Class: QgsExpressionNode¶

Abstract base class for all nodes that can appear in an expression.

Note

This is an abstract class, with methods which must be implemented by a subclass.

The following methods must be implemented: clone(), dump(), isStatic(), needsGeometry(), nodeType(), referencedColumns(), referencedFunctions(), referencedVariables()

Class Hierarchy¶

Subclasses¶

`QgsExpressionNodeBetweenOperator`	SQL-like BETWEEN and NOT BETWEEN predicates.
`QgsExpressionNodeBinaryOperator`	A binary expression operator, which operates on two values.
`QgsExpressionNodeColumnRef`	An expression node which takes its value from a feature’s field.
`QgsExpressionNodeCondition`	An expression node for CASE WHEN clauses.
`QgsExpressionNodeFunction`	An expression node for expression functions.
`QgsExpressionNodeInOperator`	An expression node for value IN or NOT IN clauses.
`QgsExpressionNodeIndexOperator`	An indexing expression operator, which allows use of square brackets [] to reference map and array items.
`QgsExpressionNodeLiteral`	An expression node for literal values.
`QgsExpressionNodeUnaryOperator`	A unary node is either negative as in boolean (not) or as in numbers (minus).

Abstract Methods

`clone`	Generate a clone of this node.
`dump`	Dump this node into a serialized (part) of an expression.
`isStatic`	Returns `True` if this node can be evaluated for a static value.
`needsGeometry`	Abstract virtual method which returns if the geometry is required to evaluate this expression.
`nodeType`	Gets the type of this node.
`referencedColumns`	Abstract virtual method which returns a list of columns required to evaluate this node.
`referencedFunctions`	Returns a set of all functions which are used in this expression.
`referencedVariables`	Returns a set of all variables which are used in this expression.

Methods

`cachedStaticValue`	Returns the node's static cached value.
`effectiveNode`	Returns a reference to the simplest node which represents this node, after any compilation optimizations have been applied.
`eval`	Evaluate this node with the given context and parent.
`hasCachedStaticValue`	Returns `True` if the node can be replaced by a static cached value.
`prepare`	Prepare this node for evaluation.

Attributes

`ntBetweenOperator`
`ntBinaryOperator`
`ntColumnRef`
`ntCondition`
`ntFunction`
`ntInOperator`
`ntIndexOperator`
`ntLiteral`
`ntUnaryOperator`
`parserFirstColumn`	First column in the parser this node was found.
`parserFirstLine`	First line in the parser this node was found.
`parserLastColumn`	Last column in the parser this node was found.
`parserLastLine`	Last line in the parser this node was found.

class qgis.core.QgsExpressionNode[source]¶

Bases: object

class NamedNode¶

Bases: object

Named node

name: str¶: Node name

node: QgsExpressionNode¶: Node

class NodeList[source]¶

Bases: object

A list of expression nodes.

append(self, node: QgsExpressionNode | None)[source]¶

Takes ownership of the provided node

Parameters:: node (Optional[QgsExpressionNode])

append(self, node: QgsExpressionNode.NamedNode | None)[source]

Adds a named node. Takes ownership of the provided node.

Parameters:: node (Optional[QgsExpressionNode.NamedNode])

at(self, i: int) → QgsExpressionNode | None[source]¶

Gets the node at position i in the list.

Parameters:: i (int)
Return type:: Optional[QgsExpressionNode]

clone(self) → QgsExpressionNode.NodeList | None[source]¶

Creates a deep copy of this list. Ownership is transferred to the caller

Return type:: Optional[QgsExpressionNode.NodeList]

count(self) → int[source]¶

Returns the number of nodes in the list.

Return type:: int

virtual dump(self) → str[source]¶

Returns a string dump of the expression node.

Return type:: str

hasNamedNodes(self) → bool[source]¶

Returns True if list contains any named nodes

Return type:: bool

list(self) → List[QgsExpressionNode]¶

Gets a list of all the nodes.

