Class: QgsExpressionFunction

class qgis.core.QgsExpressionFunction

Bases: sip.wrapper

A abstract base class for defining QgsExpression functions.

QgsExpressionFunction(fnname: str, params: int, group: str, helpText: str = ‘’, lazyEval: bool = False, handlesNull: bool = False, isContextual: bool = False) Constructor for function which uses unnamed parameters

QgsExpressionFunction(fnname: str, params: int, groups: Iterable[str], helpText: str = ‘’, lazyEval: bool = False, handlesNull: bool = False, isContextual: bool = False) Constructor for function which uses unnamed parameters and group list

New in version 3.0.

QgsExpressionFunction(fnname: str, params: Iterable[QgsExpressionFunction.Parameter], group: str, helpText: str = ‘’, lazyEval: bool = False, handlesNull: bool = False, isContextual: bool = False) Constructor for function which uses named parameter list.

New in version 2.16.

QgsExpressionFunction(fnname: str, params: Iterable[QgsExpressionFunction.Parameter], groups: Iterable[str], helpText: str = ‘’, lazyEval: bool = False, handlesNull: bool = False, isContextual: bool = False) Constructor for function which uses named parameter list and group list.

New in version 3.0.

QgsExpressionFunction(QgsExpressionFunction)

Methods

aliases

Returns a list of possible aliases for the function.

allParamsStatic

This will return True if all the params for the provided function node are static within the constraints imposed by the context within the given parent.

func

Returns result of evaluating the function.

group

Returns the first group which the function belongs to.

groups

Returns a list of the groups the function belongs to.

handlesNull

Returns True if the function handles NULL values in arguments by itself, and the default NULL value handling should be skipped.

helpText

The help text for the function.

isContextual

Returns whether the function is only available if provided by a QgsExpressionContext object.

isDeprecated

Returns True if the function is deprecated and should not be presented as a valid option to users in expression builders.

isStatic

Will be called during prepare to determine if the function is static.

lazyEval

True if this function should use lazy evaluation.

minParams

The minimum number of parameters this function takes.

name

The name of the function.

parameters

Returns the list of named parameters for the function, if set.

params

The number of parameters this function takes.

prepare

This will be called during the prepare step() of an expression if it is not static.

referencedColumns

Returns a set of field names which are required for this function.

run

Evaluates the function, first evaluating all required arguments before passing them to the function’s func() method.

usesGeometry

Does this function use a geometry object.

class Parameter

Bases: sip.wrapper

Represents a single parameter passed to a function.

New in version 2.16.

QgsExpressionFunction.Parameter(name: str, optional: bool = False, defaultValue: Any = None, isSubExpression: bool = False) Constructor for Parameter.

Parameters
  • name – parameter name, used when named parameter are specified in an expression

  • optional – set to True if parameter should be optional

  • defaultValue – default value to use for optional parameters

  • isSubExpression – set to True if this parameter is a sub-expression

QgsExpressionFunction.Parameter(QgsExpressionFunction.Parameter)

defaultValue(self)Any

Returns the default value for the parameter.

Return type

Any

isSubExpression(self)bool

Returns True if parameter argument is a separate sub-expression, and should not be checked while determining referenced columns for the expression.

New in version 3.2.

Return type

bool

name(self)str

Returns the name of the parameter.

Return type

str

optional(self)bool

Returns True if the parameter is optional.

Return type

bool

aliases(self)List[str]

Returns a list of possible aliases for the function. These include other permissible names for the function, e.g., deprecated names.

Return type

List[str]

Returns

list of known aliases

New in version 2.9.

allParamsStatic(node: QgsExpressionNodeFunction, parent: QgsExpression, context: QgsExpressionContext)bool

This will return True if all the params for the provided function node are static within the constraints imposed by the context within the given parent.

This can be used as callback for custom implementations of subclasses. It is the default for implementation for StaticFunction.isStatic.

New in version 3.0.

Parameters
Return type

bool

func(self, values: Iterable[Any], context: QgsExpressionContext, parent: QgsExpression, node: QgsExpressionNodeFunction)Any

Returns result of evaluating the function.

Parameters
Return type

Any

Returns

result of function

group(self)str

Returns the first group which the function belongs to.

Note

consider using groups() instead, as some functions naturally belong in multiple groups

Return type

str

groups(self)List[str]

Returns a list of the groups the function belongs to.

See also

group()

New in version 3.0.

Return type

List[str]

handlesNull(self)bool

Returns True if the function handles NULL values in arguments by itself, and the default NULL value handling should be skipped.

Return type

bool

helpText(self)str

The help text for the function.

Return type

str

isContextual(self)bool

Returns whether the function is only available if provided by a QgsExpressionContext object.

New in version 2.12.

Return type

bool

isDeprecated(self)bool

Returns True if the function is deprecated and should not be presented as a valid option to users in expression builders.

New in version 3.0.

Return type

bool

isStatic(self, node: QgsExpressionNodeFunction, parent: QgsExpression, context: QgsExpressionContext)bool

Will be called during prepare to determine if the function is static. A function is static if it will return the same value for every feature with different attributes and/or geometry.

By default this will return True, if all arguments that have been passed to the function are also static.

New in version 3.0.

Parameters
Return type

bool

lazyEval(self)bool

True if this function should use lazy evaluation. Lazy evaluation functions take QgsExpression.Node objects rather than the node results when called. You can use node->eval(parent, feature) to evaluate the node and return the result Functions are non lazy default and will be given the node return value when called.

Return type

bool

minParams(self)int

The minimum number of parameters this function takes.

Return type

int

name(self)str

The name of the function.

Return type

str

parameters(self)List[QgsExpressionFunction.Parameter]

Returns the list of named parameters for the function, if set.

New in version 2.16.

Return type

List[QgsExpressionFunction.Parameter]

params(self)int

The number of parameters this function takes.

Return type

int

prepare(self, node: QgsExpressionNodeFunction, parent: QgsExpression, context: QgsExpressionContext)bool

This will be called during the prepare step() of an expression if it is not static.

This can be used by functions to do any preparation steps that might help to speedup the upcoming evaluation.

New in version 3.0.

Parameters
Return type

bool

referencedColumns(self, node: QgsExpressionNodeFunction)Set[str]

Returns a set of field names which are required for this function. May contain QgsFeatureRequest.AllAttributes to signal that all attributes are required. If in doubt this will return more fields than strictly required.

New in version 3.0.

Parameters

node (QgsExpressionNodeFunction) –

Return type

Set[str]

run(self, args: QgsExpressionNode.NodeList, context: QgsExpressionContext, parent: QgsExpression, node: QgsExpressionNodeFunction)Any

Evaluates the function, first evaluating all required arguments before passing them to the function’s func() method.

Parameters
Return type

Any

usesGeometry(self, node: QgsExpressionNodeFunction)bool

Does this function use a geometry object.

Parameters

node (QgsExpressionNodeFunction) –

Return type

bool