From 7a68636dfa06e14a4a66fd3f765f8fab14b74163 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Tue, 23 Dec 2025 18:05:48 +0100
Subject: [PATCH 01/15] allow constant in objective cost function

---
 linopy/expressions.py             |  3 ++
 linopy/model.py                   | 57 ++++++++++++++++++++++++++++++-
 linopy/objective.py               |  3 --
 test/test_linear_expression.py    | 15 ++++++++
 test/test_optimization.py         | 14 ++++++++
 test/test_quadratic_expression.py | 14 ++++++++
 6 files changed, 102 insertions(+), 4 deletions(-)

diff --git a/linopy/expressions.py b/linopy/expressions.py
index 10e243de..008f5b82 100644
--- a/linopy/expressions.py
+++ b/linopy/expressions.py
@@ -1097,6 +1097,9 @@ def empty(self) -> EmptyDeprecationWrapper:
         """
         return EmptyDeprecationWrapper(not self.size)
 
+    def drop_constant(self: GenericExpression) -> GenericExpression:
+        return self - self.const  # type: ignore
+
     def densify_terms(self: GenericExpression) -> GenericExpression:
         """
         Move all non-zero term entries to the front and cut off all-zero
diff --git a/linopy/model.py b/linopy/model.py
index 81c069ab..e313325b 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -10,9 +10,11 @@
 import os
 import re
 from collections.abc import Callable, Mapping, Sequence
+from functools import wraps
 from pathlib import Path
 from tempfile import NamedTemporaryFile, gettempdir
-from typing import Any, Literal, overload
+from typing import Any, Literal, ParamSpec, TypeVar, overload
+from warnings import warn
 
 import numpy as np
 import pandas as pd
@@ -77,6 +79,58 @@
 logger = logging.getLogger(__name__)
 
 
+P = ParamSpec("P")
+R = TypeVar("R")
+
+
+class ConstantInObjectiveWarning(UserWarning): ...
+
+
+def strip_and_replace_constant_objective(func: Callable[P, R]) -> Callable[P, R]:
+    """
+    Decorates a Model instance method.
+
+    If the model objective contains a constant term, this decorator will:
+    - Remove the constant term from the model objective
+    - Call the decorated method
+    - Add the constant term back to the model objective
+    """
+
+    @wraps(func)
+    def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
+        assert args, "Expected at least one argument (self)"
+        self = args[0]
+        assert isinstance(self, Model), (
+            f"First argument must be a Model instance, got {type(self)}"
+        )
+        if not bool(self.objective.expression.has_constant):
+            return func(*args, **kwargs)
+
+        warn(
+            "The objective function contains a constant term. This will be temporarily removed",
+            ConstantInObjectiveWarning,
+        )
+
+        # Modify the model objective to drop the constant term
+        model = self
+        constant = self.objective.expression.const
+        model.objective.expression = self.objective.expression.drop_constant()
+        args = (model, *args[1:])  # type: ignore
+        result = func(*args, **kwargs)
+
+        # Re-add the constant term
+        model.objective.expression = model.objective.expression + constant
+        if model.objective.value is not None:
+            model.objective.set_value(model.objective.value + constant)
+        warn(
+            "The objective function constant term has been re-added to the result",
+            ConstantInObjectiveWarning,
+        )
+        return result
+
+    return wrapper
+
+
 class Model:
     """
     Linear optimization model.
@@ -1107,6 +1161,7 @@ def get_problem_file(
         ) as f:
             return Path(f.name)
 
+    @strip_and_replace_constant_objective
     def solve(
         self,
         solver_name: str | None = None,
diff --git a/linopy/objective.py b/linopy/objective.py
index b1449270..7d0daa95 100644
--- a/linopy/objective.py
+++ b/linopy/objective.py
@@ -189,9 +189,6 @@ def expression(
         if len(expr.coord_dims):
             expr = expr.sum()
 
-        if (expr.const != 0.0) and not np.isnan(expr.const):
-            raise ValueError("Constant values in objective function not supported.")
-
         self._expression = expr
 
     @property
diff --git a/test/test_linear_expression.py b/test/test_linear_expression.py
index a75ace3f..dfed5f7c 100644
--- a/test/test_linear_expression.py
+++ b/test/test_linear_expression.py
@@ -1230,6 +1230,21 @@ def test_cumsum(m: Model, multiple: float) -> None:
     cumsum.nterm == 2
 
 
+def test_drop_constant(x: Variable) -> None:
+    """Test that constants are removed"""
+    expr_a = 2 * x
+    expr_b = expr_a + [1, 2]
+    expr_c = expr_b + float("nan")
+    for expr in [expr_a, expr_b, expr_c]:
+        expr = 2 * x + 10
+        expr_2 = expr.drop_constant()
+
+        assert all(expr_2.const.values == 0.0), (
+            f"Expected constant 0.0, got {expr_2.const.values}"
+        )
+        assert not bool(expr_2.has_constant)
+
+
 def test_simplify_basic(x: Variable) -> None:
     """Test basic simplification with duplicate terms."""
     expr = 2 * x + 3 * x + 1 * x
diff --git a/test/test_optimization.py b/test/test_optimization.py
index 12399a4e..416136c6 100644
--- a/test/test_optimization.py
+++ b/test/test_optimization.py
@@ -20,6 +20,7 @@
 from linopy import GREATER_EQUAL, LESS_EQUAL, Model, solvers
 from linopy.common import to_path
 from linopy.expressions import LinearExpression
+from linopy.model import ConstantInObjectiveWarning
 from linopy.solver_capabilities import (
     SolverFeature,
     get_available_solvers_with_feature,
@@ -955,6 +956,19 @@ def test_model_resolve(
     assert np.isclose(model.objective.value or 0, 5.25)
 
 
+def test_model_with_constant_in_objective(model: Model) -> None:
+    objective = model.objective.expression + 1
+    model.add_objective(expr=objective, overwrite=True)
+
+    with pytest.warns(ConstantInObjectiveWarning):
+        status, _ = model.solve(solver_name="highs")
+    assert status == "ok"
+    # x = -0.1, y = 1.7
+    assert model.objective.value == 4.3
+    assert model.objective.expression.const == 1
+    assert model.objective.expression.solution == 4.3
+
+
 @pytest.mark.parametrize(
     "solver,io_api,explicit_coordinate_names", [p for p in params if "direct" not in p]
 )
diff --git a/test/test_quadratic_expression.py b/test/test_quadratic_expression.py
index fc1bb25f..a1904e24 100644
--- a/test/test_quadratic_expression.py
+++ b/test/test_quadratic_expression.py
@@ -312,6 +312,20 @@ def test_quadratic_expression_constant_to_polars() -> None:
     assert all(arr.to_numpy() == df["const"].to_numpy())
 
