**Course Title:** PyQt6 Application Development **Section Title:** Understanding the Model-View-Controller (MVC) Pattern **Topic:** Creating custom models and proxy models In the previous topics, we covered the basics of the Model-View-Controller (MVC) pattern in PyQt6 and worked with pre-built models and views. However, in real-world applications, you often need to create custom models and proxy models to handle complex data structures and filtering. In this topic, we will explore how to create custom models and proxy models using PyQt6. **What are custom models?** A custom model is a class that inherits from `QAbstractItemModel` or one of its subclasses and provides a custom implementation for handling data. Custom models are useful when you need to display data that doesn't fit into the pre-built model classes, such as a hierarchical data structure or a dynamic data set. **Creating a custom model** To create a custom model, you need to override the virtual functions of `QAbstractItemModel`. These functions include: * `data()`: Returns the data stored under the given role for the item referred to by the index. * `itemData()`: Returns a dictionary mapping model roles to data indices. * `index()`: Returns the index of the item in the model specified by the given row, column, and parent index. * `parent()`: Returns the parent index for the given child index. * `rowCount()`: Returns the number of rows under the given parent. * `columnCount()`: Returns the number of columns under the given parent. Here's an example of a simple custom model that displays a list of names and ages: ```python import sys from PyQt6.QtCore import QAbstractItemModel, Qt from PyQt6.QtWidgets import QApplication, QListView class CustomModel(QAbstractItemModel): def __init__(self, data): super().__init__() self._data = data def data(self, index, role): if role == Qt.ItemDataRole.DisplayRole: return self._data[index.row()][0] def itemData(self, index): if index.isValid(): return {Qt.ItemDataRole.DisplayRole: self._data[index.row()][0], Qt.ItemDataRole.UserRole: self._data[index.row()][1]} else: return {} def index(self, row, column, parent): return self.createIndex(row, column) def parent(self, index): return self.createIndex(0, 0) def rowCount(self, parent): return len(self._data) def columnCount(self, parent): return 1 if __name__ == "__main__": app = QApplication(sys.argv) model = CustomModel([["John", 25], ["Jane", 30], ["Bob", 35]]) view = QListView() view.setModel(model) view.show() sys.exit(app.exec()) ``` **What are proxy models?** A proxy model is a class that inherits from `QAbstractProxyModel` and provides a layer of abstraction between a source model and a view. Proxy models are useful when you need to filter or transform data before it's displayed in a view. **Creating a proxy model** To create a proxy model, you need to override the virtual functions of `QAbstractProxyModel`. These functions include: * `sourceModel()`: Returns the source model for the proxy model. * `setSourceModel()`: Sets the source model for the proxy model. * `index()`: Returns the index of the item in the proxy model specified by the given row, column, and parent index. * `mapToSource()`: Returns the source index for the given proxy index. * `mapFromSource()`: Returns the proxy index for the given source index. Here's an example of a simple proxy model that filters out items that contain a certain string: ```python import sys from PyQt6.QtCore import QAbstractProxyModel, Qt from PyQt6.QtWidgets import QApplication, QListView class ProxyModel(QAbstractProxyModel): def __init__(self, sourceModel, filterString): super().__init__() self._sourceModel = sourceModel self._filterString = filterString def setSourceModel(self, sourceModel): self._sourceModel = sourceModel self.invalidate() def index(self, row, column, parent): return self.createIndex(row, column) def mapToSource(self, proxyIndex): return self._sourceModel.index(proxyIndex.row(), proxyIndex.column()) def mapFromSource(self, sourceIndex): return self.createIndex(sourceIndex.row(), sourceIndex.column()) def rowCount(self, parent): return self._sourceModel.rowCount() def filterAcceptsRow(self, sourceRow, sourceParent): return self._filterString not in self._sourceModel.data(self._sourceModel.index(sourceRow, 0), Qt.ItemDataRole.DisplayRole) if __name__ == "__main__": app = QApplication(sys.argv) sourceModel = CustomModel([["John", 25], ["Jane", 30], ["Bob", 35]]) proxyModel = ProxyModel(sourceModel, "J") view = QListView() view.setModel(proxyModel) view.show() sys.exit(app.exec()) ``` **Conclusion** In this topic, we covered how to create custom models and proxy models in PyQt6. We explored how to override the virtual functions of `QAbstractItemModel` and `QAbstractProxyModel` to provide custom implementations for handling data. We also saw examples of simple custom models and proxy models. By mastering these concepts, you'll be able to create complex and dynamic data structures that can be displayed in a PyQt6 view. **External Links** * [Qt Documentation: Model/View Programming](https://doc.qt.io/qt-6/model-view-programming.html) * [Qt Documentation: QAbstractItemModel](https://doc.qt.io/qt-6/qabstractitemmodel.html) * [Qt Documentation: QAbstractProxyModel](https://doc.qt.io/qt-6/qabstractproxymodel.html) **Leave a Comment/Ask for Help** If you have any questions or need help with creating custom models or proxy models in PyQt6, feel free to leave a comment below.