Model.assign_coords: coordinate reassignment on an existing model

[!NOTE]
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Describe the feature you'd like to see

A public API to reassign coordinate values on an existing model, e.g.

m.assign_coords(snapshot=new_snapshots)

which replaces the coordinate values of every variable and constraint carrying that dimension (same length, new values), mirroring xarray.Dataset.assign_coords semantics.

Context

In rolling-horizon workflows with the persistent solver interface (#718), the model structure stays identical between iterations while the window data shifts. The diff machinery already anticipates this via ignore_dims={"snapshot"}, and Variable.update() / Constraint.update() (#727) cover the data side. What is missing is moving the coordinate labels themselves, which is required so that solution/dual assignment lands on the right index after the window advances:

import linopy
import pandas as pd

sns0 = pd.date_range("2026-01-01", periods=3, freq="h", name="snapshot")
sns1 = sns0 + pd.Timedelta("1h")

m = linopy.Model()
x = m.add_variables(coords=[sns0], name="x")
m.add_constraints(x >= 0, name="c")

# desired:
# m.assign_coords(snapshot=sns1)

# today this needs private mutation per container:
for _, v in m.variables.items():
    if "snapshot" in v.dims:
        v.data["snapshot"] = sns1
for _, c in m.constraints.items():
    if "snapshot" in c.dims:
        c.data["snapshot"] = sns1

Pitfall the library should own

The obvious workaround v._data = v.data.assign_coords(snapshot=sns1) silently reorders the dataset's variables (the reassigned coord moves to the end), which broke dim inference in Model.assign_result (xr.DataArray(values, var.coords) without explicit dims). Exactly this kind of footgun is why coordinate reassignment should be a library-owned, order-safe operation rather than user code.

Scope

• validate new values: same length per dimension, index-like
• apply across all variables and constraints carrying the dimension (CSR-backed constraints included)
• preserve dataset variable order
• no relabeling, no reindexing, no shape change — values only

Benchmarked in a PyPSA-Eur rolling-horizon prototype (in-place window advance + persistent HiGHS): 1.76x end-to-end speedup on warm iterations with bitwise-identical objectives vs full rebuild.

Start at the proposed Model.assign_coords entry point and trace how Variable.update(), Constraint.update(), and CSR-backed constraints store coordinate data. Check Model.assign_result for the ordering-sensitive behavior described in the issue. Done means validated same-length index-like values are applied to all relevant variables and constraints without relabeling, reindexing, shape changes, or dataset variable reordering.