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@FBumann FBumann commented May 21, 2026

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The strict v1 semantic convention for linopy — predictable coordinate alignment and NaN handling.

This is the master PR for the new semantic convention in linopy. It starts with our Design & transitioning goals, which is carried out in our New Semantics spec. Both files are tracked in this branch. WHat you read is the current state.

Both might change until this PR is merged

The concrete rollout checklist — the three stages (opt-in → default → 1.0) and their per-stage status — is tracked in arithmetics-design/open-items.md.

Scope

The convention ships behind linopy.options["semantics"]v1 opt-in, legacy the default. This PR carries the design, spec, tests and implementation; documentation notebooks follow separately.

Testing

All tests in linopy will be executed for both semantics.
Differing behaviour will be tested using pytest.markers.

This will increase ci time temporarily until v1 is released.

Defered to a follow up PR

Docs: Documentation, Migration guide, Release notes


Note

Issue links below added by AI. These are the milestone issues the v1
convention fixes; they auto-close when this PR merges. #703 (extend xarray
method coverage) stays open — it is a deferred follow-up (§13), not fixed here.

Closes #714
Closes #713
Closes #712
Closes #711
Closes #708
Closes #670
Closes #586
Closes #570
Closes #295
Closes #257
Closes #736
Closes #256

FBumann and others added 2 commits May 21, 2026 14:13
The design goals and transitioning goals for linopy's v1 arithmetic
convention, under arithmetics-design/goals.md. The convention itself and
the bug catalogue (meta issue #714) follow separately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Placeholder for the v1 convention document, to be written. Goals are in
arithmetics-design/goals.md; the bug catalogue is the meta issue #714.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
@FBumann
FBumann force-pushed the feat/arithmetic-convention branch from 40af9d6 to 1e336a4 Compare May 21, 2026 12:48
@FBumann FBumann mentioned this pull request May 21, 2026
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FBumann and others added 3 commits May 21, 2026 20:39
Flesh out convention.md from the placeholder into the full spec —
thirteen numbered sections in three groups: absence (§1–§7), coordinate
alignment (§8–§11), and constraints and reductions (§12–§13). Covers the
strict exact-match alignment model and the propagate-don't-fill
NaN/absence convention.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The convention governs coordinate alignment, absence/NaN handling,
constraints, and reductions — not just arithmetic operators — so
retitle convention.md and goals.md to "The v1 convention".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Introduce linopy.options["semantics"] — legacy (default) or v1 — with
LinopySemanticsWarning, a FutureWarning shown to users by default and
exported at top level. Add the autouse `semantics` conftest fixture
that runs every test under both conventions, plus legacy/v1 markers
to pin a test to one.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
@FBumann FBumann changed the title feat: v1 arithmetic convention feat: v1 semantic convention May 22, 2026
FBumann and others added 6 commits May 23, 2026 12:54
`_align_constant` branches on `options["semantics"]`: v1 uses exact
alignment via `xr.align(join="exact")`; legacy keeps the size-aware
positional/left-join behaviour and emits `LinopySemanticsWarning` when
v1 would diverge. `_add_constant`/`_apply_constant_op` raise on a NaN
in a user-supplied constant under v1, warn under legacy.

`Variable.__mul__(DataArray)` now routes through `to_linexpr() * other`
so the LinearExpression checks fire; the scalar fast-path is preserved
(a NaN scalar diverts to the expression path so v1 raises).

Marks the bug-class test groups `TestCoordinateAlignment` (#708/#586/
#550), `TestConstraintCoordinateAlignment`, `TestNaNMasking`,
`test_auto_mask_constraint_model`, and four piecewise NaN-padding tests
as `@pytest.mark.legacy` — they assert the very behaviour v1 forbids.
v1 coverage of those bug classes accretes via later slices.

`test/test_legacy_violations.py` (new) adds 22 paired tests covering
§5/§8/§9 plus the PyPSA #1683 `0*inf=NaN` case.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
`merge` now pre-validates that all operands agree on the labels of
every shared *user* dimension before concatenating. Helper dims
(`_term`, `_factor`) and the concat dim itself are excluded — those
legitimately vary between operands. v1 raises on mismatch; legacy
keeps current size-based override/outer behaviour and emits
`LinopySemanticsWarning` when v1 would diverge.

The check uses a new `_merge_shared_user_coords_differ` helper. The
existing override/outer decision is unchanged for the actual
`xr.concat` call — the new check only gates whether legacy/v1 accept
the merge, never how the concat itself runs.

Adds 8 paired tests for var+var, var-var, expr+expr, broadcast guard,
and warning emission on the merge path.

