The code snippets on this page need the following imports if you’re outside the pyqgis console:

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from qgis.core import (
    edit,
    QgsExpression,
    QgsExpressionContext,
    QgsFeature,
    QgsFeatureRequest,
    QgsField,
    QgsFields,
    QgsVectorLayer,
    QgsPointXY,
    QgsGeometry,
    QgsProject,
    QgsExpressionContextUtils
)

11. Espressioni, Filtraggio e Calcolo di Valori

QGIS offre supporto per l’analisi di espressioni SQL. Solo un piccolo sottoinsieme della sintassi SQL é supportato. Le espressioni possono essere valutate sia come predicati booleani (che restituiscono Vero o Falso) o come funzioni (che restituiscono un valore scalare). Vedi Espressioni nel Manuale dell’Utente per una lista completa delle funzioni presenti.

Sono supportati tre tipi base:

  • numero – sia numeri interi che decimali, e.g. 123, 3.14

  • stringa – devono essere racchiuse tra apici singoli: 'hello world'

  • riferimento a colonna – durante la valutazione, il riferimento é sostituito con il valore del campo. I nomi non sono racchiusi tra apici.

Sono disponibili le seguenti operazioni:

  • operatori aritmetici: +, -, *, /, ^

  • parentesi: per forzare la precedenza tra operatori: (1 + 1) * 3

  • somma e sottrazione unari: -12, +5

  • funzioni matematiche: sqrt, sin, cos, tan, asin, acos, atan

  • funzioni di conversione: to_int, to_real, to_string, to_date

  • funzioni sulla geometria: $area, $length

  • funzioni di manipolazione della geometria: $x, $y, $geometry, num_geometries, centroid

Sono supportati i seguenti predicati:

  • comparazione: =, !=, >, >=, <, <=

  • pattern matching: LIKE (usando % e _), ~ (espressioni regolari)

  • predicati logici: AND, OR, NOT

  • controllo di valori NULL: IS NULL, IS NOT NULL

Esempi di predicati:

  • 1 + 2 = 3

  • sin(angolo) > 0

  • 'Hello' LIKE 'He%'

  • (x > 10 AND y > 10) OR z = 0

Esempi di espressioni scalari:

  • 2 ^ 10

  • sqrt(val)

  • $length + 1

11.1. Analisi di Espressioni

The following example shows how to check if a given expression can be parsed correctly:

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exp = QgsExpression('1 + 1 = 2')
assert(not exp.hasParserError())

exp = QgsExpression('1 + 1 = ')
assert(exp.hasParserError())

assert(exp.parserErrorString() == '\nsyntax error, unexpected $end')

11.2. Valutazione di Espressioni

Expressions can be used in different contexts, for example to filter features or to compute new field values. In any case, the expression has to be evaluated. That means that its value is computed by performing the specified computational steps, which can range from simple arithmetic to aggregate expressions.

11.2.1. Espressioni Base

This basic expression evaluates to 1, meaning it is true:

exp = QgsExpression('1 + 1 = 2')
assert(exp.evaluate()) # exp.evaluate() returns 1 and assert() recognizes this as True

11.2.2. Espressioni con geometrie

To evaluate an expression against a feature, a QgsExpressionContext object has to be created and passed to the evaluate function in order to allow the expression to access the feature’s field values.

The following example shows how to create a feature with a field called «Column» and how to add this feature to the expression context.

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fields = QgsFields()
field = QgsField('Column')
fields.append(field)
feature = QgsFeature()
feature.setFields(fields)
feature.setAttribute(0, 99)

exp = QgsExpression('"Column"')
context = QgsExpressionContext()
context.setFeature(feature)
assert(exp.evaluate(context) == 99)

The following is a more complete example of how to use expressions in the context of a vector layer, in order to compute new field values:

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from qgis.PyQt.QtCore import QVariant

# create a vector layer
vl = QgsVectorLayer("Point", "Companies", "memory")
pr = vl.dataProvider()
pr.addAttributes([QgsField("Name", QVariant.String),
                  QgsField("Employees",  QVariant.Int),
                  QgsField("Revenue", QVariant.Double),
                  QgsField("Rev. per employee", QVariant.Double),
                  QgsField("Sum", QVariant.Double),
                  QgsField("Fun", QVariant.Double)])
vl.updateFields()

# add data to the first three fields
my_data = [
    {'x': 0, 'y': 0, 'name': 'ABC', 'emp': 10, 'rev': 100.1},
    {'x': 1, 'y': 1, 'name': 'DEF', 'emp': 2, 'rev': 50.5},
    {'x': 5, 'y': 5, 'name': 'GHI', 'emp': 100, 'rev': 725.9}]

for rec in my_data:
    f = QgsFeature()
    pt = QgsPointXY(rec['x'], rec['y'])
    f.setGeometry(QgsGeometry.fromPointXY(pt))
    f.setAttributes([rec['name'], rec['emp'], rec['rev']])
    pr.addFeature(f)

vl.updateExtents()
QgsProject.instance().addMapLayer(vl)

# The first expression computes the revenue per employee.
# The second one computes the sum of all revenue values in the layer.
# The final third expression doesn’t really make sense but illustrates
# the fact that we can use a wide range of expression functions, such
# as area and buffer in our expressions:
expression1 = QgsExpression('"Revenue"/"Employees"')
expression2 = QgsExpression('sum("Revenue")')
expression3 = QgsExpression('area(buffer($geometry,"Employees"))')

# QgsExpressionContextUtils.globalProjectLayerScopes() is a convenience
# function that adds the global, project, and layer scopes all at once.
# Alternatively, those scopes can also be added manually. In any case,
# it is important to always go from “most generic” to “most specific”
# scope, i.e. from global to project to layer
context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(vl))

with edit(vl):
    for f in vl.getFeatures():
        context.setFeature(f)
        f['Rev. per employee'] = expression1.evaluate(context)
        f['Sum'] = expression2.evaluate(context)
        f['Fun'] = expression3.evaluate(context)
        vl.updateFeature(f)

print( f['Sum'])

876.5

11.2.3. Filtering a layer with expressions

L’esempio seguente puó essere usato per filtrare un layer e restituire qualsiasi geometria che soddisfi il predicato.

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layer = QgsVectorLayer("Point?field=Test:integer",
                           "addfeat", "memory")

layer.startEditing()

for i in range(10):
    feature = QgsFeature()
    feature.setAttributes([i])
    assert(layer.addFeature(feature))
layer.commitChanges()

expression = 'Test >= 3'
request = QgsFeatureRequest().setFilterExpression(expression)

matches = 0
for f in layer.getFeatures(request):
   matches += 1

assert(matches == 7)

11.3. Handling expression errors

Expression-related errors can occur during expression parsing or evaluation:

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exp = QgsExpression("1 + 1 = 2")
if exp.hasParserError():
   raise Exception(exp.parserErrorString())

value = exp.evaluate()
if exp.hasEvalError():
   raise ValueError(exp.evalErrorString())