solid-skills

```python
# Inventory Management System - SOLID Principles Implementation

class InventoryManager:
    def __init__(self, inventory_repository):
        self.inventory_repository = inventory_repository

    def update_inventory(self, item_id, quantity):
        if not self._validate_quantity(quantity):
            raise ValueError("Invalid quantity")
        self.inventory_repository.update(item_id, quantity)

    def _validate_quantity(self, quantity):
        return quantity > 0

# Unit Tests
import unittest
from unittest.mock import MagicMock

class TestInventoryManager(unittest.TestCase):
    def setUp(self):
        self.mock_repository = MagicMock()
        self.inventory_manager = InventoryManager(self.mock_repository)

    def test_update_inventory_valid_quantity(self):
        self.inventory_manager.update_inventory(1, 10)
        self.mock_repository.update.assert_called_once_with(1, 10)

    def test_update_inventory_invalid_quantity(self):
        with self.assertRaises(ValueError):
            self.inventory_manager.update_inventory(1, -5)

# SOLID Principles Explanation
- **Single Responsibility Principle (SRP)**: The InventoryManager class has a single responsibility of managing inventory updates.
- **Open/Closed Principle (OCP)**: The class is open for extension (e.g., adding new validation rules) but closed for modification.
- **Liskov Substitution Principle (LSP)**: The InventoryManager can be substituted with any class that adheres to the same interface.
- **Interface Segregation Principle (ISP)**: The class depends on a specific interface (InventoryRepository) rather than a general one.
- **Dependency Inversion Principle (DIP)**: The class depends on abstractions (InventoryRepository) rather than concrete implementations.
```