Fix factor selection via TOX_FACTORS environment variable (#3557)

The argparse action type for `-f`/`--factors` is `_AppendAction` with
`nargs="+"`, which produces `list[list[str]]` at runtime. But `get_type()`
only inferred `list[str]`, so `get_env_var()` produced a flat list of strings.
When `_parse_factors()` iterated over this, it treated each character as a
separate factor instead of each string.

Fix `get_type()` to detect `nargs` on `_AppendAction` and return the correct
nested type `list[list[str]]`. The existing `Convert` infrastructure already
handles nested generics — semicolons separate OR groups, commas separate AND
factors within a group.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: worksbyfriday

docs/changelog/3557.bugfix.rst

@@ -0,0 +1,2 @@
+Fix factor selection via ``TOX_FACTORS`` environment variable producing wrong results because ``append`` + ``nargs="+"``
+actions need nested list types for proper env var conversion - by :user:`Fridayai700`.

src/tox/config/cli/parser.py

@@ -66,7 +66,10 @@ def get_type(action: Action) -> type[Any]:
         of_type: type[Any] | None = getattr(action, "of_type", None)
         if of_type is None:
             if isinstance(action, argparse._AppendAction):  # noqa: SLF001
-                of_type = list[action.type]  # ty: ignore[invalid-type-form] # runtime generic from argparse action type
+                if action.nargs in ("+", "*") or (isinstance(action.nargs, int) and action.nargs > 1):
+                    of_type = list[list[action.type]]  # ty: ignore[invalid-type-form] # nargs produces list per invocation
+                else:
+                    of_type = list[action.type]  # ty: ignore[invalid-type-form] # runtime generic from argparse action type
             elif isinstance(action, argparse._StoreAction) and action.choices:  # noqa: SLF001
                 loc = locals()
                 loc["Literal"] = Literal

tests/session/test_env_select.py

@@ -152,6 +152,32 @@ def test_factor_select(
     outcome.assert_out_err("{}\n".format("\n".join(expect_envs)), "")
 
 
+@pytest.mark.parametrize(
+    ("env_value", "expect_envs"),
+    [
+        ("cov", ("py310-django20-cov", "py310-django21-cov", "py39-django20-cov", "py39-django21-cov")),
+        ("py39,django20", ("py39-django20-cov", "py39-django20")),
+        ("py39;py310", ("py310-django20-cov", "py310-django20", "py310-django21-cov", "py310-django21",
+                        "py39-django20-cov", "py39-django20", "py39-django21-cov", "py39-django21")),
+    ],
+)
+def test_factor_select_via_env_var(
+    tox_project: ToxProjectCreator,
+    monkeypatch: MonkeyPatch,
+    env_value: str,
+    expect_envs: tuple[str, ...],
+) -> None:
+    ini = """
+        [tox]
+        env_list = py3{10,9}-{django20,django21}{-cov,}
+        """
+    monkeypatch.setenv("TOX_FACTORS", env_value)
+    project = tox_project({"tox.ini": ini})
+    outcome = project.run("l", "--no-desc")
+    outcome.assert_success()
+    outcome.assert_out_err("{}\n".format("\n".join(expect_envs)), "")
+
+
 def test_tox_skip_env(tox_project: ToxProjectCreator, monkeypatch: MonkeyPatch) -> None:
     monkeypatch.setenv("TOX_SKIP_ENV", "m[y]py")
     project = tox_project({"tox.ini": "[tox]\nenv_list = py3{10,9},mypy"})