Reclassifies as `@pytest.mark.legacy`: `test_non_aligned_variables`
(deliberately disjoint coords), `test_linear_expression_sum` /
`test_linear_expression_sum_with_const` (assert `v.loc[:9]+v.loc[10:]`
merges), `TestJoinParameter` cases that build `a*b` from mismatched-
coord vars, and two SOS2 reformulation tests. File-level legacy mark
on `test_piecewise_constraints.py` + `test_piecewise_feasibility.py`
until `linopy/piecewise.py` itself is made v1-aware (tracked as
Slice P).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Variable.to_linexpr() now produces a LinearExpression whose absent
slots (labels == -1) carry NaN coeffs and NaN const under v1, so
downstream arithmetic has something to propagate. The expression
constant operators (_add_constant, _apply_constant_op) no longer
fillna(0) self.const / self.coeffs under v1 — NaN flows through.
`merge` sums const along _term with skipna=False under v1, so a slot
that's absent in any operand stays absent in the result. Legacy paths
keep the silent-fill behaviour verbatim.

LinearExpression.isnull() now returns `const.isnull()` under v1: a
slot is absent iff its const is NaN. ``vars == -1`` is a dead-term
signal (the slot can still be a present constant after fillna),
not a slot-level absence marker. Legacy keeps the historical
``(vars == -1).all() & const.isnull()`` formula for byte-for-byte
compatibility.

Variable.fillna(numeric) now returns a LinearExpression (a constant
isn't a variable). Variable.fillna(Variable) stays Variable, as
before.

Adds 11 tests for §6 propagation (mul/add/sub/div preserve absence,
absent-vs-zero distinguishable, present + absent propagates) and §7
resolution (fillna numeric on expr / Variable, present-zero revival).

Reclassifies test_masked_variable_model as @pytest.mark.legacy — its
assertion "x bound to 10 at masked-y slots" only holds because legacy
collapses absent y to 0. The v1 way is x + y.fillna(0) >= 10; a
counterpart test in test_legacy_violations.py pins this.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The convention spec names ``reindex`` and ``reindex_like`` among the
absence-creating mechanisms (alongside ``mask=``, ``.where()``,
``.shift()``, and ``.unstack()``), but master only had them on
``LinearExpression``. Add them on ``Variable``, with the sentinel
fill values (``labels=-1``, ``lower=upper=NaN``) so new positions
slot cleanly into §6 propagation.

The methods work the same way under both semantics — under legacy
the sentinels exist but downstream arithmetic still collapses them
back to 0 (the #712 bug), so the user-visible effect of reindex-as-
absence only really lands under v1.

Adds 5 tests: extend with absent, subset drops, reindex_like with
another Variable, and the §4 + §6 hand-off (a reindex-introduced
absent flows through ``* 3`` and is visible via ``isnull()``).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Slice C propagated NaN const cleanly but left the storage half-absent
after a merge: `(1*x) + xs` at the absent slot kept the `1*x` term's
valid coefficient and label even though `const` was NaN there. The
§1/§2 promise "absence is one concept, whatever the dtype" only holds
if `const.isnull()` at a slot ⇒ every term at that slot has
`coeffs = NaN`, `vars = -1`.

Add `_absorb_absence(ds)` and call it at the end of `merge` under v1.
The constant-operand paths (`_add_constant`, `_apply_constant_op`)
don't need explicit absorption — their NaN-propagation naturally
preserves the invariant when the input is already v1-compliant
(NaN * anything = NaN; dead terms stay dead). Only `merge` opens the
gap by concatenating one operand's live term with another operand's
absent slot along `_term`.

`convention.md` §2 now states the invariant explicitly and introduces
the *dead term* terminology, so `fillna(value)` reviving a slot while
leaving the sentinel term in place reads as a feature, not a glitch.

Adds `test_outer_fillna_then_add_collapses_to_just_added` pinning
`(x + y.shift()).fillna(0) + x` — at the previously-absent slot the
result has exactly one live term (`1·x[0]`) with `const = 0`,
algebraically equal to `x[0]`. At present slots all three terms stay
live (`2·x[i] + y[i-1]`), so fillna placement is load-bearing — moving
it inside (`x + y.shift().fillna(0) + x`) would double-count `x` at
the absent slot.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
`.add/.sub/.mul/.div/.le/.ge/.eq` already accepted a `join=`
argument; this slice's job is just §12's RHS handling under v1.

`to_constraint` branches on `options["semantics"]`. Under v1 it
skips the legacy `reindex_like(self.const, fill_value=NaN)` step
that silently padded a subset RHS, so a coord mismatch with the
LHS now flows through `self.sub(rhs)` and gets caught by §8's
exact alignment. A NaN in a user-supplied constant RHS raises at
construction (§5) — including the PyPSA #1683 case of
`min_pu * nominal_fix` with `p_nom=inf` and `p_min_pu=0`. An
absent slot in the LHS (propagated from §6) still produces a NaN
RHS at that row; downstream auto-mask drops the constraint there,
which is exactly §12's "absent slot yields no row."

Legacy keeps the old auto-mask path verbatim and adds a
`LinopySemanticsWarning` whenever a NaN RHS is observed, so users
get the rollout signal without behaviour change.

