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+# Factory Pattern
+
+## Overview
+
+The Factory pattern allows dynamic creation of instances without specifying the exact class. Each module uses a factory to decouple creation from usage.
+
+## Structure
+
+```python
+# In module __init__.py (e.g., data_module/dataset/__init__.py)
+DATASET_FACTORY: Dict[str, type] = {}
+
+def DatasetFactory(data_name: str):
+    """Create dataset instance by name."""
+    dataset = DATASET_FACTORY.get(data_name, None)
+    if dataset is None:
+        # Fallback to default
+        dataset = DATASET_FACTORY.get('simple')
+    return dataset
+```
+
+## Usage
+
+```python
+# Consumer code doesn't need to know concrete class
+dataset = DatasetFactory(cfg.dataset.name)
+```
+
+## Benefits
+
+- **Loose coupling**: Consumer doesn't import concrete classes
+- **Extensibility**: Add new types without changing consumer code
+- **Fallback handling**: Graceful degradation for unknown types
+- **Centralized registry**: Single source of truth for available types
+
+## Implementation Details
+
+1. Define factory dict at module level
+2. Factory function handles lookup and fallback
+3. Return class (not instance) for deferred initialization
+4. None result triggers fallback to default implementation
+
+## Common Patterns
+
+```python
+# With config integration
+def DatasetFactory(cfg):
+    data_name = cfg.dataset.name
+    dataset_cls = DATASET_FACTORY.get(data_name)
+    if dataset_cls is None:
+        raise ValueError(f"Unknown dataset: {data_name}")
+    return dataset_cls(cfg)
+```