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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). 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

>>> 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
1

Expressions with features

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

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

You can also use QgsExpression.prepare() if you need check more than 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)
>>> bool(value)
True

Handling errors

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

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

print value

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 Exception(exp.parserErrorString())
  exp.prepare(layer.pendingFields())
  for feature in layer.getFeatures():
    value = exp.evaluate(feature)
    if exp.hasEvalError():
      raise ValueError(exp.evalErrorString())
    if bool(value):
      yield feature

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