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

Methods

currentPreviewText	Returns the current expression result preview text.
evalError	Will be set to True if the current expression text reported an eval error with the context.
expressionContext	Returns the expression context for the widget.
parserError	Will be set to True if the current expression text reports a parser error with the context.
parserErrors	Returns the expression parser errors
rootNode	Returns the root node of the expression
setCurrentFeature	sets the current feature used
setCustomPreviewGenerator	Sets the widget to run using a custom preview generator.
setExpressionContext	Sets the expression context for the widget.
setExpressionText	Sets the expression
setGeomCalculator	Sets geometry calculator used in distance/area calculations.
setLayer	Sets the layer used in the preview

Signals

evalErrorChanged	Will be set to True if the current expression text reported an eval error with the context.
expressionParsed	Emitted when the user changes the expression in the widget.
parserErrorChanged	Will be set to True if the current expression text reported a parser error with the context.
toolTipChanged	Emitted whenever the tool tip changed

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

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

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(isValid: bool)

Emitted when the user changes the expression in the widget. Users of this widget should connect to this signal to decide if to let the user continue.

Parameters:

isValid (bool) – Is True if the expression the user has typed is valid.

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

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

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(toolTip: str)

Emitted whenever the tool tip changed

Parameters:

toolTip (str)