 
+def test_drop_constant(x: Variable) -> None:
+    """Test that constants are removed"""
+    expr_a = 2 * x * x
+    expr_b = expr_a + 1
+    for expr in [expr_a, expr_b]:
+        expr = 2 * x + 10
+        expr_2 = expr.drop_constant()
+
+        assert all(expr_2.const.values == 0.0), (
+            f"Expected constant 0.0, got {expr_2.const.values}"
+        )
+        assert not bool(expr_2.has_constant)
+
+
 def test_quadratic_expression_to_matrix(model: Model, x: Variable, y: Variable) -> None:
     expr: QuadraticExpression = x * y + x + 5  # type: ignore
 

From f51b340482f7d1d5571e7865b61204e5271bc737 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Tue, 23 Dec 2025 18:07:10 +0100
Subject: [PATCH 02/15] update docs

---
 doc/release_notes.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/release_notes.rst b/doc/release_notes.rst
index 730d3e04..1583a4fd 100644
--- a/doc/release_notes.rst
+++ b/doc/release_notes.rst
@@ -3,6 +3,7 @@ Release Notes
 
 .. Upcoming Version
 
+* Allow constant values in objective cost function
 * Add support for SOS1 and SOS2 (Special Ordered Sets) constraints via ``Model.add_sos_constraints()`` and ``Model.remove_sos_constraints()``
 * Add simplify method to LinearExpression to combine duplicate terms
 * Add convenience function to create LinearExpression from constant

From a7522d6b634768b1a91dead9041bb38bd63c6066 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Tue, 23 Dec 2025 18:17:33 +0100
Subject: [PATCH 03/15] fix tests

---
 linopy/model.py           | 10 ++++++++--
 test/test_model.py        |  5 ++---
 test/test_objective.py    |  3 +--
 test/test_optimization.py | 31 ++++++++++++++++++++++++++++++-
 4 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index e313325b..d7f3cef9 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -116,9 +116,15 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
         constant = self.objective.expression.const
         model.objective.expression = self.objective.expression.drop_constant()
         args = (model, *args[1:])  # type: ignore
-        result = func(*args, **kwargs)
 
-        # Re-add the constant term
+        try:
+            result = func(*args, **kwargs)
+        except Exception as e:
+            # Even if there is an exception, make sure the model returns to it's original state
+            model.objective.expression = model.objective.expression + constant
+            raise e
+
+        # Re-add the constant term to return the model objective to the original expression
         model.objective.expression = model.objective.expression + constant
         if model.objective.value is not None:
             model.objective.set_value(model.objective.value + constant)
diff --git a/test/test_model.py b/test/test_model.py
index c363fe4c..063b70dd 100644
--- a/test/test_model.py
+++ b/test/test_model.py
@@ -82,9 +82,8 @@ def test_objective() -> None:
     assert m.objectiverange.min() == 2
     assert m.objectiverange.max() == 2
 
-    # test objective with constant which is not supported
-    with pytest.raises(ValueError):
-        m.objective = m.objective + 3
+    # test objective with constant which is supported
+    m.objective = m.objective + 3
 
 
 def test_remove_variable() -> None:
diff --git a/test/test_objective.py b/test/test_objective.py
index d869175a..80b2021a 100644
--- a/test/test_objective.py
+++ b/test/test_objective.py
@@ -192,5 +192,4 @@ def test_repr(linear_objective: Objective, quadratic_objective: Objective) -> No
 def test_objective_constant() -> None:
     m = Model()
     linear_expr = LinearExpression(None, m) + 1
-    with pytest.raises(ValueError):
-        m.objective = Objective(linear_expr, m)
+    m.objective = Objective(linear_expr, m)
diff --git a/test/test_optimization.py b/test/test_optimization.py
index 416136c6..ec4e653a 100644
--- a/test/test_optimization.py
+++ b/test/test_optimization.py
@@ -956,7 +956,7 @@ def test_model_resolve(
     assert np.isclose(model.objective.value or 0, 5.25)
 
 
-def test_model_with_constant_in_objective(model: Model) -> None:
+def test_model_with_constant_in_objective_feasible(model: Model) -> None:
     objective = model.objective.expression + 1
     model.add_objective(expr=objective, overwrite=True)
 
@@ -969,6 +969,35 @@ def test_model_with_constant_in_objective(model: Model) -> None:
     assert model.objective.expression.solution == 4.3
 
 
+def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
+    objective = model.objective.expression + 1
+    model.add_objective(expr=objective, overwrite=True)
+    model.add_constraints([(1, "x")], "<=", 0)
+    model.add_constraints([(1, "y")], "<=", 0)
+
+    with pytest.warns(ConstantInObjectiveWarning):
+        status, condition = model.solve(solver_name="highs")
+
+    assert condition == "infeasible"
+    # Even though the problem was not solved, the constant term should still be accessible
+    assert model.objective.expression.const == 1
+
+
+def test_model_with_constant_in_objective_error(model: Model) -> None:
+    objective = model.objective.expression + 1
+    model.add_objective(expr=objective, overwrite=True)
+    model.add_constraints([(1, "x")], "<=", 0)
+    model.add_constraints([(1, "y")], "<=", 0)
+
+    try:
+        status, condition = model.solve(solver_name="apples")
+    except AssertionError:
+        pass
+
+    # Even if something goes wrong, the model objective should return to the correct state
+    assert model.objective.expression.const == 1
+
+
 @pytest.mark.parametrize(
     "solver,io_api,explicit_coordinate_names", [p for p in params if "direct" not in p]
 )

From 14b603c034326f321bf7a085cac424e055feafb8 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Tue, 23 Dec 2025 18:28:24 +0100
Subject: [PATCH 04/15] fix mypy

---
 linopy/model.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/linopy/model.py b/linopy/model.py
index d7f3cef9..0aac9ebb 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -104,6 +104,7 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
             f"First argument must be a Model instance, got {type(self)}"
         )
         if not bool(self.objective.expression.has_constant):
+            # Continue as normal if there is no constant term
             return func(*args, **kwargs)
 
         warn(
@@ -113,7 +114,7 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
 
         # Modify the model objective to drop the constant term
         model = self
-        constant = self.objective.expression.const
+        constant = float(self.objective.expression.const.values)
         model.objective.expression = self.objective.expression.drop_constant()
         args = (model, *args[1:])  # type: ignore
 

From 14497bf21022bd9132a6a7efb386c7cc4ab9a8c5 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Thu, 25 Dec 2025 15:57:22 +0100
Subject: [PATCH 05/15] made objective constnat logic more strict

---
 linopy/model.py           | 42 +++++++++++++++++++++++++++++----------
 linopy/objective.py       |  7 +++++++
 test/test_optimization.py | 16 ++++++++++-----
 3 files changed, 50 insertions(+), 15 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index 0aac9ebb..fec3855f 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -86,6 +86,9 @@
 class ConstantInObjectiveWarning(UserWarning): ...
 
