Need to use an expression?
|
+-- Filter features? --> QgsFeatureRequest.setFilterExpression()
|
+-- Calculate field values? --> Field Calculator pattern (edit session + evaluate per feature)
|
+-- Drive labeling? --> QgsPalLayerSettings.fieldName + isExpression = True
|
+-- Drive symbology? --> QgsProperty.fromExpression() on symbol/renderer properties
|
+-- Evaluate standalone? --> QgsExpression.evaluate(context)
|
+-- Need custom logic? --> @qgsfunction decorator + QgsExpression.registerFunction()

from qgis.core import (
    QgsExpression, QgsExpressionContext, QgsExpressionContextUtils
)

exp = QgsExpression('"population" * 1.05')
if exp.hasParserError():
    raise ValueError(f"Parse error: {exp.parserErrorString()}")

context = QgsExpressionContext()
context.appendScopes(
    QgsExpressionContextUtils.globalProjectLayerScopes(layer)
)

for feature in layer.getFeatures():
    context.setFeature(feature)
    value = exp.evaluate(context)
    if exp.hasEvalError():
        raise ValueError(f"Eval error: {exp.evalErrorString()}")
    print(f"Feature {feature.id()}: {value}")

from qgis.core import QgsFeatureRequest

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from qgis.core import (
    edit, QgsExpression, QgsExpressionContext,
    QgsExpressionContextUtils
)

exp = QgsExpression('"population" * 2')
context = QgsExpressionContext()
context.appendScopes(
    QgsExpressionContextUtils.globalProjectLayerScopes(layer)
)

with edit(layer):
    for f in layer.getFeatures():
        context.setFeature(f)
        f['doubled_pop'] = exp.evaluate(context)
        layer.updateFeature(f)

from qgis.core import QgsPalLayerSettings, QgsVectorLayerSimpleLabeling

settings = QgsPalLayerSettings()
settings.fieldName = '"name" || \' (\' || "population" || \')\''
settings.isExpression = True
settings.enabled = True

text_format = settings.format()
text_format.setSize(12)
settings.setFormat(text_format)

labeling = QgsVectorLayerSimpleLabeling(settings)
layer.setLabeling(labeling)
layer.setLabelsEnabled(True)
layer.triggerRepaint()

from qgis.core import QgsProperty

symbol = layer.renderer().symbol()

undefined

from qgis.core import qgsfunction, QgsExpression

@qgsfunction(args='auto', group='Custom', referenced_columns=[])
def population_density(population, area_km2, feature, parent):
    """
    Calculate population density per square kilometer.
    <p>Usage: population_density("pop_field", "area_field")</p>
    """
    if area_km2 and area_km2 > 0:
        return population / area_km2
    return None

---

Global scope        → @qgis_version, @user_full_name, custom global vars
  Project scope     → @project_title, @project_crs, custom project vars
    Layer scope     → @layer_name, @layer_id, layer fields
      Feature scope → Feature attributes, $geometry, $id, $currentfeature

from qgis.core import QgsExpressionContextUtils

---

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**ALWAYS** use the `filter` parameter in aggregates to limit the scope. Aggregating an entire large layer without a filter is a performance bottleneck.

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