Change variable name delimiter from | to --

[FBumann]

Motivation

The current variable/constraint naming convention uses | as delimiter (e.g., flow|rate, storage|charge). This is problematic:

• Typing ergonomics: On German QWERTZ keyboards, | requires AltGr+< — one of the most awkward key combos. Even on US keyboards it requires Shift.
• -- is better: No shift key on any layout, visually distinct, unambiguous in Python/xarray string contexts.

Delimiter comparison

Delimiter	Example	Shift? (US)	Shift? (DE)	Verdict
| (current)	flow|rate	Yes	AltGr (painful)	Hard to type
--	flow--rate	No	No	Easy, clear separator
.	flow.rate	No	No	Conflicts with xarray attribute access
-	flow-rate	No	No	Ambiguous with multi-word names
::	flow::rate	Yes	Yes	Still needs shift
/	flow/rate	No	Shift+7	Path separator confusion

Additional change: () for mode suffixes

Share and effect variables that carry a mode suffix (e.g., share|temporal, share|periodic) should use parentheses instead: share(temporal), share(periodic). This is consistent with the existing penalty naming convention (Bus->Penalty(temporal)).

Scope

This is a breaking change for anyone accessing variables by name in the solution dataset.

Three distinct uses of | as delimiter in the codebase:

1. VarName enum constants (structure.py): 'flow|rate' → 'flow--rate'
2. f-string name construction (throughout): f'{prefix}|share' → f'{prefix}--share'
3. Name parsing (split('|'), endswith('|flow_rate')): update to --

Not changing: clustering/base.py tuple serialization (unrelated use of |), -> penalty separator.

Files to modify (~20 source + test + doc files)

Source files:

• flixopt/structure.py — VarName enums (centralized names) + _valid_label() (add -- to forbidden chars, remove |)
• flixopt/modeling.py — ~25 constraint name f-strings
• flixopt/features.py — ~10 builder name params
• flixopt/elements.py — ~30 constraint/variable names + _fit_coords calls
• flixopt/components.py — ~35 storage constraint names
• flixopt/effects.py — ~15 effect variable/share names
• flixopt/interface.py — ~10 _fit_coords name args
• flixopt/results.py — per-element name patterns, .split('|') calls
• flixopt/statistics_accessor.py — name patterns, split calls, endswith checks
• flixopt/optimize_accessor.py — rolling horizon name patterns
• flixopt/transform_accessor.py — rsplit call + name construction
• flixopt/flow_system.py — batched_var_map values

Test files: test_flow.py, test_bus.py, test_component.py, test_storage.py, test_clustering/, test_solution_persistence.py

Documentation: docs/variable_names.md, docs/architecture/batched_modeling.md, docs/user-guide/building-models/index.md, docs/user-guide/optimization/index.md

Label validation

Element._valid_label() in structure.py currently forbids | in user labels. Update to:

• Remove '|' from not_allowed
• Add '--' to not_allowed

Verification plan

1. grep -rn "'[a-z_]*|[a-z_]" flixopt/ --include="*.py" — no remaining pipe delimiters (excluding clustering)
2. python -m pytest tests/ — all tests pass
3. python -m mkdocs build — docs build