

[tool.poetry.group.dev]
dependencies= [
    "pytest ~=3.4"
]

[[tool.poetry.source]]
name = "foo"
url = "https://foo.bar/simple/"
priority = "supplemental"

Here are the noteworthy elements:

All dependency group dependencies will be of the same format as the main dependencies declaration, i.e. PEP 508 strings. This is where the main compatibility issue will reside since it's in a location already used by Poetry.
The usage of caret (https://python-poetry.org/docs/dependency-specification/#caret-requirements) and tilde (https://python-poetry.org/docs/dependency-specification/#tilde-requirements) requirements will be removed since they are not valid PEP 440 clauses. As such, we'll transition to compatible release clauses. Note that it might no longer be the default kind of clause in 2.0 but this will be discussed in a separate issue.
Dependency options (like develop or source) will no longer be located in the dependency declaration but rather in a new section called dependency-options.
Other than that each section that has an associated PEP 621 section will be relocated and adapted to acommodate any change in format.

To ease this transition, I see a few things that should be done:

Write a migration guide in the documentation to help people understand and make the changes needed to use this new format.
Implement a migrate command to alleviate the pain of this migration and let Poetry do the heavy lifting.
(optional) Automatically detect the old format and prompt the user to let Poetry do the migration for them.

From a pure internal standpoint, the best approach would be to have two separate "format" implementations – that can be converted from one another – that can both produce a Poetry instance that can be passed around like it is today.

Support for PEP 621 #3

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