 
+class ConstantObjectiveError(Exception): ...
+
+
 def strip_and_replace_constant_objective(func: Callable[P, R]) -> Callable[P, R]:
     """
     Decorates a Model instance method.
@@ -103,14 +106,16 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
         assert isinstance(self, Model), (
             f"First argument must be a Model instance, got {type(self)}"
         )
-        if not bool(self.objective.expression.has_constant):
+        model = self
+        if not self.objective.has_constant:
             # Continue as normal if there is no constant term
             return func(*args, **kwargs)
 
-        warn(
-            "The objective function contains a constant term. This will be temporarily removed",
-            ConstantInObjectiveWarning,
-        )
+        # The objective contains a constant term
+        if not model.allow_constant_objective:
+            raise ConstantObjectiveError(
+                "Objective function contains constant terms. Please use LinearExpression.drop_constants()/QuadraticExpression.drop_constants() or set Model.allow_constant_objective=True."
+            )
 
         # Modify the model objective to drop the constant term
         model = self
@@ -129,10 +134,7 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
         model.objective.expression = model.objective.expression + constant
         if model.objective.value is not None:
             model.objective.set_value(model.objective.value + constant)
-        warn(
-            "The objective function constant term has been re-added to the result",
-            ConstantInObjectiveWarning,
-        )
+
         return result
 
     return wrapper
@@ -164,6 +166,7 @@ class Model:
     _dual: Dataset
     _status: str
     _termination_condition: str
+    _allow_constant_objective: bool
     _xCounter: int
     _cCounter: int
     _varnameCounter: int
@@ -185,6 +188,7 @@ class Model:
         # hidden attributes
         "_status",
         "_termination_condition",
+        "_allow_constant_objective",
         # TODO: move counters to Variables and Constraints class
         "_xCounter",
         "_cCounter",
@@ -236,6 +240,7 @@ def __init__(
 
         self._status: str = "initialized"
         self._termination_condition: str = ""
+        self._allow_constant_objective: bool = False
         self._xCounter: int = 0
         self._cCounter: int = 0
         self._varnameCounter: int = 0
@@ -788,6 +793,17 @@ def add_constraints(
         self.constraints.add(constraint)
         return constraint
 
+    @property
+    def allow_constant_objective(self) -> bool:
+        """
+        Whether constant terms in the objective function are allowed.
+        """
+        return self._allow_constant_objective
+
+    @allow_constant_objective.setter
+    def allow_constant_objective(self, allow: bool) -> None:
+        self._allow_constant_objective = allow
+
     def add_objective(
         self,
         expr: Variable
@@ -809,7 +825,7 @@ def add_objective(
 
         Returns
         -------
-        linopy.LinearExpression
+        linopy.LinearExpression, linopy.QuadraticExpression
             The objective function assigned to the model.
         """
         if not overwrite:
@@ -819,8 +835,14 @@ def add_objective(
             )
         if isinstance(expr, Variable):
             expr = 1 * expr
+
         self.objective.expression = expr
         self.objective.sense = sense
+        if not self.allow_constant_objective and self.objective.has_constant:
+            warn(
+                "Objective function contains constant terms but this is not allowed as Model.allow_constant_objective=False, running solve will result in an error. Please either remove constants from the expression with expr.drop_constants() or set Model.allow_constant_objective=True.",
+                ConstantInObjectiveWarning,
+            )
 
     def remove_variables(self, name: str) -> None:
         """
diff --git a/linopy/objective.py b/linopy/objective.py
index 7d0daa95..11e00b49 100644
--- a/linopy/objective.py
+++ b/linopy/objective.py
@@ -191,6 +191,13 @@ def expression(
 
         self._expression = expr
 
+    @property
+    def has_constant(self) -> bool:
+        """
+        Returns whether the objective has a constant term.
+        """
+        return bool(self.expression.has_constant)
+
     @property
     def model(self) -> Model:
         """
diff --git a/test/test_optimization.py b/test/test_optimization.py
index ec4e653a..4d0cbc06 100644
--- a/test/test_optimization.py
+++ b/test/test_optimization.py
@@ -20,7 +20,7 @@
 from linopy import GREATER_EQUAL, LESS_EQUAL, Model, solvers
 from linopy.common import to_path
 from linopy.expressions import LinearExpression
-from linopy.model import ConstantInObjectiveWarning
+from linopy.model import ConstantInObjectiveWarning, ConstantObjectiveError
 from linopy.solver_capabilities import (
     SolverFeature,
     get_available_solvers_with_feature,
@@ -958,10 +958,15 @@ def test_model_resolve(
 
 def test_model_with_constant_in_objective_feasible(model: Model) -> None:
     objective = model.objective.expression + 1
-    model.add_objective(expr=objective, overwrite=True)
 
     with pytest.warns(ConstantInObjectiveWarning):
+        model.add_objective(expr=objective, overwrite=True)
+
+    with pytest.raises(ConstantObjectiveError):
         status, _ = model.solve(solver_name="highs")
+
+    model.allow_constant_objective = True
+    status, _ = model.solve(solver_name="highs")
     assert status == "ok"
     # x = -0.1, y = 1.7
     assert model.objective.value == 4.3
@@ -975,8 +980,8 @@ def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 
-    with pytest.warns(ConstantInObjectiveWarning):
-        status, condition = model.solve(solver_name="highs")
+    model.allow_constant_objective = True
+    _, condition = model.solve(solver_name="highs")
 
     assert condition == "infeasible"
     # Even though the problem was not solved, the constant term should still be accessible
@@ -985,12 +990,13 @@ def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
 
 def test_model_with_constant_in_objective_error(model: Model) -> None:
     objective = model.objective.expression + 1
+    model.allow_constant_objective = True
     model.add_objective(expr=objective, overwrite=True)
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 
     try:
-        status, condition = model.solve(solver_name="apples")
+        _ = model.solve(solver_name="apples")
     except AssertionError:
         pass
 

From 9e7eea555020ea3b8df59bd0d349604abb81d178 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 15:03:28 +0100
Subject: [PATCH 06/15] minor changes

---
 linopy/expressions.py             |  4 ++--
 linopy/model.py                   | 13 +++++++------
 linopy/objective.py               |  2 +-
 test/test_linear_expression.py    |  2 +-
 test/test_optimization.py         | 12 ++++--------
 test/test_quadratic_expression.py |  2 +-
 6 files changed, 16 insertions(+), 19 deletions(-)

diff --git a/linopy/expressions.py b/linopy/expressions.py
index 008f5b82..a9b65455 100644
--- a/linopy/expressions.py
+++ b/linopy/expressions.py
@@ -726,8 +726,8 @@ def const(self, value: DataArray) -> None:
         self._data = assign_multiindex_safe(self.data, const=value)
 
     @property
-    def has_constant(self) -> DataArray:
-        return self.const.any()
+    def has_constant(self) -> bool:
+        return bool(self.const.any())
 
     # create a dummy for a mask, which can be implemented later
     @property
diff --git a/linopy/model.py b/linopy/model.py
index fec3855f..cc33b52c 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -14,7 +14,6 @@
 from pathlib import Path
 from tempfile import NamedTemporaryFile, gettempdir
 from typing import Any, Literal, ParamSpec, TypeVar, overload
-from warnings import warn
 
 import numpy as np
 import pandas as pd
@@ -83,9 +82,6 @@
 R = TypeVar("R")
 
 
-class ConstantInObjectiveWarning(UserWarning): ...
-
-
 class ConstantObjectiveError(Exception): ...
 
