Add useful error message for missing highspy. Allow variable objective

.gitignore

@@ -34,3 +34,6 @@ benchmark/notebooks/.ipynb_checkpoints
 benchmark/scripts/__pycache__
 benchmark/scripts/benchmarks-pypsa-eur/__pycache__
 benchmark/scripts/leftovers/
+
+# IDE
+.idea

doc/release_notes.rst

@@ -12,6 +12,7 @@ Upcoming Version
   gap tolerance.
 * Improve the mapping of termination conditions for the SCIP solver
 * Treat GLPK's `integer undefined` status as not-OK
+* Allow the objective to be set by a variable instead of an expression
 
 Version 0.5.3
 --------------

linopy/model.py

@@ -683,7 +683,8 @@ def add_constraints(
 
     def add_objective(
         self,
-        expr: LinearExpression
+        expr: Variable
+        | LinearExpression
         | QuadraticExpression
         | Sequence[tuple[ConstantLike, VariableLike]],
         overwrite: bool = False,
@@ -709,6 +710,8 @@ def add_objective(
                 "Objective already defined."
                 " Set `overwrite` to True to force overwriting."
             )
+        if isinstance(expr, Variable):
+            expr = expr * 1.0  #  Convert to expression
         self.objective.expression = expr  # type: ignore[assignment]
         self.objective.sense = sense
 

linopy/solvers.py

@@ -455,6 +455,11 @@ def solve_problem_from_file(
         status.legacy_status = first_line
 
         # Use HiGHS to parse the problem file and find the set of variable names, needed to parse solution
+        if "highs" not in available_solvers:
+            raise ModuleNotFoundError(
+                f"highspy is not installed. Please install it to use {self.solver_name.name} solver."
+            )
+
         h = highspy.Highs()
         h.readModel(path_to_string(problem_fn))
         variables = {v.name for v in h.getVariables()}

test/test_objective.py

@@ -26,6 +26,14 @@ def quadratic_objective() -> Objective:
     return m.objective
 
 
+def test_set_objective_from_variable() -> None:
+    m = Model()
+    v = m.add_variables(coords=[[1, 2, 3]])
+    m.add_objective(v)
+    assert isinstance(m.objective, Objective)
+    assert isinstance(m.objective.expression, LinearExpression)
+
+
 def test_model(linear_objective: Objective, quadratic_objective: Objective) -> None:
     assert isinstance(linear_objective.model, Model)
     assert isinstance(quadratic_objective.model, Model)

test/test_solvers.py

@@ -5,9 +5,11 @@
 @author: sid
 """
 
+import importlib
 from pathlib import Path
 
 import pytest
+from _pytest.monkeypatch import MonkeyPatch
 
 from linopy import solvers
 
@@ -64,3 +66,24 @@ def test_free_mps_solution_parsing(solver: str, tmp_path: Path) -> None:
 
     assert result.status.is_ok
     assert result.solution.objective == 30.0
+
+
+def test_highs_missing(monkeypatch: MonkeyPatch) -> None:
+    monkeypatch.setattr("linopy.solvers.available_solvers", ["cbc"])
+
+    import linopy.model
+
+    importlib.reload(linopy.model)
+
+    from linopy.model import Model
+
+    model = Model()
+    x = model.add_variables(lower=0.0, name="x")
+    model.add_constraints(x >= 0.0)
+    model.add_objective(x, sense="min")
+
+    with pytest.raises(
+        ModuleNotFoundError,
+        match="highspy is not installed. Please install it to use CBC solve",
+    ):
+        model.solve(solver_name="cbc")