Automatically convert setup.cfg => pyproject.toml

This is the initial implementation of the "configuration driver"
that indirectly reads `setup.cfg` by first converting it to a data
structure corresponding to `pyproject.toml` and then expanding it.

This idea is based on the approach defined in pypa#2685.

LIMITATION: Differently from the `legacy_setupcfg` "configuration driver",
`setupcfg` does not support reading other distutils file.
The `find_others` flag is removed because of that.

setuptools/config/pyprojecttoml.py

@@ -7,7 +7,7 @@
 from setuptools.extern import tomli
 from setuptools.extern._validate_pyproject import validate
 from setuptools.config import expand as _expand
-from setuptools.errors import OptionError
+from setuptools.errors import OptionError, FileError
 
 
 def read_configuration(filepath, expand=True, ignore_option_errors=False):
@@ -28,6 +28,9 @@ def read_configuration(filepath, expand=True, ignore_option_errors=False):
     """
     filepath = os.path.abspath(filepath)
 
+    if not os.path.isfile(filepath):
+        raise FileError(f"Configuration file {filepath!r} does not exist.")
+
     with open(filepath, "rb") as file:
         asdict = tomli.load(file)
 

setuptools/config/setupcfg.py

@@ -0,0 +1,67 @@
+"""Automatically convert ``setup.cfg`` file into a ``pyproject.toml``-equivalent
+in memory data structure, and then proceed to load the configuration.
+"""
+import os
+from typing import Union
+
+from setuptools.errors import FileError
+from setuptools.extern.ini2toml.base_translator import BaseTranslator
+from setuptools.extern.ini2toml.drivers import configparser as configparser_driver
+from setuptools.extern.ini2toml.drivers import plain_builtins as plain_builtins_driver
+from setuptools.extern.ini2toml.plugins import setuptools_pep621 as setuptools_plugin
+
+from setuptools.config import pyprojecttoml as pyproject_config
+
+
+_Path = Union[os.PathLike, str, None]
+
+
+def convert(setupcfg_file: _Path) -> dict:
+    """Convert the ``setup.cfg`` file into a data struct similar to
+    the one that would be obtained by parsing a ``pyproject.toml``
+    """
+    with open(setupcfg_file, "r") as f:
+        ini_text = f.read()
+
+    translator = BaseTranslator(
+        ini_loads_fn=configparser_driver.parse,
+        toml_dumps_fn=plain_builtins_driver.convert,
+        plugins=[setuptools_plugin.activate],
+        ini_parser_opts={},
+    )
+    return translator.translate(ini_text, profile_name="setup.cfg")
+
+
+expand_configuration = pyproject_config.expand_configuration
+
+
+def read_configuration(
+    filepath: _Path, expand: bool = True, ignore_option_errors: bool = False
+):
+    """Read given configuration file and returns options from it as a dict.
+
+    :param str|unicode filepath: Path to configuration file to get options from.
+
+    :param bool expand: Whether to expand directives and other computed values
+        (i.e. post-process the given configuration)
+
+    :param bool ignore_option_errors: Whether to silently ignore
+        options, values of which could not be resolved (e.g. due to exceptions
+        in directives such as file:, attr:, etc.).
+        If False exceptions are propagated as expected.
+
+    :rtype: dict
+    """
+    filepath = os.path.abspath(filepath)
+
+    if not os.path.isfile(filepath):
+        raise FileError(f"Configuration file {filepath!r} does not exist.")
+
+    asdict = convert(filepath)
+
+    with pyproject_config._ignore_errors(ignore_option_errors):
+        pyproject_config.validate(asdict)
+
+    if expand:
+        root_dir = os.path.dirname(filepath)
+        return expand_configuration(asdict, root_dir, ignore_option_errors)

setuptools/tests/config/test_setupcfg.py

@@ -0,0 +1,93 @@
+from textwrap import dedent
+
+from setuptools.config.setupcfg import convert, read_configuration
+
+EXAMPLE = {
+    "LICENSE": "----- MIT LICENSE TEXT PLACEHOLDER ----",
+    "README.md": "hello world",
+    "pyproject.toml": dedent("""\
+        [build-system]
+        requires = ["setuptools>=42", "wheel"]
+        build-backend = "setuptools.build_meta"
+    """),
+    "setup.cfg": dedent("""\
+        [metadata]
+        name = example-pkg
+        version = 0.0.1
+        author = Example Author
+        author_email = author@example.com
+        description = A small example package
+        long_description = file: README.md
+        long_description_content_type = text/markdown
+        url = https://github.com/pypa/sampleproject
+        project_urls =
+            Bug Tracker = https://github.com/pypa/sampleproject/issues
+        classifiers =
+            Programming Language :: Python :: 3
+            License :: OSI Approved :: MIT License
+            Operating System :: OS Independent
+
+        [options]
+        package_dir =
+            = src
+        packages = find:
+        python_requires = >=3.6
+        install_requires =
+            peppercorn
+        entry_points = file: entry_points.ini
+
+        [options.extras_require]
+        dev =
+            check-manifest
+        test =
+            coverage
+
+        [options.packages.find]
+        where = src
+    """),
+    "entry_points.ini": dedent("""\
+        [my.plugin.group]
+        add_one = example_package.example:add_one
+    """),
+    "src/example_package/__init__.py": "",
+    "src/example_package/example.py": "def add_one(number):\n    return number + 1",
+    "src/example_package/package_data.csv": "42",
+    "src/example_package/nested/__init__.py": "",
+}
+
+
+def create_project(parent_dir, files):
+    for file, content in files.items():
+        path = parent_dir / file
+        path.parent.mkdir(exist_ok=True, parents=True)
+        path.write_text(content)
+
+
+def test_convert(tmp_path):
+    create_project(tmp_path, EXAMPLE)
+    pyproject = convert(tmp_path / "setup.cfg")
+    project = pyproject["project"]
+    assert project["name"] == "example-pkg"
+    assert project["version"] == "0.0.1"
+    assert project["readme"]["file"] == "README.md"
+    assert project["readme"]["content-type"] == "text/markdown"
+    assert project["urls"]["Homepage"] == "https://github.com/pypa/sampleproject"
+    assert set(project["dependencies"]) == {"peppercorn"}
+    assert set(project["optional-dependencies"]["dev"]) == {"check-manifest"}
+    assert set(project["optional-dependencies"]["test"]) == {"coverage"}
+    setuptools = pyproject["tool"]["setuptools"]
+    from pprint import pprint
+    pprint(setuptools)
+    assert set(setuptools["dynamic"]["entry-points"]["file"]) == {"entry_points.ini"}
+    assert setuptools["packages"]["find"]["where"] == ["src"]
+    assert setuptools["packages"]["find"]["namespaces"] is False
+
+
+def test_read_configuration(tmp_path):
+    create_project(tmp_path, EXAMPLE)
+    pyproject = read_configuration(tmp_path / "setup.cfg")
+    project = pyproject["project"]
+    ep_value = "example_package.example:add_one"
+    assert project["entry-points"]["my.plugin.group"]["add_one"] == ep_value
+    setuptools = pyproject["tool"]["setuptools"]
+    assert set(setuptools["packages"]) == {"example_package", "example_package.nested"}