 
@@ -812,6 +808,7 @@ def add_objective(
         | Sequence[tuple[ConstantLike, VariableLike]],
         overwrite: bool = False,
         sense: str = "min",
+        allow_constant: bool | None = None,
     ) -> None:
         """
         Add an objective function to the model.
@@ -822,6 +819,8 @@ def add_objective(
             Expression describing the objective function.
         overwrite : False, optional
             Whether to overwrite the existing objective. The default is False.
+        allow_constant : bool, optional
+            Set the `Model.allow_constant_objective` attribute. If True, the objective is allowed to contain a constant term.
 
         Returns
         -------
@@ -838,10 +837,12 @@ def add_objective(
 
         self.objective.expression = expr
         self.objective.sense = sense
+        if allow_constant is not None:
+            self.allow_constant_objective = allow_constant
+
         if not self.allow_constant_objective and self.objective.has_constant:
-            warn(
+            raise ConstantObjectiveError(
                 "Objective function contains constant terms but this is not allowed as Model.allow_constant_objective=False, running solve will result in an error. Please either remove constants from the expression with expr.drop_constants() or set Model.allow_constant_objective=True.",
-                ConstantInObjectiveWarning,
             )
 
     def remove_variables(self, name: str) -> None:
diff --git a/linopy/objective.py b/linopy/objective.py
index 11e00b49..a810064a 100644
--- a/linopy/objective.py
+++ b/linopy/objective.py
@@ -196,7 +196,7 @@ def has_constant(self) -> bool:
         """
         Returns whether the objective has a constant term.
         """
-        return bool(self.expression.has_constant)
+        return self.expression.has_constant
 
     @property
     def model(self) -> Model:
diff --git a/test/test_linear_expression.py b/test/test_linear_expression.py
index dfed5f7c..4969e964 100644
--- a/test/test_linear_expression.py
+++ b/test/test_linear_expression.py
@@ -1242,7 +1242,7 @@ def test_drop_constant(x: Variable) -> None:
         assert all(expr_2.const.values == 0.0), (
             f"Expected constant 0.0, got {expr_2.const.values}"
         )
-        assert not bool(expr_2.has_constant)
+        assert not expr_2.has_constant
 
 
 def test_simplify_basic(x: Variable) -> None:
diff --git a/test/test_optimization.py b/test/test_optimization.py
index 4d0cbc06..5ea88e25 100644
--- a/test/test_optimization.py
+++ b/test/test_optimization.py
@@ -20,7 +20,7 @@
 from linopy import GREATER_EQUAL, LESS_EQUAL, Model, solvers
 from linopy.common import to_path
 from linopy.expressions import LinearExpression
-from linopy.model import ConstantInObjectiveWarning, ConstantObjectiveError
+from linopy.model import ConstantObjectiveError
 from linopy.solver_capabilities import (
     SolverFeature,
     get_available_solvers_with_feature,
@@ -958,9 +958,7 @@ def test_model_resolve(
 
 def test_model_with_constant_in_objective_feasible(model: Model) -> None:
     objective = model.objective.expression + 1
-
-    with pytest.warns(ConstantInObjectiveWarning):
-        model.add_objective(expr=objective, overwrite=True)
+    model.add_objective(expr=objective, overwrite=True)
 
     with pytest.raises(ConstantObjectiveError):
         status, _ = model.solve(solver_name="highs")
@@ -980,8 +978,7 @@ def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 
-    model.allow_constant_objective = True
-    _, condition = model.solve(solver_name="highs")
+    _, condition = model.solve(solver_name="highs", allow_constant=True)
 
     assert condition == "infeasible"
     # Even though the problem was not solved, the constant term should still be accessible
@@ -990,8 +987,7 @@ def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
 
 def test_model_with_constant_in_objective_error(model: Model) -> None:
     objective = model.objective.expression + 1
-    model.allow_constant_objective = True
-    model.add_objective(expr=objective, overwrite=True)
+    model.add_objective(expr=objective, overwrite=True, allow_constant=True)
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 
diff --git a/test/test_quadratic_expression.py b/test/test_quadratic_expression.py
index a1904e24..cb203b15 100644
--- a/test/test_quadratic_expression.py
+++ b/test/test_quadratic_expression.py
@@ -323,7 +323,7 @@ def test_drop_constant(x: Variable) -> None:
         assert all(expr_2.const.values == 0.0), (
             f"Expected constant 0.0, got {expr_2.const.values}"
         )
-        assert not bool(expr_2.has_constant)
+        assert not expr_2.has_constant
 
 
 def test_quadratic_expression_to_matrix(model: Model, x: Variable, y: Variable) -> None:

From 9e5847a0dd166132801e82daab163282bb489770 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 15:29:54 +0100
Subject: [PATCH 07/15] fix test

---
 linopy/model.py           | 12 ++++++------
 test/test_optimization.py | 17 ++++++++++++-----
 2 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index cc33b52c..c7c84577 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -110,7 +110,7 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
         # The objective contains a constant term
         if not model.allow_constant_objective:
             raise ConstantObjectiveError(
-                "Objective function contains constant terms. Please use LinearExpression.drop_constants()/QuadraticExpression.drop_constants() or set Model.allow_constant_objective=True."
+                "Objective function contains constant terms. Please use expr.drop_constants() or set model.allow_constant_objective=True."
             )
 
         # Modify the model objective to drop the constant term
@@ -808,7 +808,7 @@ def add_objective(
         | Sequence[tuple[ConstantLike, VariableLike]],
         overwrite: bool = False,
         sense: str = "min",
-        allow_constant: bool | None = None,
+        allow_constant_objective: bool | None = None,
     ) -> None:
         """
         Add an objective function to the model.
@@ -819,7 +819,7 @@ def add_objective(
             Expression describing the objective function.
         overwrite : False, optional
             Whether to overwrite the existing objective. The default is False.
-        allow_constant : bool, optional
+        allow_constant_objective : bool, optional
             Set the `Model.allow_constant_objective` attribute. If True, the objective is allowed to contain a constant term.
 
