fix: align Variable.fix() value to the variable's coordinates

fix() converted the value with as_dataarray().broadcast_like(self.labels),
which aligns only by dimension name and so worked solely for the default
`dim_0`. On a named dimension, a positional value (list/array) gained a
spurious `dim_0` and broadcast across the real dimension instead of onto it,
silently building a wrong fix constraint (one fixing every entry to every
value).

Use broadcast_to_coords against the variable's own coords — the same coords-
aware alignment add_variables uses for lower/upper: scalars broadcast,
positional inputs land on the right dimension, named pandas/xarray inputs
align by coordinate value, and a mismatch raises an error naming the variable.

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

Author: FBumann

doc/release_notes.rst

@@ -6,6 +6,7 @@ Upcoming Version
 
 * Add documentation about `LinearExpression.where` with `drop=True`. Add `BaseExpression.variable_names` property.
 * Add ``BaseExpression.has_terms`` property: boolean array, true at slots with at least one live term (`#741 <https://github.com/PyPSA/linopy/issues/741>`_).
+* ``Variable.fix(value)`` now places ``value`` correctly on variables with named dimensions; previously array values could be misaligned.
 
 **Features**
 

linopy/variables.py

@@ -31,7 +31,7 @@
 from xarray.core.utils import Frozen
 
 import linopy.expressions as expressions
-from linopy.alignment import as_dataarray, broadcast_to_coords
+from linopy.alignment import broadcast_to_coords
 from linopy.common import (
     LabelPositionIndex,
     LocIndexer,
@@ -1368,7 +1368,9 @@ def fix(
                 )
                 raise ValueError(msg) from None
 
-        value = as_dataarray(value).broadcast_like(self.labels)
+        value = broadcast_to_coords(
+            value, self.coords, label=f"fix() for variable '{self.name}'"
+        )
 
         if self.attrs.get("integer") or self.attrs.get("binary"):
             value = value.round(0)

test/test_fix_relax.py

@@ -482,3 +482,17 @@ def test_unrelax_after_roundtrip(
         m2.variables["z"].unrelax()
         assert m2.variables["z"].attrs["binary"]
         assert "z" not in m2._relaxed_registry
+
+
+def test_fix_aligns_positional_value_to_named_dimension() -> None:
+    # fix() delegates alignment to the (separately tested) broadcast_to_coords;
+    # this only guards that it passes the variable's own coords, so a positional
+    # value lands on the named dimension instead of gaining a spurious dim_0.
+    m = Model()
+    m.add_variables(
+        lower=-5, upper=5, coords=[pd.Index([2020, 2030, 2040], name="time")], name="t"
+    )
+    m.variables["t"].fix([1.0, 2.0, 3.0])
+    con = m.constraints[f"{FIX_CONSTRAINT_PREFIX}t"]
+    assert con.rhs.dims == ("time",)
+    np.testing.assert_array_almost_equal(con.rhs.values, [1.0, 2.0, 3.0])