fix(solvers): probe Xpress license with xpress.init

doc/release_notes.rst

@@ -92,6 +92,7 @@ Upcoming Version
 
 **Bug fixes**
 
+* Probe the Xpress license via ``xpress.init()`` in ``Xpress._license_probe`` so unlicensed Xpress installations are accurately excluded from ``licensed_solvers`` and reported in ``check_solver_licenses``. (:issue:`994`, :pull:`1028`)
 * ``Variable.get_solver_attribute`` for Gurobi now maps values by position for the direct API. It parsed labels from the solver's variable names, which returned the values of the wrong variables when names were off and a variable was masked. (`#978 <https://github.com/PyPSA/linopy/issues/978>`__)
 * ``densify_terms`` no longer raises on expressions without coordinate dimensions (``expr.sum(drop_zeros=True)`` over all dimensions) and now works on ``QuadraticExpression``, where it previously indexed the ``_factor`` axis as the term axis.
 * ``sum()`` over a dimension no longer raises when another dimension of the expression has size 0; it returns an expression without terms over the kept coordinates, as summing over the empty dimension itself already did. (https://github.com/PyPSA/linopy/issues/906)

linopy/solvers.py

@@ -306,6 +306,7 @@ def __getattr__(self, attr: str) -> Any:
 coptpy = _LazyModule("coptpy")
 cupdlpx = _LazyModule("cupdlpx")
 cuopt = _LazyModule("cuopt")
+xpress = _LazyModule("xpress")
 
 
 def _has_module(name: str) -> bool:
@@ -2665,6 +2666,10 @@ class Xpress(Solver[None]):
     def is_available(cls) -> bool:
         return _has_module("xpress")
 
+    @classmethod
+    def _license_probe(cls) -> None:
+        xpress.init()
+
     def _apply_var_bounds(
         self, ctx: Any, indices: np.ndarray, lower: np.ndarray, upper: np.ndarray
     ) -> None:

test/test_available_solvers.py

@@ -238,3 +238,23 @@ class _FakeMindoptpy:
     cls._license_probe()
 
     assert events == ["env_init", "env_dispose"]
+
+
+def test_xpress_license_probe(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    cls = _solver_class_for("xpress")
+    assert cls is not None
+
+    events: list[str] = []
+
+    class _FakeXpress:
+        @staticmethod
+        def init() -> None:
+            events.append("init")
+
+    monkeypatch.setattr(solvers_mod, "xpress", _FakeXpress)
+
+    cls._license_probe()
+
+    assert events == ["init"]