fix: GLPK objective parsing (#818)

* Fix GLPK objective parsing

* Add release note

* Refactor GLPK objective parsing into helper method

* Add GLPK objective parser tests

* Keep GLPK class docstring intact

* Move GLPK objective token below public attributes

Author: danielelerede-oet

doc/release_notes.rst

@@ -29,6 +29,7 @@ Upcoming Version
 
 **Bug fixes**
 
+* Fix GLPK objective parsing. (https://github.com/PyPSA/linopy/pull/818)
 * LP file export now honors bounds tightened below ``[0, 1]`` on a binary variable via the ``.lower``/``.upper`` setters after creation (e.g. ``upper = 0``). Previously such bounds were written only by ``io_api="direct"`` and dropped by ``io_api="lp"``. (https://github.com/PyPSA/linopy/issues/776)
 * Freezing an empty constraint group (e.g. an empty ``isel`` slice) no longer raises ``ValueError: cannot reshape array of size 0``. ``Model(freeze_constraints=True)`` and ``Constraint.freeze()`` now round-trip zero-row constraints losslessly.
 * ``Variable.where`` no longer raises ``ValueError: exact match required for all data variable names`` once a solution is attached (after ``Model.solve``) or the variable is fixed. The fill value now covers auxiliary data variables (``solution``, stashed bounds) instead of only ``labels``/``lower``/``upper``.

linopy/solvers.py

@@ -1382,6 +1382,26 @@ class GLPK(Solver[None]):
         }
     )
 
+    _OBJECTIVE_TOKEN: ClassVar[re.Pattern[str]] = re.compile(
+        r"[-+]?(?:\d+(?:\.\d*)?|\.\d+)(?:[eE][-+]?\d+)?"
+    )
+
+    @classmethod
+    def _parse_objective(cls, text: str) -> float:
+        """
+        Extract the objective value from a GLPK ``Objective:`` line.
+
+        GLPK reports the objective as ``<name> = <value> (MINimum)``. Anchoring
+        on the ``=`` drops the objective name before matching the first
+        float/scientific-notation token, so surrounding text can no longer
+        corrupt the parsed value.
+        """
+        _, _, tail = text.rpartition("=")
+        match = cls._OBJECTIVE_TOKEN.search(tail)
+        if match is None:
+            raise ValueError(f"Could not parse objective value: {text!r}")
+        return float(match.group(0))
+
     @classmethod
     @functools.cache
     def is_available(cls) -> bool:
@@ -1472,7 +1492,7 @@ def read_until_break(f: io.TextIOWrapper) -> Generator[str, None, None]:
         info_io = io.StringIO("".join(read_until_break(f))[:-2])
         info = pd.read_csv(info_io, sep=":", index_col=0, header=None)[1]
         condition = info.Status.lower().strip()
-        objective = float(re.sub(r"[^0-9\.\+\-e]+", "", info.Objective))
+        objective = self._parse_objective(info.Objective)
 
         termination_condition = CONDITION_MAP.get(condition, condition)
         status = Status.from_termination_condition(termination_condition)

test/test_solvers.py

@@ -570,6 +570,41 @@ def test_assign_result_without_solver_kwarg_leaves_solver_unset(self) -> None:
         assert m.solver is None
 
 
+@pytest.mark.parametrize(
+    "objective_text, expected",
+    [
+        (" obj = 1234.56 (MINimum)", 1234.56),
+        (" obj = 0 (MINimum)", 0.0),
+        (" obj = -3.5 (MAXimum)", -3.5),
+        (" obj = +42 (MINimum)", 42.0),
+        (" obj = .5 (MINimum)", 0.5),
+        (" obj = 1.5e+06 (MAXimum)", 1.5e6),
+        (" obj = -2E-3 (MINimum)", -2e-3),
+        (" net_present_value = 1234.5 (MINimum)", 1234.5),
+        (" obj1 = 1234.56 (MINimum)", 1234.56),
+        (" c2e = -7.5e2 (MAXimum)", -750.0),
+        ("  3.5 (MINimum)", 3.5),
+        ("  -1.5e3 (MAXimum)", -1500.0),
+        ("  0 (MINimum)", 0.0),
+    ],
+)
+def test_parse_glpk_objective(objective_text: str, expected: float) -> None:
+    assert solvers.GLPK._parse_objective(objective_text) == pytest.approx(expected)
+
+
+@pytest.mark.parametrize(
+    "objective_text",
+    [
+        " obj = (MINimum)",
+        " unbounded",
+        "",
+    ],
+)
+def test_parse_glpk_objective_no_value_raises(objective_text: str) -> None:
+    with pytest.raises(ValueError, match="Could not parse objective value"):
+        solvers.GLPK._parse_objective(objective_text)
+
+
 mosek_installed = pytest.importorskip("mosek", reason="Mosek is not installed")