Avoid false warnings about missing UV_CONSTRAINT variable (#162)

- Fixes issue where UV_CONSTRAINT warning was disabled even when
  defined
- Avoids repeated messages on each environment_variables use

src/tox_uv/_venv.py

@@ -51,6 +51,7 @@ def __init__(self, create_args: ToxEnvCreateArgs) -> None:
         self._executor: Execute | None = None
         self._installer: UvInstaller | None = None
         self._created = False
+        self._displayed_uv_constraint_warning = False
         super().__init__(create_args)
 
     def register_config(self) -> None:
@@ -183,12 +184,14 @@ 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
+        if "UV_CONSTRAINT" not in env and not self._displayed_uv_constraint_warning:
+            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
+                    )
+                    self._displayed_uv_constraint_warning = True
+                    break
         return env
 
     def _default_pass_env(self) -> list[str]:

tests/test_tox_uv_venv.py

@@ -396,5 +396,27 @@ def test_uv_pip_constraints(tox_project: ToxProjectCreator) -> None:
     result = project.run()
     result.assert_success()
     assert (
-        "Found PIP_CONSTRAINTS defined, you may want to also define UV_CONSTRAINT to match pip behavior." in result.out
+        result.out.count(
+            "Found PIP_CONSTRAINTS defined, you may want to also define UV_CONSTRAINT to match pip behavior."
+        )
+        == 1
+    ), "Warning should be found once and only once in output."
+
+
+def test_uv_pip_constraints_no(tox_project: ToxProjectCreator) -> None:
+    project = tox_project({
+        "tox.ini": f"""
+            [testenv]
+            package=skip
+            setenv=
+                PIP_CONSTRAINTS={os.devnull}
+                UV_CONSTRAINT={os.devnull}
+            commands=python --version
+            """
+    })
+    result = project.run()
+    result.assert_success()
+    assert (
+        "Found PIP_CONSTRAINTS defined, you may want to also define UV_CONSTRAINT to match pip behavior."
+        not in result.out
     )