Adds 11 paired tests: TestNamedMethodJoin (inner/outer/left across
.add/.mul/.le, plus a "bare op still raises" guard) and
TestConstraintRHS (subset RHS raises, NaN RHS raises, PyPSA #1683
on the constraint side, §6→§12 hand-off where the absent LHS slot
yields NaN RHS, plus the paired legacy auto-mask documentation and
warning-emission tests).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
@FBumann

FBumann commented May 23, 2026

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Coming next, in order:

  • Slice F — §11 auxiliary-coordinate conflicts. Raise on non-dimension-coord conflict during alignment (covers Auxiliary non-dimension coordinates leak into expressions and break alignment #295). Small.
  • Slice G — §13 reductions audit. Most reductions already use skipna=True; need a focused audit + tests for sum / mean / groupby / resample / coarsen and the objective. Likely mostly tests.
  • Slice P — linopy/piecewise.py and linopy/sos_reformulation.py. Internal callers build expressions by .isel(...)-slicing along a piece dim and comparing the two slices (delta_hi <= delta_lo); the slices share the dim with different coords, which v1 §8 rejects. Until this lands, test_piecewise_constraints.py + test_piecewise_feasibility.py carry a module-level pytestmark = pytest.mark.legacy and two SOS2 tests are method-marked legacy. Fixing piecewise removes those marks.

Then a final pass on docs (user-facing migration / rollout — deferred so far).

FBumann and others added 4 commits May 23, 2026 20:44
Three internal patterns were violating §8 / §11:

1. ``_add_incremental`` in ``linopy/piecewise.py`` builds
   ``delta_hi <= delta_lo`` from two ``.isel(piece_dim=slice)`` slices
   of the same variable. ``drop=True`` is a no-op for slice indexers
   so ``piece_dim`` stays on both with *different* labels (first n-1
   vs last n-1 of piece_index) — v1 §8 rejects. Relabel the high
   slice onto the low slice's labels so the comparison aligns by
   label (the explicit-positional path of §10). Same fix for
   ``binary_hi <= delta_lo``.

2. ``_incremental_weighted`` computes ``bp0 = bp.isel({dim: 0})``
   without ``drop=True``, leaving the breakpoint dim as a scalar
   coord on the resulting expression. When that expression appears
   as the RHS of ``links.eq_expr == ...`` it conflicts with the LHS,
   which has no such coord — §11 aux-coord conflict. Add ``drop=True``.

3. ``reformulate_sos2`` builds its first/last constraints from
   scalar isels at different positions on ``sos_dim`` (``x``/``M`` at
   ``n-1`` paired with ``z`` at ``n-2``, etc.). All without
   ``drop=True``, so the scalar ``sos_dim`` coord differs across
   operands — §11 aux-coord conflict. Add ``drop=True`` to all three
   sites.

Removes the module-level ``pytestmark = pytest.mark.legacy`` from
``test_piecewise_constraints.py`` and ``test_piecewise_feasibility.py``
and the method-level marks from the two SOS2 multidim tests. Suite is
+598 tests under v1 vs Slice E (legacy → v1 broadened coverage),
0 failures under either semantics.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
§13 falls out of xarray's ``skipna=True`` default; no code changes
needed. Adds 4 tests so future drift is caught: sum over a dim,
sum without a dim, sum of all-absent (the zero expression), and
groupby.sum across heterogeneously-present groups.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Adds `_conflicting_aux_coord(datasets)` and wires it into both
`merge` and `_align_constant`. When two operands carry an aux coord
of the same name with disagreeing values, v1 raises with a pointer
to the explicit resolutions (``.drop_vars(...)`` or
``.assign_coords(...)``). xarray silently drops the conflict — the
#295 bug — and legacy keeps that behaviour but now emits a
`LinopySemanticsWarning`. The helper guards against string-dtype
coord values (no `equal_nan=True` there) so the multiindex case
keeps working.

`_merge_shared_user_coords_differ` refactored to compare bare
``d.indexes[k]`` instead of ``d.coords[k]``: aux coords no longer
leak into the §8 check, so §11 owns aux-coord conflicts cleanly
and §8 owns dim-coord mismatches with a separate message.

Convention §11 expanded from one paragraph: aux coords are
validated and propagated but never computed with — they describe
the data, they don't enter the math. Goal #4 in `goals.md` picks
this up: user-attached auxiliary coordinates are the user's,
linopy never silently rewrites them.

`test_linear_expression.py::test_merge` adds ``drop=True`` to its
``.sel`` setup — the test was leaving a leftover scalar coord that
v1 now correctly catches as a §11 conflict; the fix preserves the
test's intent of exercising merge with differing term counts.

Conflict-raising tests (TestAuxCoordConflict) cover expr+const,
var+var, scalar-isel-without-drop, the ``drop=True`` escape hatch,
plus the paired legacy left-wins documentation and warning-emission
tests. Propagation guarantees land in a follow-up.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Regression coverage on the half of §11 that wasn't tested before:
non-conflicting aux coords carry through every binary operator and
into constraints. xarray already preserves them; the tests guard
against future drift (e.g. a reduction or helper accidentally
dropping a non-dim coord).

TestAuxCoordPropagation covers ``3*v``, ``v+5`` (single-operand,
fast paths), ``v+v`` with matching aux (the merge path), ``v<=10``
(the constraint path), ``x*a`` / ``x+a`` / ``x/a`` / ``x<=a`` where
only the constant DataArray carries the coord (the
``_align_constant`` path), and the var+var case where only one side
has the coord. Together: every operator times every "one side / both
sides" arrangement, since only conflicts on both sides raise.

