refac: introduce consistent convention for linopy operations with subsets and supersets (PyPSA#572)

* refac: introduce consistent convention for linopy operations with subsets and supersets

* move scalar addition to add_constant

* add overwriting logic to add constant

* add join parameter to control alignment in operations

* Add le, ge, eq methods with join parameter for constraints

Add le(), ge(), eq() methods to LinearExpression and Variable classes,
mirroring the pattern of add/sub/mul/div methods. These methods support
the join parameter for flexible coordinate alignment when creating constraints.

* Extract constant alignment logic into _align_constant helper

Consolidate repetitive alignment handling in _add_constant and
_apply_constant_op into a single _align_constant method. This
eliminates code duplication and makes the alignment behavior
(handling join parameter, fill_value, size-aware defaults) testable
and maintainable in one place.

* update notebooks

* update release notes

* fix types

* add regression test

* fix numpy array dim mismatch in constraints and add RHS dim tests

numpy_to_dataarray no longer inflates ndim beyond arr.ndim, fixing
lower-dim numpy arrays as constraint RHS. Also reject higher-dim
constant arrays (numpy/pandas) consistently with DataArray behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* remove pandas reindexing warning

* Fix mypy errors: type ignores for xr.align/merge, match override signature, add test type hints

* remove outdated warning tests

* reintroduce expansions of extra rhs dims, fix multiindex alignment

* refactor test fixtures and use sign constants

* add tests for pandas series subset/superset

* test: add TestMissingValues for same-shape constants with NaN entries

* Fix broken test imports, stray docstring char, and incorrect test assertion from fixture refactor

* Fill NaN with neutral elements in expression arithmetic, preserve NaN as 'no constraint' in RHS

- Fill NaN with 0 (add/sub) or fill_value (mul/div) in _add_constant/_apply_constant_op
- Fill NaN coefficients with 0 in Variable.to_linexpr
- Restore NaN mask in to_constraint() so subset RHS still signals unconstrained positions

* Fix CI doctest collection by deferring linopy import in test/conftest.py

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: FabianHofmann

.gitignore

@@ -50,3 +50,4 @@ benchmark/scripts/leftovers/
 # direnv
 .envrc
 AGENTS.md
+coverage.xml

CLAUDE.md

@@ -110,27 +110,6 @@ When modifying the codebase, maintain consistency with these patterns and ensure
 * Always create a feature branch for new features or bug fixes.
 * Use the github cli (gh) to interact with the Github repository.
 
-### GitHub Claude Code Integration
-
-This repository includes Claude Code GitHub Actions for automated assistance:
-
-1. **Automated PR Reviews** (`claude-code-review.yml`):
-   - Automatically reviews PRs only when first created (opened)
-   - Subsequent reviews require manual `@claude` mention
-   - Focuses on Python best practices, xarray patterns, and optimization correctness
-   - Can run tests and linting as part of the review
-   - **Skip initial review by**: Adding `[skip-review]` or `[WIP]` to PR title, or using draft PRs
-
-2. **Manual Claude Assistance** (`claude.yml`):
-   - Trigger by mentioning `@claude` in any:
-     - Issue comments
-     - Pull request comments
-     - Pull request reviews
-     - New issue body or title
-   - Claude can help with bug fixes, feature implementation, code explanations, etc.
-
-**Note**: Both workflows require the `ANTHROPIC_API_KEY` secret to be configured in the repository settings.
-
 
 ## Development Guidelines
 
@@ -140,3 +119,4 @@ This repository includes Claude Code GitHub Actions for automated assistance:
 4. Use type hints and mypy for type checking.
 5. Always write tests into the `test` directory, following the naming convention `test_*.py`.
 6. Always write temporary and non git-tracked code in the `dev-scripts` directory.
+7. In test scripts use linopy assertions from the testing.py module where useful (assert_linequal, assert_varequal, etc.)

doc/index.rst

@@ -111,6 +111,7 @@ This package is published under MIT license.
    creating-variables
    creating-expressions
    creating-constraints
+   coordinate-alignment
    sos-constraints
    piecewise-linear-constraints
    piecewise-linear-constraints-tutorial

doc/release_notes.rst

@@ -4,6 +4,12 @@ Release Notes
 Upcoming Version
 ----------------
 
+* Harmonize coordinate alignment for operations with subset/superset objects:
+  - Multiplication and division fill missing coords with 0 (variable doesn't participate)
+  - Addition and subtraction of constants fill missing coords with 0 (identity element) and pin result to LHS coords
+  - Comparison operators (``==``, ``<=``, ``>=``) fill missing RHS coords with NaN (no constraint created)
+  - Fixes crash on ``subset + var`` / ``subset + expr`` reverse addition
+  - Fixes superset DataArrays expanding result coords beyond the variable's coordinate space
 * Add ``add_piecewise_constraints()`` with SOS2, incremental, LP, and disjunctive formulations (``linopy.piecewise(x, x_pts, y_pts) == y``).
 * Add ``linopy.piecewise()`` to create piecewise linear function descriptors (`PiecewiseExpression`) from separate x/y breakpoint arrays.
 * Add ``linopy.breakpoints()`` factory for convenient breakpoint construction from lists, Series, DataFrames, DataArrays, or dicts. Supports slopes mode.