Fix --skip-missing-interpreters (#2793)

Closes #2649

Author: q0w

docs/changelog/2649.bugfix.rst

@@ -0,0 +1 @@
+Fix ``--skip-missing-interpreters`` behaviour - by :user:`q0w`.

src/tox/session/cmd/run/single.py

@@ -46,6 +46,7 @@ def _evaluate(tox_env: RunToxEnv, no_test: bool) -> tuple[bool, int, list[Outcom
             code, outcomes = run_commands(tox_env, no_test)
         except Skip as exception:
             LOGGER.warning("skipped because %s", exception)
+            code = 0
             skipped = True
         except ToxBackendFailed as exception:
             LOGGER.error("%s", exception)

src/tox/tox_env/python/runner.py

@@ -13,6 +13,7 @@
 from tox.tox_env.package import Package
 from tox.tox_env.python.pip.req_file import PythonDeps
 
+from ...config.loader.str_convert import StrConvert
 from ..api import ToxEnvCreateArgs
 from ..runner import RunToxEnv
 from .api import Python
@@ -32,10 +33,17 @@ def register_config(self) -> None:
             default=PythonDeps("", root),
             desc="Name of the python dependencies as specified by PEP-440",
         )
+
+        def skip_missing_interpreters_post_process(value: bool) -> bool:
+            if getattr(self.options, "skip_missing_interpreters", "config") != "config":
+                return StrConvert().to_bool(self.options.skip_missing_interpreters)
+            return value
+
         self.core.add_config(
             keys=["skip_missing_interpreters"],
             default=True,
             of_type=bool,
+            post_process=skip_missing_interpreters_post_process,
             desc="skip running missing interpreters",
         )
 

tests/tox_env/python/test_python_runner.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import sys
 from pathlib import Path
 
 import pytest
@@ -121,3 +122,14 @@ def test_extras_are_normalized(
     result = project.run("c", "-e", "py", "--root", str(demo_pkg_inline), "-k", "extras")
     result.assert_success()
     assert result.out == f"[testenv:py]\nextras = {used_extra}\n"
+
+
+@pytest.mark.parametrize(
+    ("config", "cli", "expected"),
+    [("false", "true", True), ("true", "false", False), ("false", "config", False), ("true", "config", True)],
+)
+def test_config_skip_missing_interpreters(tox_project: ToxProjectCreator, config: str, cli: str, expected: str) -> None:
+    py_ver = ".".join(str(i) for i in sys.version_info[0:2])
+    project = tox_project({"tox.ini": f"[tox]\nenvlist=py4,py{py_ver}\nskip_missing_interpreters={config}"})
+    result = project.run("--skip-missing-interpreters", cli)
+    assert result.code == 0 if expected else 1