Runs under both semantics — the legacy behaviour matches the v1
behaviour for the non-conflict cases.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
FBumann and others added 7 commits May 23, 2026 22:17
… solve

Fills the convention-coverage gaps surfaced by review of the branch:

- §1/§2 dead-term storage invariant: pin that after a merge with an
  absent slot, coeffs=NaN AND vars=-1, not just const=NaN. The existing
  propagation tests read through isnull() which only checks const, so a
  regression in _absorb_absence would have passed them. Multi-operand
  variant catches binary-only-absorption regressions.
- §12 equality: mirror the existing <=/>= TestConstraintRHS coverage for
  ==. Subset RHS raises, NaN RHS raises, absence in LHS drops the row.
- §11 extra operators: add mul-constant and == constraint cases to the
  existing TestAuxCoordConflict. The class already covered +-constant
  and var+var; these extend coverage to the other call-site shapes.
- §13 scope note: mean/resample/coarsen aren't yet on LinearExpression
  (tracked in #703); the spec text is the rule those will follow when
  implemented. Docstring note in TestReductionsSkipAbsent makes this
  explicit so the gap doesn't read as missing coverage.
- End-to-end v1 solve: test_masked_variable_model_v1_drops_constraint
  pins the v1 outcome at the solver layer — con0 masked at absent
  slots (solver-independent) and x bound to 0 where the constraint
  still binds. _v1_fillna_binds confirms the §7 escape hatch recovers
  the legacy outcome. Catches the regression where v1 silently
  produces wrong solutions instead of raising.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Pulls the seven v1-specific helpers and the user-NaN message out of
``expressions.py`` and into a dedicated ``linopy/semantics.py`` module
— a single home for "what v1 means" that imports cleanly from
``config`` and ``constants`` only. Adds a tiny ``is_v1()`` predicate
so the 16 scattered ``options["semantics"] == V1_SEMANTICS`` checks
collapse to a one-line call.

Helpers (renamed to drop the leading underscore now that they're a
real module API): ``check_user_nan_scalar``, ``check_user_nan_array``,
``dim_coords_differ`` (was ``_shared_coords_differ`` — clearer name,
matches ``merge_shared_user_coords_differ``), ``merge_shared_user_coords_differ``,
``conflicting_aux_coord``, ``absorb_absence``, plus ``is_v1``.

No behaviour change — same checks, same warnings, same raises. The
diff is mechanical: imports flipped, two local ``is_v1 = options[...]``
bindings replaced by the imported predicate, one missed
``_USER_NAN_MESSAGE`` reference in ``to_constraint`` routed through
``check_user_nan_array`` for consistency. ``expressions.py`` shrinks
by ~105 lines.

Future v1-only API surface (e.g. exposing ``is_v1()`` as
``linopy.is_v1()`` for downstream code) and the eventual legacy
removal at 1.0 both reduce to deletions of ``semantics.py`` and its
import sites.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Three test clusters in ``test_legacy_violations.py`` had near-identical
``test_add_X``, ``test_mul_X``, ``test_div_X`` triples that varied only
by which binary operator they exercised. Collapse each into a single
``@pytest.mark.parametrize("op", ...)`` test:

- TestExactAlignmentConstant: same-size-different-labels and
  subset-constant raises, parameterized over add/sub/mul/div.
- TestUserNaNRaises: NaN-DataArray raises over add/sub/mul/div, NaN
  scalar over add/sub/mul (div scalar shares the same ``_apply_constant_op``
  code path as mul, but ``x / nan`` trips ``__div__``'s unary-negate
  TypeError before our check fires; the dispatch needs a separate
  fix that's not worth pulling into this refactor).
- TestAbsencePropagation: ``shifted OP scalar`` preserves absence,
  parameterized over add/sub/mul/div. Adds a per-op present-slot
  value check so the parameterization broadens rather than narrows
  the assertion.

Adds a module-level ``_OPS`` dict mapping name → ``operator``
callable so the parameter is the readable name (``"add"``,
``"div"``) while the test still calls the actual operator.

Cuts ~50 lines off ``test_legacy_violations.py`` and makes adding a
new operator a one-line change. Test IDs become e.g.
``test_same_size_different_labels_raises[v1-add]`` — slightly less
self-describing than the explicit-method names but cheap to read.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Both methods had v1 and legacy logic interleaved via a ``fillna0``
closure that was identity under v1 and ``da.fillna(0)`` under legacy.
Pull them apart into:

- ``_add_constant`` / ``_apply_constant_op`` — two-line dispatchers.
- ``*_v1`` — v1's implementation, reads as a single coherent story.
- ``*_legacy`` — legacy's implementation, ``# LEGACY: remove at 1.0``
  marker on each.

At 1.0 the removal is mechanical: delete the ``_legacy`` methods and
inline the ``_v1`` body into the dispatcher (or rename it back to the
public name). Future readers don't have to mentally subtract the
legacy branches to understand what v1 does.

