**Course Title:** PySide6 Application Development **Section Title:** Model-View-Controller (MVC) Architecture **Topic:** Custom models and proxy models **Introduction** In the previous topics, we covered the basics of MVC architecture and how to use Qt models to handle data in PySide6 applications. However, there are cases where the built-in Qt models do not provide the desired functionality, and custom models are required. In this topic, we will delve into the world of custom models and proxy models in PySide6, exploring how to create and use these models in your applications. **Understanding Custom Models** A custom model in PySide6 is a class that inherits from `QAbstractItemModel` or one of its subclasses. Custom models allow you to define your own data structures and behavior for models, giving you more flexibility and control over data handling. Here's an example of a simple custom model that stores a list of items: ```python from PySide6.QtCore import QAbstractItemModel, QModelIndex class CustomListModel(QAbstractItemModel): def __init__(self): super().__init__() self.items = [] def rowCount(self, parent=QModelIndex()): return len(self.items) def columnCount(self, parent=QModelIndex()): return 1 def data(self, index, role=Qt.DisplayRole): if role == Qt.DisplayRole: return self.items[index.row()] return None def index(self, row, column=0, parent=QModelIndex()): return self.createIndex(row, column) def column(self, index, column=0): return self.createIndex(index.row(), column) def parent(self, index): return QModelIndex() class Food: def __init__(self, name): self.name = name # Usage: model = CustomListModel() model.items = [Food("Pizza"), Food("Burger"), Food("Sushi")] # Display data using the model for i in range(model.rowCount()): print(model.data(model.index(i)).name) ``` **Understanding Proxy Models** Proxy models in PySide6 are classes that inherit from `QAbstractProxyModel`. They wrap existing models and manipulate the data or behavior of the original model. Proxy models are useful for performing data transformations, filtering, or sorting. Here's an example of a simple proxy model that filters items from an original model based on a specific condition: ```python from PySide6.QtCore import QAbstractProxyModel class FilterProxyModel(QAbstractProxyModel): def __init__(self, original_model): super().__init__() self.original_model = original_model self.filter_role = Qt.DisplayRole def rowCount(self, parent=QModelIndex()): return self.original_model.rowCount(parent) def columnCount(self, parent=QModelIndex()): return self.original_model.columnCount(parent) def data(self, index, role=Qt.DisplayRole): if role == self.filter_role: # Apply filter logic here if index.row() % 2 == 0: # For example, show only even indices return self.original_model.data(index) return self.original_model.data(index) # Usage: custom_model = CustomListModel() # ... populate custom model with data ... proxy_model = FilterProxyModel(custom_model) # Display filtered data for i in range(proxy_model.rowCount()): print(proxy_model.data(proxy_model.index(i)).name) ``` **Best Practices for Custom Models and Proxy Models** 1. **Custom models should inherit from **QAbstractItemModel**:** This base class provides essential methods and properties for custom models to work correctly with Qt views. 2. **Property proxy models should inherit from **QAbstractProxyModel**:** This base class provides essential methods and properties for proxy models to work correctly with Qt views. 3. **Follow Qt naming conventions:** Adhere to Qt naming conventions for model class names and methods to ensure compatibility with Qt views and other components. 4. **Implement model methods correctly:** Ensure that custom models implement all required methods, including **rowCount()**, **columnCount()**, **data()**, and **index()**, to work correctly with Qt views. **Conclusion** In this topic, we covered custom models and proxy models in PySide6, exploring how to create and use these models to manipulate and handle data in your applications. You now understand the basics of custom models, proxy models, and best practices for creating and working with these models. You can apply your knowledge of custom models to create unique and flexible data handling solutions for your PySide6 applications. **What's Next** In the next topic, [Introduction to Qt Stylesheets (CSS-like theming)](link-to-next-topic) from the section **Styling and Theming Applications**, we will dive into the world of CSS-like theming in Qt applications. You will learn how to create modern, engaging user interfaces with Qt stylesheets and CSS. **Comment or Require Help?** If you have any questions or need help with custom models or proxy models in PySide6, please [leave a comment below](link-to-comment-section). **External Resources:** * [Qt 6: Custom Models](https://doc.qt.io/qt-6/model-view-programming.html#custom-models) * [Qt 6: Proxy Models](https://doc.qt.io/qt-6/model-view-programming.html#proxy-models) * [PySide6 Documentation: QAbstractItemModel](https://doc.qt.io/qtforpython-6/PySide6/QtCore/QAbstractItemModel.html) By following this course material and applying the concepts learned, you will be well-equipped to handle the challenges of creating effective and efficient data models for your PySide6 applications.