add linear expression from constant

Author: Robbie Muir

linopy/expressions.py

@@ -1647,6 +1647,26 @@ def process_one(
 
         return merge(exprs, cls=cls) if len(exprs) > 1 else exprs[0]
 
+    @classmethod
+    def from_constant(cls, model: Model, constant: ConstantLike) -> LinearExpression:
+        """
+        Create a linear expression from a constant value or series
+
+        Parameters
+        ----------
+        model : linopy.Model
+            The model to which the constant expression will belong.
+        constant : int/float/array_like
+            The constant value for the linear expression.
+
+        Returns
+        -------
+        linopy.LinearExpression
+            A linear expression representing the constant value.
+        """
+        const_da = as_dataarray(constant)
+        return LinearExpression(const_da, model)
+
 
 class QuadraticExpression(BaseExpression):
     """

test/test_linear_expression.py

@@ -1123,6 +1123,21 @@ def test_linear_expression_from_tuples_bad_calls(
         LinearExpression.from_tuples(10)
 
 
+def test_linear_expression_from_constant_scalar(m: Model) -> None:
+    expr = LinearExpression.from_constant(model=m, constant=10)
+    assert isinstance(expr, LinearExpression)
+    assert (expr.const == 10).all()
+
+
+def test_linear_expression_from_constant_array(m: Model) -> None:
+    arr = pd.Series(index=pd.Index([0, 1], name="t"), data=[10, 20])
+    expr = LinearExpression.from_constant(model=m, constant=arr)
+    assert isinstance(expr, LinearExpression)
+    assert list(expr.coords.keys())[0] == "t"
+    assert expr.nterm == 0
+    assert (expr.const.values == [10, 20]).all()
+
+
 def test_linear_expression_sanitize(x: Variable, y: Variable, z: Variable) -> None:
     expr = 10 * x + y + z
     assert isinstance(expr.sanitize(), LinearExpression)