Add ``LEGACY: remove at 1.0`` marker comments at the other mixed
sites in ``expressions.py`` so ``grep`` finds every place that needs
touching: ``_align_constant``'s size-aware default fallback,
``to_constraint``'s auto-mask fallthrough, ``LinearExpression.isnull``'s
historical AND, and the two warn-on-divergence sites in ``merge``.

New ``arithmetics-design/legacy-removal.md`` is the master checklist
for the 1.0 cut: every file, function, test, doc edit, and the safe
order to do them in. The intent is that the eventual legacy removal
takes an afternoon, not a week of grep-archaeology.

No behaviour change — same checks, same warns, same raises. Suite is
7282 passed, 0 failures under both semantics.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Two distinct CI failures both rooted in the v1 harness commit:

1. **Test collection crash on every linopy/*.py module.** ``test/conftest.py``
   imported ``linopy.config`` at module top, which loaded linopy from
   site-packages before pytest's ``--doctest-modules`` collection walked
   the source tree. The resulting __file__ mismatch broke all 22 module
   collections. ``pyproject.toml`` already documents this exact failure
   mode in the ``filterwarnings`` block. Fix: keep the constant *values*
   (``"legacy"`` / ``"v1"``) inline in conftest as ``_LEGACY_SEMANTICS``
   etc. so the parametrize decorator doesn't force an import, and defer
   the ``LinopySemanticsWarning`` / ``options`` import into the fixture
   body. The original import comment in pyproject is now mirrored at
   the top of conftest.

2. **mypy: 72 "no-untyped-def" errors in test_legacy_violations.py.**
   The new tests were missing parameter type annotations on the
   fixture-injected params (``x``, ``xs``, ``op``, ``unsilenced``,
   ``subset``, ``A``, ``da_aux_B``, ...). ``disallow_untyped_defs`` is
   set globally, so test files need them too. Filled in the types
   (``Variable``, ``str``, ``None``, ``xr.DataArray``, ``pd.Index``),
   added an ``isinstance(result, LinearExpression)`` narrowing in
   ``test_variable_fillna_zero_revives_slot_as_present_zero`` so mypy
   can pick the right branch of ``fillna``'s return union.

Local: 7282 passed, 0 failures under both semantics; ``mypy .``
Success.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Three v1 raises were under-informative — naming the rule violated but
not the operand, dim, or values involved. Make each message carry the
information the helper already has:

- **§5 user-NaN**: the old message conflated the two intents the user
  might have had — *data error* (fix with ``.fillna(value)``) vs
  *intended absence* (mark on the variable with ``mask=`` / ``.where``
  / ``.reindex`` / ``.shift``). The new message separates them and
  points each to its own remedy.
- **§8 merge mismatch**: rename ``merge_shared_user_coords_differ``
  (bool) to ``merge_shared_user_coord_mismatch`` (tuple ``(dim, left,
  right) | None``). Raise text now includes the offending dim name and
  both sides' labels (truncated), plus the full set of resolution
  paths from §10: ``.sel`` / ``.reindex`` / ``.assign_coords`` /
  ``linopy.align`` / ``join=`` on ``.add`` / ``.sub`` / ``.mul`` /
  ``.div`` / ``.le`` / ``.ge`` / ``.eq``.
- **§11 aux-coord conflict**: ``conflicting_aux_coord`` returns
  ``(name, left_vals, right_vals) | None``. Raise text includes the
  coord name, both value snippets, and all three resolution paths
  (``.drop_vars`` / ``.assign_coords`` / ``isel(drop=True)`` —
  ``.assign_coords`` was previously omitted). The text is now
  centralized in ``semantics.py`` so the two raise sites in
  ``expressions.py`` (``_align_constant`` and ``merge``) share one
  voice instead of paraphrasing each other.

New ``TestErrorMessageContent`` pins the rich content in three tests
— that the §5 message names both intents, that the §8 message names
the dim and both label lists, and that the §11 message names the
coord, both value lists, and lists all three §11 fixes (the
``.assign_coords`` omission would have slipped through ``match=
"Auxiliary coordinate"`` substrings).

Section references (``§5``, ``§8``, ``§11``) deliberately omitted
from user-visible text — spec jargon, not a navigation aid for
downstream callers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Closes the small-but-real holes in the §1–§13 coverage map. New tests
mostly, plus one code fix that the test surfaced.

§4 — absence creation
  - test_where_creates_absence: §4 names ``.where(cond)`` but only
    ``mask=`` / ``.reindex`` were tested.
  - test_unstack_creates_absence_at_missing_combinations: the
    non-rectangular MultiIndex case (``stack`` preserves, ``unstack``
    fills) is the asymmetry that earns its own test. Hit a real bug
    on the way — ``Variable.unstack`` was producing float NaN in the
    integer ``labels`` field instead of the ``FILL_VALUE`` sentinel
    (-1), violating §2. Fixed by passing ``fill_value=_fill_value``
    to the underlying ``Dataset.unstack`` (same pattern as ``shift``).
    Audited the rest of the varwrap calls — only ``shift`` and
    ``unstack`` introduce new positions; the others either preserve
    shape (``assign_*``, ``rename``, ``swap_dims``, ``set_index``,
    ``roll``, ``stack``), select existing positions (``sel`` /
    ``isel`` / ``drop_*``), or broadcast existing data without fill
    (``broadcast_like``, ``expand_dims``).
  - test_data_preserving_methods_do_not_create_absence: parameterized
    over ``.roll`` / ``.sel`` / ``.isel``, regression-guards §4's
    explicit contrast against the creators.

