Set always PYTHONIOENCODING to utf-8 (#2646)

Resolves #2422

Author: gaborbernat

docs/changelog/2422.bugfix.rst

@@ -0,0 +1,2 @@
+Always set environment variable ``PYTHONIOENCODING`` to ``utf-8`` to ensure tox works under Windows custom encodings
+- by :user:`gaborbernat`.

src/tox/config/sets.py

@@ -219,6 +219,7 @@ def __init__(self, conf: Config, section: Section, env_name: str) -> None:
     def register_config(self) -> None:
         def set_env_post_process(values: SetEnv) -> SetEnv:
             values.update(self.default_set_env_loader(), override=False)
+            values.update({"PYTHONIOENCODING": "utf-8"}, override=True)
             return values
 
         def set_env_factory(raw: object) -> SetEnv:

tests/config/test_set_env.py

@@ -60,9 +60,9 @@ def func(tox_ini: str, extra_files: dict[str, Any] | None = None, from_cwd: Path
 def test_set_env_default(eval_set_env: EvalSetEnv) -> None:
     set_env = eval_set_env("")
     keys = list(set_env)
-    assert keys == ["PIP_DISABLE_PIP_VERSION_CHECK"]
+    assert keys == ["PIP_DISABLE_PIP_VERSION_CHECK", "PYTHONIOENCODING"]
     values = [set_env.load(k) for k in keys]
-    assert values == ["1"]
+    assert values == ["1", "utf-8"]
 
 
 def test_set_env_self_key(eval_set_env: EvalSetEnv, monkeypatch: MonkeyPatch) -> None:
@@ -120,7 +120,7 @@ def test_set_env_replacer(eval_set_env: EvalSetEnv, monkeypatch: MonkeyPatch) ->
     monkeypatch.setenv("MAGIC", "\nb=2\n")
     set_env = eval_set_env("[testenv]\npackage=skip\nset_env=a=1\n {env:MAGIC}")
     env = {k: set_env.load(k) for k in set_env}
-    assert env == {"PIP_DISABLE_PIP_VERSION_CHECK": "1", "a": "1", "b": "2"}
+    assert env == {"PIP_DISABLE_PIP_VERSION_CHECK": "1", "a": "1", "b": "2", "PYTHONIOENCODING": "utf-8"}
 
 
 def test_set_env_honor_override(eval_set_env: EvalSetEnv) -> None:
@@ -148,6 +148,7 @@ def test_set_env_environment_file(eval_set_env: EvalSetEnv) -> None:
         "C": "1",
         "E": '"1"',
         "F": "",
+        "PYTHONIOENCODING": "utf-8",
     }
 
 

tests/session/cmd/test_show_config.py

@@ -160,7 +160,7 @@ def test_show_config_select_only(tox_project: ToxProjectCreator) -> None:
 def test_show_config_alias(tox_project: ToxProjectCreator) -> None:
     outcome = tox_project({"tox.ini": ""}).run("c", "-e", "py", "-k", "setenv")
     outcome.assert_success()
-    assert "set_env = " in outcome.out
+    assert "set_env =" in outcome.out
 
 
 def test_show_config_description_normalize(tox_project: ToxProjectCreator) -> None: