fix(io): write LP bounds for tightened binary variables

doc/release_notes.rst

@@ -4,6 +4,10 @@ Release Notes
 Upcoming Version
 ----------------
 
+**Bug fixes**
+
+* 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)
+
 Version 0.8.0
 -------------
 

linopy/io.py

@@ -33,6 +33,7 @@
     from highspy.highs import Highs
 
     from linopy.model import Model
+    from linopy.variables import Variable
 
 
 logger = logging.getLogger(__name__)
@@ -235,6 +236,17 @@ def objective_to_file(
         objective_write_quadratic_terms(f, quads, print_variable)
 
 
+def _binary_has_nondefault_bounds(var: Variable) -> bool:
+    """
+    Whether a binary variable carries bounds other than the implied (0, 1).
+
+    Scans the raw bound values (a single vectorised pass each), so masked
+    slots are tolerated: a false positive only routes the variable through
+    the bounds loop, where masked labels are dropped before writing.
+    """
+    return bool((var.lower.values != 0).any() or (var.upper.values != 1).any())
+
+
 def bounds_to_file(
     m: Model,
     f: BufferedWriter,
@@ -250,8 +262,10 @@ def bounds_to_file(
         + list(m.variables.integers)
         + list(m.variables.semi_continuous)
         + [
-            n for n in m.variables.binaries if m.variables[n].fixed
-        ]  # fixed binaries need bounds
+            n
+            for n in m.variables.binaries
+            if _binary_has_nondefault_bounds(m.variables[n])
+        ]
     )
     if not len(list(names)):
         return

test/test_io.py

@@ -515,6 +515,64 @@ def test_to_file_lp_mixed_sign_constraints(tmp_path: Path) -> None:
     assert "=" in content
 
 
+def test_to_file_lp_binary_default_bounds_omitted(tmp_path: Path) -> None:
+    """A binary with the implied [0, 1] bounds gets no bounds section."""
+    m = Model()
+    b = m.add_variables(binary=True, coords=[pd.RangeIndex(3, name="t")], name="b")
+    m.add_constraints(b.sum() >= 1, name="c")
+    m.add_objective(b.sum())
+
+    fn = tmp_path / "binary_default.lp"
+    m.to_file(fn)
+    assert "bounds" not in fn.read_text()
+
+
+def test_to_file_lp_binary_tightened_bounds(tmp_path: Path) -> None:
+    """
+    Per-element bounds tighter than [0, 1] on a binary reach the LP file.
+
+    Regression test for https://github.com/PyPSA/linopy/issues/776: the LP
+    export used to emit binaries only in the `binary` section (implied
+    [0, 1]), diverging from the direct API which honored the bounds.
+    """
+    m = Model()
+    x = m.add_variables(binary=True, coords=[pd.RangeIndex(4, name="t")], name="x")
+    x.upper = pd.Series([1, 1, 0, 0], index=pd.RangeIndex(4, name="t"))
+    m.add_constraints(x.sum() >= 2, name="atleast2")
+    m.add_objective(-1 * x.sum())
+
+    fn = tmp_path / "binary_tightened.lp"
+    m.to_file(fn)
+    content = fn.read_text()
+
+    bounds_section = content.split("bounds")[1].split("binary")[0]
+    labels = m.variables["x"].labels.values
+    for label in labels[2:]:
+        assert f"x{label} <= +0.0" in bounds_section
+
+
+@pytest.mark.skipif(not available_solvers, reason="No solver installed")
+def test_lp_and_direct_agree_on_binary_bounds(tmp_path: Path) -> None:
+    """The LP and direct paths see the same feasible set for tightened binaries."""
+    solver = available_solvers[0]
+
+    def build() -> Model:
+        m = Model()
+        x = m.add_variables(binary=True, coords=[pd.RangeIndex(4, name="t")], name="x")
+        x.upper = pd.Series([1, 1, 0, 0], index=pd.RangeIndex(4, name="t"))
+        m.add_constraints(x.sum() >= 2, name="atleast2")
+        m.add_objective(-1 * x.sum())
+        return m
+
+    m_direct = build()
+    m_direct.solve(solver_name=solver, io_api="direct")
+
+    m_lp = build()
+    m_lp.solve(solver_name=solver, io_api="lp")
+
+    assert m_direct.objective.value == m_lp.objective.value == -2
+
+
 def test_to_file_lp_frozen_vs_mutable(tmp_path: Path) -> None:
     """Test that frozen and mutable constraints produce identical LP output."""
     m_frozen = Model()