field_expression_widget_demo.py

class TestDialog(QDialog):
    
    def __init__(self, iface):
        QDialog.__init__(self)
        self.iface = iface
        self.exp_widget = QgsFieldExpressionWidget(self)
        self.line_edit = QLineEdit(self)
        self.btn = QPushButton('Run', self)
        self.list_widget = QListWidget(self)
        self.layout = QVBoxLayout()
        self.layout.addWidget(self.exp_widget)
        self.layout.addWidget(self.btn)
        self.layout.addWidget(self.line_edit)
        self.layout.addWidget(self.list_widget)
        self.setGeometry(500, 300, 500, 300)
        
        self.setLayout(self.layout)
        self.btn.clicked.connect(self.get_expression)
        
        self.layer = self.iface.activeLayer()
        self.exp_widget.setLayer(self.layer)
        self.iface.currentLayerChanged.connect(self.active_layer_changed)
        
        
    def active_layer_changed(self, lyr):
        if lyr is None:
            return
        self.layer = lyr
        self.exp_widget.setExpression('')
        self.exp_widget.setLayer(self.layer)
                
    def get_expression(self):
        self.line_edit.clear()
        self.list_widget.clear()
        exp = QgsExpression(self.exp_widget.asExpression())
        self.line_edit.setText(str(exp))
        context = QgsExpressionContext()
        context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(self.layer))
        items = []
        for f in self.layer.getFeatures():
            context.setFeature(f)
            val = exp.evaluate(context)
            items.append(str(val))
        self.list_widget.addItems(items)

    def closeEvent(self, e):
        self.iface.currentLayerChanged.disconnect(self.active_layer_changed)
                
dlg = TestDialog(iface)
dlg.show()