Return type:: List[QgsExpressionNode]

names(self) → List[str][source]¶

Returns a list of names for nodes. Unnamed nodes will be indicated by an empty string in the list.

Return type:: List[str]

reserve(self, size: int)[source]¶

Reserves size for the node list.

Added in version 3.34.

Parameters:: size (int)

class NodeType¶: Bases: int

cachedStaticValue(self) → Any[source]¶

Returns the node’s static cached value. Only valid if hasCachedStaticValue() is True.

See also

cachedStaticValue()

Added in version 3.18.

Return type:: bool

abstract isStatic(self, parent: QgsExpression | None, context: QgsExpressionContext | None) → bool[source]¶

Returns True if this node can be evaluated for a static value. This is used during the prepare() step and in case it returns True, the value of this node will already be evaluated and the result cached (and therefore not re-evaluated in subsequent calls to eval()). In case this returns True, prepareNode() will never be called.

Parameters:

parent (Optional[QgsExpression])
context (Optional[QgsExpressionContext])

Return type:

bool

abstract needsGeometry(self) → bool[source]¶

Abstract virtual method which returns if the geometry is required to evaluate this expression.

This needs to call :py:func:`~QgsExpressionNode.needsGeometry` recursively on any child nodes.

Return type:: bool
Returns:: True if a geometry is required to evaluate this expression

abstract nodeType(self) → QgsExpressionNode.NodeType[source]¶

Gets the type of this node.

Return type:: QgsExpressionNode.NodeType
Returns:: The type of this node

ntBetweenOperator = 8¶

ntBinaryOperator = 1¶

ntColumnRef = 5¶

ntCondition = 6¶

ntFunction = 3¶

ntInOperator = 2¶

ntIndexOperator = 7¶

ntLiteral = 4¶

ntUnaryOperator = 0¶

parserFirstColumn: int¶: First column in the parser this node was found.

Note

This might not be complete for all nodes. Currently only QgsExpressionNode has this complete

parserFirstLine: int¶: First line in the parser this node was found.

Note

This might not be complete for all nodes. Currently only QgsExpressionNode has this complete

parserLastColumn: int¶: Last column in the parser this node was found.

Note

This might not be complete for all nodes. Currently only QgsExpressionNode has this complete

parserLastLine: int¶: Last line in the parser this node was found.

Note

This might not be complete for all nodes. Currently only QgsExpressionNode has this complete

prepare(self, parent: QgsExpression | None, context: QgsExpressionContext | None) → bool[source]¶

Prepare this node for evaluation. This will check if the node content is static and in this case cache the value. If it’s not static it will call prepareNode() to allow the node to do initialization work like for example resolving a column name to an attribute index.

Parameters:

parent (Optional[QgsExpression])
context (Optional[QgsExpressionContext])

Return type:

bool

abstract referencedColumns(self) → Set[str]¶

Abstract virtual method which returns a list of columns required to evaluate this node.

When reimplementing this, you need to return any column that is required to evaluate this node and in addition recursively collect all the columns required to evaluate child nodes.

Warning

If the expression has been prepared via a call to QgsExpression.prepare(), or a call to QgsExpressionNode.prepare() for a node has been made, then some nodes in the expression may have been determined to evaluate to a static pre-calculatable value. In this case the results will omit attribute indices which are used by these pre-calculated nodes, regardless of their actual referenced columns. If you are seeking to use these functions to introspect an expression you must take care to do this with an unprepared expression node.

Return type:: Set[str]
Returns:: A list of columns required to evaluate this expression

abstract referencedFunctions(self) → Set[str]¶

Returns a set of all functions which are used in this expression.

Note

In contrast to the referencedColumns() function this method is not affected by any previous calls to QgsExpressionNode.prepare().

Return type:: Set[str]

abstract referencedVariables(self) → Set[str]¶

Returns a set of all variables which are used in this expression.

Note

In contrast to the referencedColumns() function this method is not affected by any previous calls to QgsExpressionNode.prepare().

Return type:: Set[str]