§10 — named-method join= argument
  - test_add_join_override_aligns_positionally: positional-mode is the
    surprising one in the join= set; pin it explicitly.
  - test_reindex_like_resolves_mismatch_before_bare_op and
    test_assign_coords_resolves_mismatch_before_bare_op: §10 names
    these as the canonical user fixes; pin that the post-fix bare
    operator actually accepts the once-mismatched operand.

§11 — auxiliary-coordinate conflicts
  - test_assign_coords_resolves_conflict: §11 lists three escape
    hatches; only ``.drop_vars`` / ``isel(drop=True)`` were tested.
  - test_multi_operand_merge_aux_conflict_raises: the merge-path
    check inspects all operands; a 3-way ``v + w + u`` with the
    third disagreeing exercises that.

§12 — constraints follow the same rules
  - Parameterize the existing subset / NaN / absence-propagation
    tests in ``TestConstraintRHS`` over the three signs (``le`` /
    ``ge`` / ``eq``) via a new module-level ``_SIGNS`` dispatch.
    Folds the previous ``<=`` and ``==`` duplicates together and
    fills in ``>=`` for each rule (which was the explicit gap).
    The PyPSA #1683 test stays separate — it's tied to ``>=`` by
    the real-world case it documents.

Suite: 7303 passed, 515 skipped, 0 failures under both semantics.
``mypy .`` clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
* fix(v1): don't warn on exactly-aligned @/dot under legacy (#849)

The legacy alignment path in LinearExpression._align_constant warned
whenever the constant operand's size dict differed from the expression's.
But a size difference can come purely from disjoint dims (e.g. `x @ C`,
where C carries its own contracted-out dim) — that is ordinary
broadcasting, not a shared-dim misalignment. v1 accepts it, so the legacy
LinopySemanticsWarning was a false positive on already-aligned operands.

Warn only when first_mismatched_dim reports a real shared-dim
disagreement, mirroring the sizes-equal branch, and collapse the
duplicated warn/branch into one.

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

* refactor: simplify legacy _align_constant branch (single return)

Name the aligned result and return once instead of repeating the
(self.const, ..., False) triple, and move the "positional when sizes
match" comment onto the branch it describes. Behaviour unchanged.

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

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…entions (#847, #848) (#851)

* fix(v1): make Variable.fillna(scalar) resolve absence under both conventions (#847, #848)

Variable.fillna(<scalar>) is the documented resolution for absent slots
(from shift/where/reindex/mask), but under legacy it misbehaved two ways:

- #847: it warned. fillna internally calls to_linexpr(), whose legacy path
  emits the masked-variable LinopySemanticsWarning — even though fillna is
  itself the resolution that warning points to. So the documented fix
  couldn't be written warning-free on both conventions.
- #848: it silently dropped the fill value. Legacy to_linexpr marks absent
  const as 0 (not NaN), so the subsequent LinearExpression.fillna had
  nothing to fill; fillna(5) left 0 at absent slots while v1 put 5.

The v1 path already behaved correctly, so this is a pure legacy workaround
(marked LEGACY: remove at 1.0): keep the clean one-liner under v1, and under
legacy place the value at the -1 labels directly and skip the absence warning.
Result: `var.shift(1).fillna(v)` is now a single form, identical under both
conventions (same vars/const; only the immaterial phantom coeff differs), so
downstream migrating to v1 no longer has to version-gate the expression.

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

* docs(v1): note legacy no-op of LinearExpression.fillna on absent slots

Document why var.to_linexpr().fillna(v) is a no-op under legacy (absence is
already materialised as const=0, so there is no NaN to fill) and point at
Variable.fillna as the cross-convention resolution. No behavior change.

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

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Polish for the #717 "polish the tests" checklist, scoped to the v1/legacy
groupby-MultiIndex divergence tests in test_linear_expression.py:

- Replace the manual try/finally toggle of options["semantics"] in
  test_group_multiindex_reset_index_matches_v1 with the `with options as o`
  context manager, so semantics state is restored even if the assertion
  fails (it was the only test bypassing the conftest marker mechanism).
- Swap internal `.data.indexes` / `.data.coords` reaches for the public
  `.indexes` / `.coords` accessors, matching the already-public sibling
  tests in the same class.

No behaviour change; 584 passed, ruff clean.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
FBumann added a commit that referenced this pull request Jul 20, 2026
…sages

Follow-up polish for the #717 "polish the tests" checklist:

- Move the four masked-addend §6 tests (absence propagation of a fully
  masked term in a sum) out of test_linear_expression.py into
  TestAbsencePropagation in test_legacy_violations.py, where the rest of
  the §6 coverage lives. They now share one `ab_all_masked` fixture instead
  of rebuilding an inline two-variable model four times.
- Add `match="only supported for"` to the four bare `pytest.raises(ValueError)`
  guards on the `use_fallback=True` DataFrame-grouper path, so they assert
  *why* the fallback raises rather than accepting any ValueError. The fifth
  guard (issue #351) keeps its precise `(KeyError, IndexError)` — its message
  is an incidental numpy index error, brittle to pin.

