fix(constraints): freeze empty constraint groups without reshape error (#783)

* fix(constraints): freeze empty constraint groups without reshape error

CSRConstraint.from_mutable reshaped con.vars with an inferred -1
dimension. For an empty constraint group (zero rows and zero terms) the
vars array has size 0, and NumPy refuses to infer a (0, -1) reshape,
raising "cannot reshape array of size 0 into shape (0,newaxis)". This
broke the documented lossless freeze round-trip for legitimately empty
groups (e.g. shifted-time difference constraints at n_time == 1).

Pass the explicit _term count instead of -1; NumPy accepts a (0, 0)
reshape and the rest of the method already handles zero-row input.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* chore: code review

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Ketchp

doc/release_notes.rst

@@ -13,6 +13,7 @@ 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)
+* 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.
 
 Version 0.8.0
 -------------

linopy/constraints.py

@@ -1081,7 +1081,7 @@ def from_mutable(
         # Build active_mask aligned with con_labels (rows in csr)
         # Use same filter as to_matrix: label != -1 AND at least one var != -1
         labels_flat = con.labels.values.ravel()
-        vars_flat = con.vars.values.reshape(len(labels_flat), -1)
+        vars_flat = con.vars.values.reshape(len(labels_flat), con.nterm)
         active_mask = (labels_flat != -1) & (vars_flat != -1).any(axis=1)
         rhs = con.rhs.values.ravel()[active_mask]
         sign_vals = con.sign.values.ravel()

test/test_constraint.py

@@ -104,6 +104,27 @@ def test_empty_constraints_repr() -> None:
     Model().constraints.__repr__()
 
 
+@pytest.mark.parametrize("freeze_constraints", [True, False])
+def test_constraint_handles_empty_rows(freeze_constraints: bool) -> None:
+    """An empty constraint group must be accepted and solve cleanly."""
+
+    m = Model(freeze_constraints=freeze_constraints)
+    x = m.add_variables(
+        lower=0.0,
+        coords=[range(3), range(2)],
+        dims=["time", "product"],
+        name="x",
+    )
+    empty = x.isel(time=range(1, 1))
+    c = m.add_constraints(empty == 0, name="empty")
+    assert isinstance(c, linopy.constraints.ConstraintBase)
+    assert c.size == 0
+    # Solving a model with only an empty constraint group is also fine.
+    m.add_objective(x.sum())
+    m.solve("highs", io_api="direct", output_flag=False)
+    assert m.status == "ok"
+
+
 def test_cannot_create_constraint_without_variable() -> None:
     model = linopy.Model()
     with pytest.raises(ValueError):