Expressions, Filtering and Calculating Values¶

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).

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
geometry functions: $area, $length
conversion functions: to int, to real, to string

And 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

Compatibility note: mathematical, geometry, conversion functions and power operator ^ are available from QGIS 1.4.

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¶

>>> exp = QgsExpression('1 + 1 = 2')
>>> exp.hasParserError()
False
>>> exp = QgsExpression('1 + 1 = ')
>>> exp.hasParserError()
True
>>> exp.parserErrorString()
PyQt4.QtCore.QString(u'syntax error, unexpected $end')

Evaluating Expressions¶

Basic Expressions¶

>>> exp = QgsExpression('1 + 1 = 2')
>>> value = exp.evaluate()
>>> value.toInt()
(1, True)

Expressions with features¶

The following example will evaluate the given expression against a feature. “Column” is a name of the field in the layer.

>>> exp = QgsExpression('Column = 99')
>>> value = exp.evaluate(feature, layer.pendingFields())
>>> value.toBool()
True

You can also use QgsExpression.prepare() if you need check more then one feature. Using QgsExpression.prepare() will increase the speed that evaluate takes to run.

>>> exp = QgsExpression('Column = 99')
>>> exp.prepare(layer.pendingFields())
>>> value = exp.evaluate(feature)
>>> value.toBool()
True

Hanlding errors¶

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

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

value.toInt()

Examples¶

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

def where(layer, exp):
  print "Where"
  exp = QgsExpression(exp)
  if exp.hasParserError():
    raise Expection(exp.parserErrorString())
  exp.prepare(layer.pendingFields())
  feature = QgsFeature()
  layer.select(layer.pendingAllAttributesList())
  while layer.nextFeature(feature):
    value = exp.evaluate(feature)
  if exp.hasEvalError():
    raise ValueError(exp.evalErrorString())
    if value.toBool():
      yield feature

layer = qgis.utils.iface.activeLayer()
for f in where(layer, 'Test > 1.0'):
  print f + " Matches expression"

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