QGIS/python/plugins/processing/algs/qgis/ExtractByExpression.py

# -*- coding: utf-8 -*-

"""
***************************************************************************
    ExtractByExpression.py
    ---------------------
    Date                 : October 2016
    Copyright            : (C) 2016 by Nyall Dawson
***************************************************************************
*                                                                         *
*   This program is free software; you can redistribute it and/or modify  *
*   it under the terms of the GNU General Public License as published by  *
*   the Free Software Foundation; either version 2 of the License, or     *
*   (at your option) any later version.                                   *
*                                                                         *
***************************************************************************
"""

__author__ = 'Nyall Dawson'
__date__ = 'October 2016'
__copyright__ = '(C) 2016, Nyall Dawson'

# This will get replaced with a git SHA1 when you do a git archive

__revision__ = '$Format:%H$'

import processing
from qgis.core import QgsExpression, QgsVectorLayer, QgsFeatureRequest
from processing.core.GeoAlgorithmExecutionException import GeoAlgorithmExecutionException
from processing.core.parameters import ParameterVector
from processing.core.outputs import OutputVector
from processing.core.GeoAlgorithm import GeoAlgorithm
from processing.core.parameters import ParameterString, ParameterExpression
from processing.tools import dataobjects


class ExtractByExpression(GeoAlgorithm):

    INPUT = 'INPUT'
    EXPRESSION = 'EXPRESSION'
    OUTPUT = 'OUTPUT'

    def defineCharacteristics(self):
        self.name, self.i18n_name = self.trAlgorithm('Extract by expression')
        self.group, self.i18n_group = self.trAlgorithm('Vector selection tools')

        self.addParameter(ParameterVector(self.INPUT,
                                          self.tr('Input Layer')))
        self.addParameter(ParameterExpression(self.EXPRESSION,
                                              self.tr("Expression"), parent_layer=self.INPUT))
        self.addOutput(OutputVector(self.OUTPUT, self.tr('Extracted (expression)')))

    def processAlgorithm(self, progress):
        layer = dataobjects.getObjectFromUri(self.getParameterValue(self.INPUT))
        expression_string = self.getParameterValue(self.EXPRESSION)
        writer = self.getOutputFromName(self.OUTPUT).getVectorWriter(layer.fields(), layer.wkbType(), layer.crs())

        expression = QgsExpression(expression_string)
        if not expression.hasParserError():
            req = QgsFeatureRequest().setFilterExpression(expression_string)
        else:
            raise GeoAlgorithmExecutionException(expression.parserErrorString())

        for f in layer.getFeatures(req):
            writer.addFeature(f)

        del writer