         Returns
@@ -837,12 +837,12 @@ def add_objective(
 
         self.objective.expression = expr
         self.objective.sense = sense
-        if allow_constant is not None:
-            self.allow_constant_objective = allow_constant
+        if allow_constant_objective is not None:
+            self.allow_constant_objective = allow_constant_objective
 
         if not self.allow_constant_objective and self.objective.has_constant:
             raise ConstantObjectiveError(
-                "Objective function contains constant terms but this is not allowed as Model.allow_constant_objective=False, running solve will result in an error. Please either remove constants from the expression with expr.drop_constants() or set Model.allow_constant_objective=True.",
+                "Objective function contains constant terms but this is not allowed as Model.allow_constant_objective=False. Either remove constants from the expression with expr.drop_constants() or pass allow_constant_objective=True.",
             )
 
     def remove_variables(self, name: str) -> None:
diff --git a/test/test_optimization.py b/test/test_optimization.py
index 5ea88e25..c1a877eb 100644
--- a/test/test_optimization.py
+++ b/test/test_optimization.py
@@ -948,7 +948,7 @@ def test_model_resolve(
     # add another constraint after solve
     model.add_constraints(model.variables.y >= 3)
 
-    status, condition = model.solve(
+    status, _ = model.solve(
         solver, io_api=io_api, explicit_coordinate_names=explicit_coordinate_names
     )
     assert status == "ok"
@@ -958,7 +958,14 @@ def test_model_resolve(
 
 def test_model_with_constant_in_objective_feasible(model: Model) -> None:
     objective = model.objective.expression + 1
-    model.add_objective(expr=objective, overwrite=True)
+
+    with pytest.raises(ConstantObjectiveError):
+        model.add_objective(
+            expr=objective, overwrite=True, allow_constant_objective=False
+        )
+
+    model.add_objective(expr=objective, overwrite=True, allow_constant_objective=True)
+    model.allow_constant_objective = False
 
     with pytest.raises(ConstantObjectiveError):
         status, _ = model.solve(solver_name="highs")
@@ -974,11 +981,11 @@ def test_model_with_constant_in_objective_feasible(model: Model) -> None:
 
 def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
     objective = model.objective.expression + 1
-    model.add_objective(expr=objective, overwrite=True)
+    model.add_objective(expr=objective, overwrite=True, allow_constant_objective=True)
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 
-    _, condition = model.solve(solver_name="highs", allow_constant=True)
+    _, condition = model.solve(solver_name="highs")
 
     assert condition == "infeasible"
     # Even though the problem was not solved, the constant term should still be accessible
@@ -987,7 +994,7 @@ def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
 
 def test_model_with_constant_in_objective_error(model: Model) -> None:
     objective = model.objective.expression + 1
-    model.add_objective(expr=objective, overwrite=True, allow_constant=True)
+    model.add_objective(expr=objective, overwrite=True, allow_constant_objective=True)
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 

From ee6f1ec69a542ac88d43ca7b63534cdfb7a6707c Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 16:28:36 +0100
Subject: [PATCH 08/15] refactor

---
 linopy/model.py           | 44 +++++++++++----------------------------
 test/test_optimization.py | 29 ++++++++++++++------------
 2 files changed, 28 insertions(+), 45 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index c7c84577..98848509 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -108,11 +108,6 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
             return func(*args, **kwargs)
 
         # The objective contains a constant term
-        if not model.allow_constant_objective:
-            raise ConstantObjectiveError(
-                "Objective function contains constant terms. Please use expr.drop_constants() or set model.allow_constant_objective=True."
-            )
-
         # Modify the model objective to drop the constant term
         model = self
         constant = float(self.objective.expression.const.values)
@@ -162,7 +157,6 @@ class Model:
     _dual: Dataset
     _status: str
     _termination_condition: str
-    _allow_constant_objective: bool
     _xCounter: int
     _cCounter: int
     _varnameCounter: int
@@ -184,7 +178,6 @@ class Model:
         # hidden attributes
         "_status",
         "_termination_condition",
-        "_allow_constant_objective",
         # TODO: move counters to Variables and Constraints class
         "_xCounter",
         "_cCounter",
@@ -236,7 +229,6 @@ def __init__(
 
         self._status: str = "initialized"
         self._termination_condition: str = ""
-        self._allow_constant_objective: bool = False
         self._xCounter: int = 0
         self._cCounter: int = 0
         self._varnameCounter: int = 0
@@ -277,9 +269,10 @@ def objective(
         self, obj: Objective | LinearExpression | QuadraticExpression
     ) -> Objective:
         if not isinstance(obj, Objective):
-            obj = Objective(obj, self)
-
-        self._objective = obj
+            expr = obj
+        else:
+            expr = obj.expression
+        self.add_objective(expr=expr, overwrite=True, allow_constant=False)
         return self._objective
 
     @property
@@ -789,17 +782,6 @@ def add_constraints(
         self.constraints.add(constraint)
         return constraint
 
-    @property
-    def allow_constant_objective(self) -> bool:
-        """
-        Whether constant terms in the objective function are allowed.
-        """
-        return self._allow_constant_objective
-
-    @allow_constant_objective.setter
-    def allow_constant_objective(self, allow: bool) -> None:
-        self._allow_constant_objective = allow
-
     def add_objective(
         self,
         expr: Variable
@@ -808,7 +790,7 @@ def add_objective(
         | Sequence[tuple[ConstantLike, VariableLike]],
         overwrite: bool = False,
         sense: str = "min",
-        allow_constant_objective: bool | None = None,
+        allow_constant: bool = False,
     ) -> None:
         """
         Add an objective function to the model.
@@ -819,8 +801,8 @@ def add_objective(
             Expression describing the objective function.
         overwrite : False, optional
             Whether to overwrite the existing objective. The default is False.
-        allow_constant_objective : bool, optional
-            Set the `Model.allow_constant_objective` attribute. If True, the objective is allowed to contain a constant term.
+        allow_constant: bool, optional
+            If True, the objective is allowed to contain a constant term. The default is False
 
         Returns
         -------
@@ -835,16 +817,14 @@ def add_objective(
         if isinstance(expr, Variable):
             expr = 1 * expr
 
-        self.objective.expression = expr
-        self.objective.sense = sense
-        if allow_constant_objective is not None:
-            self.allow_constant_objective = allow_constant_objective
-
-        if not self.allow_constant_objective and self.objective.has_constant:
+        if not allow_constant and expr.has_constant:
             raise ConstantObjectiveError(
-                "Objective function contains constant terms but this is not allowed as Model.allow_constant_objective=False. Either remove constants from the expression with expr.drop_constants() or pass allow_constant_objective=True.",
+                "Objective contains constant term. Either remove constants from the expression with expr.drop_constants() or use model.add_objective(..., allow_constant=True).",
             )
 
