Translate PIP_CONSTRAINT(S) into UV_CONSTRAINTS if needed

Related: astral-sh/uv#10686

src/tox_uv/_venv.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import logging
 import sys
 from abc import ABC
 from functools import cached_property
@@ -41,6 +42,7 @@
     "system",
     "only-system",
 ]
+logger = logging.getLogger(__name__)
 
 
 class UvVenv(Python, ABC):
@@ -178,6 +180,12 @@ def environment_variables(self) -> dict[str, str]:
         env = super().environment_variables
         env.pop("UV_PYTHON", None)  # UV_PYTHON takes precedence over VIRTUAL_ENV
         env["VIRTUAL_ENV"] = str(self.venv_dir)
+        for pip_var in ("PIP_CONSTRAINT", "PIP_CONSTRAINTS"):
+            if pip_var in env:
+                logger.warning(
+                    "Found %s defined, you may want to also define UV_CONSTRAINT to match pip behavior.", pip_var
+                )
+                break
         return env
 
     def _default_pass_env(self) -> list[str]:

tests/test_tox_uv_venv.py

@@ -339,3 +339,17 @@ def test_uv_python_set(tox_project: ToxProjectCreator, monkeypatch: pytest.Monke
     })
     result = project.run("-vv")
     result.assert_success()
+
+
+def test_uv_pip_constraints(tox_project: ToxProjectCreator) -> None:
+    project = tox_project({
+        "tox.ini": (
+            "[testenv]\npackage=skip\ndeps=setuptools\n"
+            "setenv=\n\tPIP_CONSTRAINTS=/dev/null\n"
+            "commands=python -c 'import setuptools'"
+        )
+    })
+    result = project.run()
+    result.assert_success()
+    assert "PIP_CONSTRAINTS" in result.out
+    assert "UV_CONSTRAINT" in result.out