Allow passing config directory without filename (#2550)

Fixes #2340

Author: ssbarnea

docs/changelog/2340.bugfix.rst

@@ -0,0 +1 @@
+Allow passing config directory without filename. - by :user:`ssbarnea`.

docs/config.rst

@@ -79,8 +79,9 @@ examples first and use this page as a reference.
 
 .. code-block:: toml
 
-    [tool:tox]
+    [tool.tox]
     legacy_tox_ini = """
+        [tox]
         min_version = 4.0
         env_list =
             py310

src/tox/config/cli/parser.py

@@ -316,7 +316,7 @@ def add_core_arguments(parser: ArgumentParser) -> None:
         default=None,
         type=Path,
         of_type=Optional[Path],
-        help="configuration file for tox (if not specified will discover one)",
+        help="configuration file/folder for tox (if not specified will discover one)",
     )
     parser.add_argument(
         "--workdir",

src/tox/config/source/discover.py

@@ -26,6 +26,17 @@ def discover_source(config_file: Path | None, root_dir: Path | None) -> Source:
         src = _locate_source()
         if src is None:
             src = _create_default_source(root_dir)
+    elif config_file.is_dir():
+        src = None
+        for src_type in SOURCE_TYPES:
+            candidate: Path = config_file / src_type.FILENAME
+            try:
+                src = src_type(candidate)
+                break
+            except ValueError:
+                continue
+        if src is None:
+            raise HandledError(f"could not find any config file in {config_file}")
     else:
         src = _load_exact_source(config_file)
     return src

tests/config/cli/test_cli_ini.py

@@ -11,7 +11,10 @@
 
 from tox.config.cli.ini import IniConfig
 from tox.config.cli.parse import get_options
+from tox.config.cli.parser import Parsed
 from tox.config.loader.api import Override
+from tox.config.main import Config
+from tox.config.source import discover_source
 from tox.pytest import CaptureFixture, LogCaptureFixture, MonkeyPatch
 from tox.session.env_select import CliEnv
 from tox.session.state import State
@@ -199,3 +202,31 @@ def test_cli_ini_with_interpolated(tmp_path: Path, monkeypatch: MonkeyPatch) ->
     monkeypatch.setenv("TOX_CONFIG_FILE", str(to))
     conf = IniConfig()
     assert conf.get("a", str)
+
+
+@pytest.mark.parametrize(
+    ("conf_arg", "filename", "content"),
+    [
+        pytest.param("", "tox.ini", "[tox]", id="ini-dir"),
+        pytest.param("tox.ini", "tox.ini", "[tox]", id="ini"),
+        pytest.param("", "setup.cfg", "[tox:tox]", id="cfg-dir"),
+        pytest.param("setup.cfg", "setup.cfg", "[tox:tox]", id="cfg"),
+        pytest.param("", "pyproject.toml", '[tool.tox]\nlegacy_tox_ini = """\n[tox]\n"""\n', id="toml-dir"),
+        pytest.param("pyproject.toml", "pyproject.toml", '[tool.tox]\nlegacy_tox_ini = """\n[tox]\n"""\n', id="toml"),
+    ],
+)
+def test_conf_arg(tmp_path: Path, conf_arg: str, filename: str, content: str) -> None:
+    dest = tmp_path / "c"
+    dest.mkdir()
+    if filename:
+        cfg = dest / filename
+        cfg.write_bytes(content.encode(encoding="utf-8"))
+
+    config_file = dest / conf_arg
+    source = discover_source(config_file, None)
+
+    Config.make(
+        Parsed(work_dir=dest, override=[], config_file=config_file, root_dir=None),
+        pos_args=[],
+        source=source,
+    )