feat!: rename label → id, add IdList[T], redesign Flow constructor

CHANGELOG.md

@@ -54,7 +54,29 @@ Until here -->
 
 ## [7.0.0] - Unreleased
 
-**Summary**: Performance release with **up to 67x faster model building** for large systems through batched/vectorized operations.
+**Summary**: Performance release with **up to 67x faster model building** for large systems through batched/vectorized operations. Renames `label` to `id` across the API with deprecation support, introduces `IdList[T]` as the standard container, and redesigns the `Flow` constructor.
+
+### ✨ Added
+
+- **`IdList[T]` container** (`flixopt/id_list.py`): New generic frozen ordered container replacing `FlowContainer`, `ElementContainer`, `ResultsContainer`, and `CarrierContainer`. Provides dict-like access by primary key, short-key fallback, or positional index, with helpful error messages including close-match suggestions.
+
+### 💥 Breaking Changes
+
+- **`label` renamed to `id`**: All element constructors now use `id` instead of `label`. The old `label` parameter and `.label` / `.label_full` properties are deprecated and will be removed in v8.0.0. Use `.id` everywhere.
+- **`Flow` constructor redesigned**: `bus` is now the first positional argument; `flow_id` (optional) sets the short name, defaulting to the bus name. Old forms `Flow(label, bus)` and `Flow(label, bus=...)` remain deprecated until v8.0.0.
+- **`Flow.id` returns qualified name**: `Flow.id` now returns `component(flow_id)` (e.g., `Boiler(Q_fu)`) instead of just the short name. Use `flow.flow_id` for the short name.
+- **`Flow.flow_id`**: New public property for the short flow identifier (e.g., `'Q_fu'`). This replaces the internal `_short_id` for Flow objects.
+- **Container classes replaced**: `FlowContainer`, `ElementContainer`, `ResultsContainer` replaced by `IdList`. `EffectCollection` and `CarrierContainer` now inherit from `IdList`. Access patterns (`[]`, `in`, `keys()`, `values()`, `items()`, `get()`) are preserved.
+
+### 🗑️ Deprecated
+
+The following items are deprecated and will be removed in **v8.0.0**:
+
+- `Element(label=...)` — use `Element(id=...)` instead
+- `Flow(id=...)` — use `Flow(flow_id=...)` instead
+- `.label` property — use `.id` instead
+- `.label_full` property — use `.id` instead
+- `Flow(label, bus)` positional form — use `Flow(bus, flow_id=...)` instead
 
 ### 🚀 Performance
 

flixopt/__init__.py

@@ -31,10 +31,11 @@
 )
 from .config import CONFIG
 from .core import TimeSeriesData
-from .effects import PENALTY_EFFECT_LABEL, Effect
+from .effects import PENALTY_EFFECT_ID, PENALTY_EFFECT_LABEL, Effect
 from .elements import Bus, Flow
 from .flow_system import FlowSystem
 from .flow_system_status import FlowSystemStatus
+from .id_list import IdList
 from .interface import InvestParameters, Piece, Piecewise, PiecewiseConversion, PiecewiseEffects, StatusParameters
 from .optimization import Optimization, SegmentedOptimization
 from .plot_result import PlotResult
@@ -48,6 +49,8 @@
     'Flow',
     'Bus',
     'Effect',
+    'IdList',
+    'PENALTY_EFFECT_ID',
     'PENALTY_EFFECT_LABEL',
     'Source',
     'Sink',