+        objective = Objective(expression=expr, model=self, sense=sense)
+        self._objective = objective
+
     def remove_variables(self, name: str) -> None:
         """
         Remove all variables stored under reference name `name` from the model.
diff --git a/test/test_optimization.py b/test/test_optimization.py
index c1a877eb..7bb05be8 100644
--- a/test/test_optimization.py
+++ b/test/test_optimization.py
@@ -956,21 +956,24 @@ def test_model_resolve(
     assert np.isclose(model.objective.value or 0, 5.25)
 
 
-def test_model_with_constant_in_objective_feasible(model: Model) -> None:
+def test_constant_not_allowed_unless_specified_explicitly(model: Model) -> None:
     objective = model.objective.expression + 1
 
     with pytest.raises(ConstantObjectiveError):
-        model.add_objective(
-            expr=objective, overwrite=True, allow_constant_objective=False
-        )
-
-    model.add_objective(expr=objective, overwrite=True, allow_constant_objective=True)
-    model.allow_constant_objective = False
+        model.add_objective(expr=objective, overwrite=True, allow_constant=False)
+    with pytest.raises(ConstantObjectiveError):
+        model.add_objective(expr=objective, overwrite=True)
 
     with pytest.raises(ConstantObjectiveError):
-        status, _ = model.solve(solver_name="highs")
+        model.objective = objective
+
+    model.add_objective(expr=objective, overwrite=True, allow_constant=True)
+
+
+def test_constant_feasible(model: Model) -> None:
+    objective = model.objective.expression + 1
+    model.add_objective(expr=objective, overwrite=True, allow_constant=True)
 
-    model.allow_constant_objective = True
     status, _ = model.solve(solver_name="highs")
     assert status == "ok"
     # x = -0.1, y = 1.7
@@ -979,9 +982,9 @@ def test_model_with_constant_in_objective_feasible(model: Model) -> None:
     assert model.objective.expression.solution == 4.3
 
 
-def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
+def test_constant_infeasible(model: Model) -> None:
     objective = model.objective.expression + 1
-    model.add_objective(expr=objective, overwrite=True, allow_constant_objective=True)
+    model.add_objective(expr=objective, overwrite=True, allow_constant=True)
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 
@@ -992,9 +995,9 @@ def test_model_with_constant_in_objective_infeasible(model: Model) -> None:
     assert model.objective.expression.const == 1
 
 
-def test_model_with_constant_in_objective_error(model: Model) -> None:
+def test_constant_error(model: Model) -> None:
     objective = model.objective.expression + 1
-    model.add_objective(expr=objective, overwrite=True, allow_constant_objective=True)
+    model.add_objective(expr=objective, overwrite=True, allow_constant=True)
     model.add_constraints([(1, "x")], "<=", 0)
     model.add_constraints([(1, "y")], "<=", 0)
 

From 8eb9fdc37e4f7285938d8080db29192e0900ea05 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 17:06:23 +0100
Subject: [PATCH 09/15] more changes

---
 linopy/model.py           | 46 +++++++++++++++++++++++++++++----------
 linopy/objective.py       | 13 ++++++++---
 test/test_model.py        | 26 +++++++++++++++++++---
 test/test_objective.py    |  6 -----
 test/test_optimization.py | 15 -------------
 5 files changed, 68 insertions(+), 38 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index 98848509..da5aac10 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -268,11 +268,7 @@ def objective(self) -> Objective:
     def objective(
         self, obj: Objective | LinearExpression | QuadraticExpression
     ) -> Objective:
-        if not isinstance(obj, Objective):
-            expr = obj
-        else:
-            expr = obj.expression
-        self.add_objective(expr=expr, overwrite=True, allow_constant=False)
+        self.add_objective(expr=obj, overwrite=True, allow_constant=False)
         return self._objective
 
     @property
@@ -782,14 +778,36 @@ def add_constraints(
         self.constraints.add(constraint)
         return constraint
 
+    @overload
+    def add_objective(
+        self,
+        expr: Objective,
+        sense: None = None,
+        overwrite: bool = False,
+        allow_constant: bool = False,
+    ) -> None: ...
+
+    @overload
     def add_objective(
         self,
         expr: Variable
         | LinearExpression
         | QuadraticExpression
         | Sequence[tuple[ConstantLike, VariableLike]],
+        sense: Literal["min", "max"] | None = None,
+        overwrite: bool = False,
+        allow_constant: bool = False,
+    ) -> None: ...
+
+    def add_objective(
+        self,
+        expr: Variable
+        | LinearExpression
+        | QuadraticExpression
+        | Sequence[tuple[ConstantLike, VariableLike]]
+        | Objective,
+        sense: Literal["min", "max"] | None = None,
         overwrite: bool = False,
-        sense: str = "min",
         allow_constant: bool = False,
     ) -> None:
         """
@@ -797,8 +815,9 @@ def add_objective(
 
         Parameters
         ----------
-        expr : linopy.LinearExpression, linopy.QuadraticExpression
+        expr : linopy.Variable, linopy.LinearExpression, linopy.QuadraticExpression, Objective
             Expression describing the objective function.
+        sense: "min" or "max", the sense to optimize for. Defaults to min. Cannot be set if passing Objective directly
         overwrite : False, optional
             Whether to overwrite the existing objective. The default is False.
         allow_constant: bool, optional
@@ -814,15 +833,20 @@ def add_objective(
                 "Objective already defined."
                 " Set `overwrite` to True to force overwriting."
             )
-        if isinstance(expr, Variable):
-            expr = 1 * expr
 
-        if not allow_constant and expr.has_constant:
+        if isinstance(expr, Objective):
+            assert sense is None, "Cannot set sense if objective object is passed"
+            objective = expr
+            assert objective.model == self
+        else:
+            sense = sense or "min"
+            objective = Objective(expression=expr, model=self, sense=sense)
+
+        if not allow_constant and objective.expression.has_constant:
             raise ConstantObjectiveError(
                 "Objective contains constant term. Either remove constants from the expression with expr.drop_constants() or use model.add_objective(..., allow_constant=True).",
             )
 
-        objective = Objective(expression=expr, model=self, sense=sense)
         self._objective = objective
 
     def remove_variables(self, name: str) -> None:
diff --git a/linopy/objective.py b/linopy/objective.py
index a810064a..fcd2709c 100644
--- a/linopy/objective.py
+++ b/linopy/objective.py
@@ -9,7 +9,7 @@
 
 import functools
 from collections.abc import Callable, Sequence
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, Literal
 
 import numpy as np
 import polars as pl
@@ -24,6 +24,7 @@
 
 from linopy import expressions
 from linopy.types import ConstantLike
+from linopy.variables import Variable
 
 if TYPE_CHECKING:
     from linopy.expressions import LinearExpression, QuadraticExpression
@@ -64,13 +65,19 @@ class Objective:
 
     def __init__(
         self,
-        expression: expressions.LinearExpression | expressions.QuadraticExpression,
+        expression: Variable
+        | expressions.LinearExpression
+        | expressions.QuadraticExpression,
         model: Model,
-        sense: str = "min",
+        sense: Literal["min", "max"] = "min",
     ) -> None:
         self._model: Model = model
         self._value: float | None = None
 
