Pass through CI as __TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI

docs/changelog/3442.feature.rst

@@ -0,0 +1 @@
+Add ``__TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI``, which passes through the ``CI`` variable if present. This is intended for use by other libraries to detect if tox is running under CI.

docs/config.rst

@@ -561,6 +561,8 @@ Base options
             - ✅
             - ❌
 
+   If the environment variable ``CI`` is present, ``__TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI`` will be set to the value of ``CI``. The ``CI`` variable itself will not be passed through.
+
    More environment variable-related information can be found in :ref:`environment variable substitutions`.
 
 .. conf::

src/tox/tox_env/api.py

@@ -379,6 +379,8 @@ def environment_variables(self) -> dict[str, str]:
         result["TOX_ENV_NAME"] = self.name
         result["TOX_WORK_DIR"] = str(self.core["work_dir"])
         result["TOX_ENV_DIR"] = str(self.conf["env_dir"])
+        if (ci := os.environ.get("CI")) is not None:
+            result["__TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI"] = ci
         return result
 
     @staticmethod

tests/tox_env/test_ci_passthrough.py

@@ -0,0 +1,29 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+import pytest
+
+if TYPE_CHECKING:
+    from tox.pytest import ToxProjectCreator
+
+
+@pytest.mark.parametrize("value", ["1", "0", "", "arbitrary_value"])
+def test_ci_passthrough_present(value: str, tox_project: ToxProjectCreator, monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("CI", value)
+    prj = tox_project({"tox.ini": "[testenv]\npackage=skip\ncommands=python -c 'print(0)'\n"})
+    execute_calls = prj.patch_execute(lambda _r: 0)
+    result = prj.run("r", "-e", "py")
+    result.assert_success()
+    req = execute_calls.call_args[0][3]
+    assert req.env["__TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI"] == value
+
+
+def test_ci_passthrough_absent(tox_project: ToxProjectCreator, monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("CI", raising=False)
+    prj = tox_project({"tox.ini": "[testenv]\npackage=skip\ncommands=python -c 'print(0)'\n"})
+    execute_calls = prj.patch_execute(lambda _r: 0)
+    result = prj.run("r", "-e", "py")
+    result.assert_success()
+    req = execute_calls.call_args[0][3]
+    assert "__TOX_ENVIRONMENT_VARIABLE_ORIGINAL_CI" not in req.env