No behaviour change; test_legacy_violations.py 294 passed,
test_linear_expression.py green, ruff clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…sages (#853)

Follow-up polish for the #717 "polish the tests" checklist:

- Move the four masked-addend §6 tests (absence propagation of a fully
  masked term in a sum) out of test_linear_expression.py into
  TestAbsencePropagation in test_legacy_violations.py, where the rest of
  the §6 coverage lives. They now share one `ab_all_masked` fixture instead
  of rebuilding an inline two-variable model four times.
- Add `match="only supported for"` to the four bare `pytest.raises(ValueError)`
  guards on the `use_fallback=True` DataFrame-grouper path, so they assert
  *why* the fallback raises rather than accepting any ValueError. The fifth
  guard (issue #351) keeps its precise `(KeyError, IndexError)` — its message
  is an incidental numpy index error, brittle to pin.

No behaviour change; test_legacy_violations.py 294 passed,
test_linear_expression.py green, ruff clean.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
FBumann added a commit that referenced this pull request Jul 20, 2026
…rop dead code

Three follow-ups from the #717 review:

- §5 user-NaN check on the scalar fast path used `isinstance(other, float)`,
  which misses `np.float32`/`np.float16` NaN scalars (they don't subclass
  Python `float`). Such a scalar was silently added/multiplied into the
  expression instead of raising (v1) / warning (legacy). Add
  `semantics.is_nan_scalar` (float | np.floating) and use it at all four
  scalar sites.

- `conform_merge_dims` called `Index.get_indexer` on a shared dim with
  non-unique labels, raising an opaque `InvalidIndexError` — a regression
  vs. master, which aligned duplicate labels positionally. Guard on
  `idx.is_unique` so a non-unique differing index is reported as a §8
  mismatch: legacy aligns positionally + warns, v1 raises the canonical
  "Coordinate mismatch" ValueError.

- Remove the unused `LEGACY_SEMANTICS_MESSAGE` constant and the stale
  `EvolvingAPIWarning` filter cells in the piecewise notebooks (obsolete
  under `semantics="v1"`; the warning was renamed to LinopySemanticsWarning).

Adds regression tests for the NaN-scalar dtypes and the duplicate-label
merge on both conventions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…rop dead code (#854)

Three follow-ups from the #717 review:

- §5 user-NaN check on the scalar fast path used `isinstance(other, float)`,
  which misses `np.float32`/`np.float16` NaN scalars (they don't subclass
  Python `float`). Such a scalar was silently added/multiplied into the
  expression instead of raising (v1) / warning (legacy). Add
  `semantics.is_nan_scalar` (float | np.floating) and use it at all four
  scalar sites.

- `conform_merge_dims` called `Index.get_indexer` on a shared dim with
  non-unique labels, raising an opaque `InvalidIndexError` — a regression
  vs. master, which aligned duplicate labels positionally. Guard on
  `idx.is_unique` so a non-unique differing index is reported as a §8
  mismatch: legacy aligns positionally + warns, v1 raises the canonical
  "Coordinate mismatch" ValueError.

- Remove the unused `LEGACY_SEMANTICS_MESSAGE` constant and the stale
  `EvolvingAPIWarning` filter cells in the piecewise notebooks (obsolete
  under `semantics="v1"`; the warning was renamed to LinopySemanticsWarning).

Adds regression tests for the NaN-scalar dtypes and the duplicate-label
merge on both conventions.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…to MI row (#855)

The migration guide's fix table covered the core absence + alignment +
aux-coord + MultiIndex-dim + groupby-result cases, but three issue-backed
v1 changes were under-covered. Add rows and mirror two into the
release-notes v1 summary:

- Unlabeled operand (numpy / list / polars) pairs to dims by size;
  ambiguous or no-size-match raises (#736) — the object-scope premise.
- A reordered/mismatched groupby grouper raises rather than reindexing
  positionally (#827).
- MultiIndex-level projection: v1 rejects the MultiIndex *dimension* at
  construction, so the per-level-input case is folded into the existing
  MultiIndex row — decompose with reset_index, then project by the level
  aux coord. (The convention.md §11 example still shows the legacy-shaped
  `.get_level_values` snippet, which cannot run under v1 — flagged for the
  spec pass.)

Release notes already carry the level-projection deprecation separately,
so only #736 and the grouper point are mirrored there.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…raduation (#856)

The §11 example used `expr.indexes["snapshot"].get_level_values("period")`,
which cannot run under v1: v1 rejects a first-class `pd.MultiIndex` dimension
at construction, so `snapshot` is a flat dim with `period`/`timestep` as aux
coords and `.indexes["snapshot"]` is a flat index, not a MultiIndex. Replace it
with the working aux-coord mapping (verified on the branch) and add a one-line
note on the flat representation.