+        if isinstance(expression, Variable):
+            expression = 1 * expression
+
+        assert sense in ["min", "max"]
         self.sense: str = sense
         self.expression: (
             expressions.LinearExpression | expressions.QuadraticExpression
diff --git a/test/test_model.py b/test/test_model.py
index 063b70dd..494118ad 100644
--- a/test/test_model.py
+++ b/test/test_model.py
@@ -9,10 +9,12 @@
 from tempfile import gettempdir
 
 import numpy as np
+import pandas as pd
 import pytest
 import xarray as xr
 
 from linopy import EQUAL, Model
+from linopy.model import ConstantObjectiveError
 from linopy.testing import assert_model_equal
 
 target_shape: tuple[int, int] = (10, 10)
@@ -67,7 +69,7 @@ def test_objective() -> None:
     y = m.add_variables(lower, upper, name="y")
 
     obj1 = (10 * x + 5 * y).sum()
-    m.add_objective(obj1)
+    m.add_objective(obj1, allow_constant=True)
     assert m.objective.vars.size == 200
 
     # test overwriting
@@ -82,8 +84,9 @@ def test_objective() -> None:
     assert m.objectiverange.min() == 2
     assert m.objectiverange.max() == 2
 
-    # test objective with constant which is supported
-    m.objective = m.objective + 3
+    # test setting constant term in objective with explicitly allowing it
+    with pytest.raises(ConstantObjectiveError):
+        m.objective = m.objective + 3
 
 
 def test_remove_variable() -> None:
@@ -162,3 +165,20 @@ def test_assert_model_equal() -> None:
     m.add_objective(obj)
 
     assert_model_equal(m, m)
+
+
+def test_constant_not_allowed_in_objective_unless_specified_explicitly() -> None:
+    model = Model()
+    days = pd.Index(["Mon", "Tue", "Wed", "Thu", "Fri"], name="day")
+    x = model.add_variables(name="x", coords=[days])
+    non_linear = x + 1
+
+    with pytest.raises(ConstantObjectiveError):
+        model.add_objective(expr=non_linear, overwrite=True, allow_constant=False)
+    with pytest.raises(ConstantObjectiveError):
+        model.add_objective(expr=non_linear, overwrite=True)
+
+    with pytest.raises(ConstantObjectiveError):
+        model.objective = non_linear
+
+    model.add_objective(expr=non_linear, overwrite=True, allow_constant=True)
diff --git a/test/test_objective.py b/test/test_objective.py
index 80b2021a..cffbaa29 100644
--- a/test/test_objective.py
+++ b/test/test_objective.py
@@ -187,9 +187,3 @@ def test_repr(linear_objective: Objective, quadratic_objective: Objective) -> No
 
     assert "Linear" in linear_objective.__repr__()
     assert "Quadratic" in quadratic_objective.__repr__()
-
-
-def test_objective_constant() -> None:
-    m = Model()
-    linear_expr = LinearExpression(None, m) + 1
-    m.objective = Objective(linear_expr, m)
diff --git a/test/test_optimization.py b/test/test_optimization.py
index 7bb05be8..846b292a 100644
--- a/test/test_optimization.py
+++ b/test/test_optimization.py
@@ -20,7 +20,6 @@
 from linopy import GREATER_EQUAL, LESS_EQUAL, Model, solvers
 from linopy.common import to_path
 from linopy.expressions import LinearExpression
-from linopy.model import ConstantObjectiveError
 from linopy.solver_capabilities import (
     SolverFeature,
     get_available_solvers_with_feature,
@@ -956,20 +955,6 @@ def test_model_resolve(
     assert np.isclose(model.objective.value or 0, 5.25)
 
 
-def test_constant_not_allowed_unless_specified_explicitly(model: Model) -> None:
-    objective = model.objective.expression + 1
-
-    with pytest.raises(ConstantObjectiveError):
-        model.add_objective(expr=objective, overwrite=True, allow_constant=False)
-    with pytest.raises(ConstantObjectiveError):
-        model.add_objective(expr=objective, overwrite=True)
-
-    with pytest.raises(ConstantObjectiveError):
-        model.objective = objective
-
-    model.add_objective(expr=objective, overwrite=True, allow_constant=True)
-
-
 def test_constant_feasible(model: Model) -> None:
     objective = model.objective.expression + 1
     model.add_objective(expr=objective, overwrite=True, allow_constant=True)

From 7f89c068db37ac7b371f4e95c6ed05e5ba1911ed Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 17:26:55 +0100
Subject: [PATCH 10/15] fix mypy issues maybe

---
 doc/release_notes.rst | 2 +-
 linopy/model.py       | 9 ++++++---
 linopy/objective.py   | 4 ++--
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/doc/release_notes.rst b/doc/release_notes.rst
index 1583a4fd..60904063 100644
--- a/doc/release_notes.rst
+++ b/doc/release_notes.rst
@@ -3,7 +3,7 @@ Release Notes
 
 .. Upcoming Version
 
-* Allow constant values in objective cost function
+* Allow constant values in objective cost function. Refactored objective setting.
 * Add support for SOS1 and SOS2 (Special Ordered Sets) constraints via ``Model.add_sos_constraints()`` and ``Model.remove_sos_constraints()``
 * Add simplify method to LinearExpression to combine duplicate terms
 * Add convenience function to create LinearExpression from constant
diff --git a/linopy/model.py b/linopy/model.py
index da5aac10..862de47d 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -266,7 +266,7 @@ def objective(self) -> Objective:
 
     @objective.setter
     def objective(
-        self, obj: Objective | LinearExpression | QuadraticExpression
+        self, obj: Objective | Variable | LinearExpression | QuadraticExpression
     ) -> Objective:
         self.add_objective(expr=obj, overwrite=True, allow_constant=False)
         return self._objective
@@ -779,7 +779,7 @@ def add_constraints(
         return constraint
 
     @overload
-    def add_objective(
+    def add_objective(  # Set objective as Objective object
         self,
         expr: Objective,
         sense: None = None,
@@ -788,7 +788,7 @@ def add_objective(
     ) -> None: ...
 
