[FEATURE][processing] New algorithm to truncate tables

Irretrievably deletes all features from a table... use with caution!

Author: nyalldawson

python/plugins/processing/algs/help/qgis.yaml

@@ -540,6 +540,11 @@ qgis:texttofloat: >
 qgis:translate: >
   This algorithm moves the geometries within a layer, by offsetting them with a specified x and y displacement.
 
+qgis:truncatetable: >
+  This algorithm truncates a layer, by deleting all features from within the layer.
+
+  Warning - this algorithm modifies the layer in place, and deleted features cannot be restored!
+
 qgis:union: >
   This algorithm creates a layer containing all the features from both input layers. In the case of polygon layers, separate features are created for overlapping and non-overlapping features. The attribute table of the union layer contains attribute values from the respective input layer for non-overlapping features, and attribute values from both input layers for overlapping features.
 

python/plugins/processing/algs/qgis/QGISAlgorithmProvider.py

@@ -187,6 +187,7 @@
 from .ShortestPathLayerToPoint import ShortestPathLayerToPoint
 from .ServiceAreaFromPoint import ServiceAreaFromPoint
 from .ServiceAreaFromLayer import ServiceAreaFromLayer
+from .TruncateTable import TruncateTable
 
 pluginPath = os.path.normpath(os.path.join(
     os.path.split(os.path.dirname(__file__))[0], os.pardir))
@@ -255,7 +256,7 @@ def __init__(self):
                         RasterCalculator(), Heatmap(), Orthogonalize(),
                         ShortestPathPointToPoint(), ShortestPathPointToLayer(),
                         ShortestPathLayerToPoint(), ServiceAreaFromPoint(),
-                        ServiceAreaFromLayer()
+                        ServiceAreaFromLayer(), TruncateTable()
                         ]
 
         if hasMatplotlib:

python/plugins/processing/algs/qgis/TruncateTable.py

@@ -0,0 +1,59 @@
+# -*- coding: utf-8 -*-
+
+"""
+***************************************************************************
+    TruncateTable.py
+    -----------------------
+    Date                 : January 2017
+    Copyright            : (C) 2017 by Nyall Dawson
+    Email                : nyall dot dawson at gmail dot com
+***************************************************************************
+*                                                                         *
+*   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__ = 'January 2017'
+__copyright__ = '(C) 2017, Nyall Dawson'
+
+# This will get replaced with a git SHA1 when you do a git archive
+
+__revision__ = '$Format:%H$'
+
+from processing.core.GeoAlgorithm import GeoAlgorithm
+from processing.core.parameters import ParameterTable
+from processing.core.outputs import OutputVector
+from processing.core.GeoAlgorithmExecutionException import GeoAlgorithmExecutionException
+from processing.tools import dataobjects
+
+
+class TruncateTable(GeoAlgorithm):
+
+    INPUT = 'INPUT'
+    OUTPUT = 'OUTPUT'
+
+    def defineCharacteristics(self):
+        self.name, self.i18n_name = self.trAlgorithm('Truncate table')
+        self.tags = self.tr('empty,delete,layer,clear,features')
+        self.group, self.i18n_group = self.trAlgorithm('Vector general tools')
+
+        self.addParameter(ParameterTable(self.INPUT,
+                                         self.tr('Input Layer')))
+        self.addOutput(OutputVector(self.OUTPUT,
+                                    self.tr('Truncated layer'), True))
+
+    def processAlgorithm(self, progress):
+        file_name = self.getParameterValue(self.INPUT)
+        layer = dataobjects.getObjectFromUri(file_name)
+        provider = layer.dataProvider()
+
+        if not provider.truncate():
+            raise GeoAlgorithmExecutionException(
+                self.tr('Could not truncate table.'))
+
+        self.setOutputValue(self.OUTPUT, file_name)

python/plugins/processing/tests/testdata/expected/truncated.cpg

@@ -0,0 +1 @@
+UTF-8

python/plugins/processing/tests/testdata/expected/truncated.dbf

@@ -0,0 +1 @@
+GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]

python/plugins/processing/tests/testdata/expected/truncated.prj

@@ -0,0 +1 @@
+GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]]

python/plugins/processing/tests/testdata/expected/truncated.qpj

@@ -2118,3 +2118,15 @@ tests:
         type: vector
         in_place_result: true
 
+  - algorithm: qgis:truncatetable
+    name: Truncate table
+    params:
+      INPUT:
+        name: custom/points.shp
+        type: vector
+        in_place: true
+    results:
+      INPUT:
+        name: expected/truncated.shp
+        type: vector
+        in_place_result: true