Class: QgsExpressionPreviewWidget

A widget for previewing an expression result.

If the layer is set, one can browse across features to see the different outputs.

Added in version 3.14.

Class Hierarchy

Base classes

QWidget
QObject
QPaintDevice

class qgis.gui.QgsExpressionPreviewWidget

Bases: QWidget

__init__(parent: QWidget | None = None)

Constructor

Parameters:

parent (Optional[QWidget] = None)

currentPreviewText(self) → str

Returns the current expression result preview text.

Added in version 3.38.

Return type:

str

evalError(self) → bool

Will be set to True if the current expression text reported an eval error with the context.

Return type:

bool

signal evalErrorChanged

pyqtSignal(*types, name: str = …, revision: int = …, arguments: Sequence = …) -> PYQT_SIGNAL

types is normally a sequence of individual types. Each type is either a type object or a string that is the name of a C++ type. Alternatively each type could itself be a sequence of types each describing a different overloaded signal. name is the optional C++ name of the signal. If it is not specified then the name of the class attribute that is bound to the signal is used. revision is the optional revision of the signal that is exported to QML. If it is not specified then 0 is used. arguments is the optional sequence of the names of the signal’s arguments.

expressionContext(self) → QgsExpressionContext

Returns the expression context for the widget. The context is used for the expression preview result and for populating the list of available functions and variables.

See also

setExpressionContext()

Return type:

QgsExpressionContext

signal expressionParsed

pyqtSignal(*types, name: str = …, revision: int = …, arguments: Sequence = …) -> PYQT_SIGNAL

types is normally a sequence of individual types. Each type is either a type object or a string that is the name of a C++ type. Alternatively each type could itself be a sequence of types each describing a different overloaded signal. name is the optional C++ name of the signal. If it is not specified then the name of the class attribute that is bound to the signal is used. revision is the optional revision of the signal that is exported to QML. If it is not specified then 0 is used. arguments is the optional sequence of the names of the signal’s arguments.

parserError(self) → bool

Will be set to True if the current expression text reports a parser error with the context.

Return type:

bool

signal parserErrorChanged

pyqtSignal(*types, name: str = …, revision: int = …, arguments: Sequence = …) -> PYQT_SIGNAL

types is normally a sequence of individual types. Each type is either a type object or a string that is the name of a C++ type. Alternatively each type could itself be a sequence of types each describing a different overloaded signal. name is the optional C++ name of the signal. If it is not specified then the name of the class attribute that is bound to the signal is used. revision is the optional revision of the signal that is exported to QML. If it is not specified then 0 is used. arguments is the optional sequence of the names of the signal’s arguments.

parserErrors(self) → List[QgsExpression.ParserError]

Returns the expression parser errors

Return type:

List[QgsExpression.ParserError]

rootNode(self) → QgsExpressionNode | None

Returns the root node of the expression

Return type:

Optional[QgsExpressionNode]

setCurrentFeature(self, feature: QgsFeature)

sets the current feature used

Parameters:

feature (QgsFeature)

setCustomPreviewGenerator(self, label: str | None, choices: Iterable[Tuple[str | None, Any]], a2: Callable[..., None])

Sets the widget to run using a custom preview generator.

In this mode, the widget will call a callback function to generate a new QgsExpressionContext as the previewed object changes. This can be used to provide custom preview values for different objects (i.e. for objects which aren’t vector layer features).

Parameters:

• label (Optional[str]) – The label to display for the combo box presenting choices of objects. This should be a representative name, eg “Band” if the widget is showing choices of raster layer bands

• choices (Iterable[Tuple[Optional[str], Any]]) – A list of choices to present to the user. Each choice is a pair of a human-readable label and a QVariant representing the object to preview.

• previewContextGenerator – A function which takes a QVariant representing the object to preview, and returns a QgsExpressionContext to use for previewing the object.

Added in version 3.38.

Parameters:

a2 (Callable[..., None])

setExpressionContext(self, context: QgsExpressionContext)

Sets the expression context for the widget. The context is used for the expression preview result and to populate the list of available functions and variables.

Parameters:

context (QgsExpressionContext) – expression context

setExpressionText(self, expression: str | None)

Sets the expression

Parameters:

expression (Optional[str])

setGeomCalculator(self, da: QgsDistanceArea)

Sets geometry calculator used in distance/area calculations.

Parameters:

da (QgsDistanceArea)

setLayer(self, layer: QgsVectorLayer | None)

Sets the layer used in the preview

Parameters:

layer (Optional[QgsVectorLayer])

signal toolTipChanged

pyqtSignal(*types, name: str = …, revision: int = …, arguments: Sequence = …) -> PYQT_SIGNAL

types is normally a sequence of individual types. Each type is either a type object or a string that is the name of a C++ type. Alternatively each type could itself be a sequence of types each describing a different overloaded signal. name is the optional C++ name of the signal. If it is not specified then the name of the class attribute that is bound to the signal is used. revision is the optional revision of the signal that is exported to QML. If it is not specified then 0 is used. arguments is the optional sequence of the names of the signal’s arguments.