     @overload
-    def add_objective(
+    def add_objective(  # Set objective as expression-like with sense
         self,
         expr: Variable
         | LinearExpression
@@ -834,6 +834,9 @@ def add_objective(
                 " Set `overwrite` to True to force overwriting."
             )
 
+        if isinstance(expr, list | tuple):
+            expr: LinearExpression = self.linexpr(*expr)
+
         if isinstance(expr, Objective):
             assert sense is None, "Cannot set sense if objective object is passed"
             objective = expr
diff --git a/linopy/objective.py b/linopy/objective.py
index fcd2709c..87fd76ef 100644
--- a/linopy/objective.py
+++ b/linopy/objective.py
@@ -213,14 +213,14 @@ def model(self) -> Model:
         return self._model
 
     @property
-    def sense(self) -> str:
+    def sense(self) -> Literal["min", "max"]:
         """
         Returns the sense of the objective.
         """
         return self._sense
 
     @sense.setter
-    def sense(self, sense: str) -> None:
+    def sense(self, sense: Literal["min", "max"]) -> None:
         """
         Sets the sense of the objective.
         """

From 931b194c448014ce47c5e5a106a31b2826527490 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 17:32:47 +0100
Subject: [PATCH 11/15] fix types

---
 linopy/model.py        | 13 +++++++------
 test/test_objective.py |  2 +-
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index 862de47d..f28bb8d2 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -272,14 +272,14 @@ def objective(
         return self._objective
 
     @property
-    def sense(self) -> str:
+    def sense(self) -> Literal["min", "max"]:
         """
         Sense of the objective function.
         """
         return self.objective.sense
 
     @sense.setter
-    def sense(self, value: str) -> None:
+    def sense(self, value: Literal["min", "max"]) -> None:
         self.objective.sense = value
 
     @property
@@ -834,16 +834,17 @@ def add_objective(
                 " Set `overwrite` to True to force overwriting."
             )
 
-        if isinstance(expr, list | tuple):
-            expr: LinearExpression = self.linexpr(*expr)
-
         if isinstance(expr, Objective):
             assert sense is None, "Cannot set sense if objective object is passed"
             objective = expr
             assert objective.model == self
         else:
             sense = sense or "min"
-            objective = Objective(expression=expr, model=self, sense=sense)
+            if isinstance(expr, list | tuple):
+                expr_2: LinearExpression = self.linexpr(*expr)
+            else:
+                expr_2 = expr
+            objective = Objective(expression=expr_2, model=self, sense=sense)
 
         if not allow_constant and objective.expression.has_constant:
             raise ConstantObjectiveError(
diff --git a/test/test_objective.py b/test/test_objective.py
index cffbaa29..f886b9c4 100644
--- a/test/test_objective.py
+++ b/test/test_objective.py
@@ -69,7 +69,7 @@ def test_set_sense_via_model(
 
 def test_sense_setter_error(linear_objective: Objective) -> None:
     with pytest.raises(ValueError):
-        linear_objective.sense = "not min or max"
+        linear_objective.sense = "not min or max"  # type: ignore
 
 
 def test_variables_inherited_properties(linear_objective: Objective) -> None:

From ea1ebaed707b7f41c9af54177f12486737043694 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 17:38:11 +0100
Subject: [PATCH 12/15] maybe fix types

---
 linopy/model.py | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index f28bb8d2..1be4e598 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -838,13 +838,12 @@ def add_objective(
             assert sense is None, "Cannot set sense if objective object is passed"
             objective = expr
             assert objective.model == self
+        elif isinstance(expr, (Variable, LinearExpression, QuadraticExpression)):
+            sense = sense or "min"
+            objective = Objective(expression=expr, model=self, sense=sense)
         else:
             sense = sense or "min"
-            if isinstance(expr, list | tuple):
-                expr_2: LinearExpression = self.linexpr(*expr)
-            else:
-                expr_2 = expr
-            objective = Objective(expression=expr_2, model=self, sense=sense)
+            objective = Objective(expression=self.linexpr(*expr), model=self, sense=sense)
 
         if not allow_constant and objective.expression.has_constant:
             raise ConstantObjectiveError(

From 1d23943bfbe94f192dfb8c7239c63b2a7c8f3629 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Sun, 28 Dec 2025 16:38:22 +0000
Subject: [PATCH 13/15] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 linopy/model.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/linopy/model.py b/linopy/model.py
index 1be4e598..22860208 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -843,7 +843,9 @@ def add_objective(
             objective = Objective(expression=expr, model=self, sense=sense)
         else:
             sense = sense or "min"
-            objective = Objective(expression=self.linexpr(*expr), model=self, sense=sense)
+            objective = Objective(
+                expression=self.linexpr(*expr), model=self, sense=sense
+            )
 
         if not allow_constant and objective.expression.has_constant:
             raise ConstantObjectiveError(

From 4f07a35388d31475fb04e3eb0566bfb76abb2557 Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Sun, 28 Dec 2025 17:41:32 +0100
Subject: [PATCH 14/15] make precommit happy

---
 linopy/model.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index 1be4e598..9404eb62 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -838,12 +838,14 @@ def add_objective(
             assert sense is None, "Cannot set sense if objective object is passed"
             objective = expr
             assert objective.model == self
-        elif isinstance(expr, (Variable, LinearExpression, QuadraticExpression)):
+        elif isinstance(expr, Variable | LinearExpression | QuadraticExpression):
             sense = sense or "min"
             objective = Objective(expression=expr, model=self, sense=sense)
         else:
             sense = sense or "min"
-            objective = Objective(expression=self.linexpr(*expr), model=self, sense=sense)
+            objective = Objective(
+                expression=self.linexpr(*expr), model=self, sense=sense
+            )
 
         if not allow_constant and objective.expression.has_constant:
             raise ConstantObjectiveError(

From c184189b067ea84e80b0fc2c9b94d9489d6893ee Mon Sep 17 00:00:00 2001
From: Robbie Muir <robbiemuir7@gmail.com>
Date: Mon, 29 Dec 2025 08:35:28 +0100
Subject: [PATCH 15/15] update docstring

---
 linopy/model.py | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/linopy/model.py b/linopy/model.py
index 9404eb62..7ac7d58c 100644
--- a/linopy/model.py
+++ b/linopy/model.py
@@ -102,22 +102,21 @@ def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
         assert isinstance(self, Model), (
             f"First argument must be a Model instance, got {type(self)}"
         )
-        model = self
-        if not self.objective.has_constant:
+        model: Model = self
+        if not model.objective.has_constant:
             # Continue as normal if there is no constant term
             return func(*args, **kwargs)
 
         # The objective contains a constant term
         # Modify the model objective to drop the constant term
-        model = self
-        constant = float(self.objective.expression.const.values)
-        model.objective.expression = self.objective.expression.drop_constant()
+        constant = float(model.objective.expression.const.values)
+        model.objective.expression = model.objective.expression.drop_constant()
         args = (model, *args[1:])  # type: ignore
 
         try:
             result = func(*args, **kwargs)
         except Exception as e:
-            # Even if there is an exception, make sure the model returns to it's original state
+            # Even if there is an exception, make sure the model returns to its original state
             model.objective.expression = model.objective.expression + constant
             raise e