Expressões, FIltros e Cálculo de Valores¶

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 Expressões 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

>>> 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')

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

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

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

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

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

print value

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"