Always pass the Python implementation to uv's -p argument (#143)

* Demonstrate #142

* Always pass the Python implementation to uv

Doing so ensures that uv cannot find an unexpected implementation
(like PyPy) that matches based solely on the language version.

Fixes #142

src/tox_uv/_venv.py

@@ -182,13 +182,13 @@ def create_python_env(self) -> None:
         elif (base.major, base.minor) == sys.version_info[:2] and (sys.implementation.name.lower() == imp):
             version_spec = sys.executable
         else:
-            uv_imp = "" if (imp and imp == "cpython") else imp
+            uv_imp = imp or ""
             if not base.major:
-                version_spec = f"{uv_imp or ''}"
+                version_spec = uv_imp
             elif not base.minor:
-                version_spec = f"{uv_imp or ''}{base.major}"
+                version_spec = f"{uv_imp}{base.major}"
             else:
-                version_spec = f"{uv_imp or ''}{base.major}.{base.minor}"
+                version_spec = f"{uv_imp}{base.major}.{base.minor}"
 
         cmd: list[str] = [self.uv, "venv", "-p", version_spec, "--allow-existing"]
         if self.options.verbosity > 3:  # noqa: PLR2004

tests/test_tox_uv_venv.py

@@ -97,6 +97,42 @@ def test_uv_venv_spec_pypy(
     assert f"-p {expected_uv_pypy} " in stdout
 
 
+@pytest.mark.parametrize(
+    ("implementation", "expected_implementation", "expected_name"),
+    [
+        ("", "cpython", "cpython"),
+        ("py", "cpython", "cpython"),
+        ("pypy", "pypy", "pypy"),
+    ],
+)
+def test_uv_venv_spec_full_implementation(
+    tox_project: ToxProjectCreator,
+    implementation: str,
+    expected_implementation: str,
+    expected_name: str,
+) -> None:
+    """Validate that Python implementations are explicitly passed to uv's `-p` argument.
+
+    This test ensures that uv searches for the target Python implementation and version,
+    even if another implementation -- with the same language version --
+    is found on the path first.
+
+    This prevents a regression to a bug that occurred when PyPy 3.10 was on the PATH
+    and tox was invoked with `tox -e py3.10`:
+    uv was invoked with `-p 3.10` and found PyPy 3.10, not CPython 3.10.
+    """
+
+    project = tox_project({})
+    result = project.run("run", "-vve", f"{implementation}9.99")
+
+    # Verify that uv was invoked with the full Python implementation and version.
+    assert f" -p {expected_implementation}9.99 " in result.out
+
+    # Verify that uv interpreted the `-p` argument as a Python spec, not an executable.
+    # This confirms that tox-uv is passing recognizable, usable `-p` arguments to uv.
+    assert f"no interpreter found for {expected_name} 9.99" in result.err.lower()
+
+
 @pytest.fixture
 def other_interpreter_exe() -> pathlib.Path:  # pragma: no cover
     """Returns an interpreter executable path that is not the exact same as `sys.executable`.