Also record in open-items.md that convention.md graduates into the rendered
Sphinx docs at the 1.0 reframe (add myst-parser, toctree, internal cross-refs);
until then it stays a design-record linked by URL — the rules are permanent but
the transitional framing would only be re-done, so rendering now is wasted.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…e) (#858)

Positional indexing leaves a scalar coordinate behind: `x.isel(time=0)` drops
`time` as a dimension but keeps it as a scalar coord (the first label). So a
cyclic/boundary constraint like `x.isel(time=0) == x.isel(time=-1)` — extremely
common in storage/energy models — hits §11's aux-coord conflict under v1
(`time` = first vs last) and raises, where legacy silently dropped it.

The aux-coord row already covered this in principle, but listed only
`.drop_vars` / `.assign_coords` as fixes and didn't name the scalar-leftover
cause. Broaden the migration-guide row and add the natural fix — drop the coord
at the indexing site with `.isel(..., drop=True)` / `.sel(..., drop=True)` — and
add the same note to convention.md §11.

Verified on the branch: `x.isel(time=0) == x.isel(time=-1)` raises
"Auxiliary coordinate 'time' has conflicting values ... left=0, right=3" under
v1; `drop=True` on both sides builds cleanly.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
* test(v1): polish nan-scalar + duplicate-label merge tests (dedup, trim comments)

* test(v1): polish the #717 convention test suite

Applies the maintainer's polish guidelines across the v1/legacy semantics
tests (test_legacy_violations.py, test_convention.py):

- Assert FULL error/warning message text (hardcoded literals) instead of
  partial `match="..."` substrings, so every message change surfaces as a
  test diff. Covers all v1 ValueError raises and legacy LinopySemanticsWarning
  sites across the 17 convention classes.
- DRY: dedupe repeated operand/coord setup into fixtures; merge near-identical
  tests via @pytest.mark.parametrize (per-operator NaN/mismatch cases,
  quadratic build paths, join modes, object-scope operand kinds, ...).
- Trim essay-length docstrings/inline comments to one crisp line where the
  rationale isn't load-bearing.

Coverage is preserved or expanded (500 vs 480 collected invocations); no
tests removed. Suite green under both semantics; ruff + mypy clean.

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

---------

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

FBumann commented Jul 21, 2026

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@FabianHofmann I fixed all issues you found.

We need to decide where to place the documentation of the convention.
I would suggest moving it to the docs.
Keep convention, goals, legacy removal.
remove the rest.

@FabianHofmann

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@FabianHofmann I fixed all issues you found.

We need to decide where to place the documentation of the convention. I would suggest moving it to the docs. Keep convention, goals, legacy removal. remove the rest.

you are a hero. this sounds good, let's do it.

I added two tracker prs

I will gather feedback from those here. atm it looks like there is not big issues

FBumann added a commit to flixOpt/flixopt that referenced this pull request Jul 21, 2026
linopy's upcoming "v1" arithmetic convention (PyPSA/linopy#717) replaces
legacy's silent coordinate realignment and NaN-fill with strict, explicit
rules that raise on ambiguity. flixopt's constraint construction relied on
the legacy behaviour in three places; this makes every site explicit so the
model builds and solves identically under both
`linopy.options["semantics"] = "legacy"` and `= "v1"`.

Categories fixed:

- Coordinate mismatch on a shared dim (adjacent-step constraints). Sites that
  combine `x.isel(dim=slice(1, None))` with `x.isel(dim=slice(None, -1))`
  carried different labels on the shared dim; legacy aligned by position, v1
  raises. New `modeling._lead(var, dim)` relabels the leading slice onto the
  trailing slice's labels (linopy's recommended `.assign_coords` fix), which
  is identical under both semantics. Applied to the storage energy balance,
  on/off duration forward/backward/lb, state- and continuous-transition
  bounds, level tracking, and linked-period sizing.

- Conflicting scalar auxiliary coordinate. Endpoint constraints like
  `x.isel(time=0) == x.isel(time=-1)` leave a scalar `time`/`scenario`/
  `cluster_boundary` coord whose values disagree; legacy drops it silently,
  v1 raises. Added `drop=True` at each scalar-index site (cyclic/initial
  storage & status constraints, scenario-equality, intercluster linking,
  previous-state selection).

- Deprecated `Constraint.lhs` setter. Replaced `con.lhs +=/-=` with
  `con.update(lhs=...)` (linopy 0.8 deprecation) at the transmission-loss,
  bus-balance, and share-allocation sites.

The white-box `assert_conequal` tests in tests/deprecated reconstruct these
constraints; updated their expected expressions to the same explicit form.

Verified: full suite (tests/, excl. slow tutorials) is identical under both
semantics — 1615 passed, same 4 pre-existing clustering failures (solution
degeneracy in cyclic-cluster SOC level, unrelated to the semantics flag).

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

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@FBumann quick update, making good progress on the pypsa compatibility side. will continue tomorrow, but if it continues like that, I would say we should be able to merge this by the end of the week

@FBumann

FBumann commented Jul 21, 2026

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I also updated flixOpt here for reference including a reference of transferring patterns

flixOpt/flixopt#729

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