feat: add objective from variable (#489)

Author: FabianHofmann

doc/release_notes.rst

@@ -11,6 +11,7 @@ Version 0.5.6
 * Improved variable/expression arithmetic methods so that they correctly handle types
 * Gurobi: Pass dictionary as env argument `env={...}` through to gurobi env creation
 * Mosek: Remove explicit use of Env, use global env instead
+* Objectives can now be created from variables via `linopy.Model.add_objective`.
 
 **Breaking Changes**
 

linopy/model.py

@@ -684,7 +684,8 @@ def add_constraints(
 
     def add_objective(
         self,
-        expr: LinearExpression
+        expr: Variable
+        | LinearExpression
         | QuadraticExpression
         | Sequence[tuple[ConstantLike, VariableLike]],
         overwrite: bool = False,
@@ -710,6 +711,8 @@ def add_objective(
                 "Objective already defined."
                 " Set `overwrite` to True to force overwriting."
             )
+        if isinstance(expr, Variable):
+            expr = 1 * expr
         self.objective.expression = expr
         self.objective.sense = sense
 

test/test_objective.py

@@ -31,6 +31,13 @@ def test_model(linear_objective: Objective, quadratic_objective: Objective) -> N
     assert isinstance(quadratic_objective.model, Model)
 
 
+def test_add_objective_from_variable() -> None:
+    m = Model()
+    v = m.add_variables(coords=[[1, 2, 3]])
+    m.add_objective(v)
+    assert isinstance(m.objective, Objective)
+
+
 def test_sense(linear_objective: Objective, quadratic_objective: Objective) -> None:
     assert linear_objective.sense == "min"
     assert quadratic_objective.sense == "max"