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

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

Expressions, Filtering and Calculating Values¶

Parsing Expressions
Evaluating Expressions
Handling expression errors

QGIS has some support for parsing of SQL-like expressions. Only a small subset of SQL syntax is supported. The expressions can be evaluated either as boolean predicates (returning True or False) or as functions (returning a scalar value). See Expressions in the User Manual for a complete list of available functions.

Three basic types are supported:

number --- both whole numbers and decimal numbers, e.g. 123, 3.14
string --- they have to be enclosed in single quotes: 'hello world'
column reference --- when evaluating, the reference is substituted with the actual value of the field. The names are not escaped.

The following operations are available:

arithmetic operators: +, -, *, /, ^
parentheses: for enforcing the operator precedence: (1 + 1) * 3
unary plus and minus: -12, +5
mathematical functions: sqrt, sin, cos, tan, asin, acos, atan
conversion functions: to_int, to_real, to_string, to_date
geometry functions: $area, $length
geometry handling functions: $x, $y, $geometry, num_geometries, centroid

And the following predicates are supported:

comparison: =, !=, >, >=, <, <=
pattern matching: LIKE (using % and _), ~ (regular expressions)
logical predicates: AND, OR, NOT
NULL value checking: IS NULL, IS NOT NULL

Examples of predicates:

1 + 2 = 3
sin(angle) > 0
'Hello' LIKE 'He%'
(x > 10 AND y > 10) OR z = 0

Examples of scalar expressions:

2 ^ 10
sqrt(val)
$length + 1

Parsing Expressions ¶

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

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

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

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

Evaluating Expressions ¶

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.

Basic Expressions ¶

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

exp = QgsExpression('1 + 1 = 2')
assert(exp.evaluate())

Expressions with features ¶

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.

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:

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'])

Filtering a layer with expressions ¶

The following example can be used to filter a layer and return any feature that matches a predicate.

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)

Handling expression errors ¶

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

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

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

Expressions, Filtering and Calculating Values¶

Parsing Expressions¶

Evaluating Expressions¶

Basic Expressions¶

Expressions with features¶

Filtering a layer with expressions¶

Handling expression errors¶