Create an Iterator with a selection

Question

I'm blocked with creation of an iterator in my script. I want coordinates of selected points. If I do that, it works, it print all points coordinates

##point=vector
layer = processing.getObject(point)
it = layer.getFeatures()
for i in it:
    x = i.geometry().asPoint().x()
    y = i.geometry().asPoint().y()
    print "X Coord :  %f" %x
    print "Y Coord :  %f" %y
    print

So after I add my selection. But it crash.

##point=vector
layer = processing.getObject(point)
nbr = QgsExpression( "ID LIKE '%-1'" )
itnbr = layer.getFeatures( QgsFeatureRequest( nbr ) )
for i in itnbr:
    x = i.geometry().asPoint().x()
    y = i.geometry().asPoint().y()
    print "X Coord :  %f" %x
    print "Y Coord :  %f" %y
    print

I have an OGR error message : invalid data source

What causes that?

Answer 1

Issue is probably related to type field of your shapefile. Generally, ID values are numerical and you are using a Wildcard Character (%) with a minus sign (-) for your search. It's only logical if shapefile has a string field in attributes table. To test my approach I used a shapefile point layer (25 features) with ID field and following version of your code:

import processing

point = '/home/zeito/pyqgis_data/random_points.shp'

layer = processing.getObject(point)
nbr = QgsExpression( u"ID LIKE '%1'" )
#nbr2 = QgsExpression( u"MARKER_COL LIKE '%-ue'" )

itnbr = layer.getFeatures( QgsFeatureRequest( nbr ) )

for i in itnbr:
    x = i.geometry().asPoint().x()
    y = i.geometry().asPoint().y()
    print "X Coord :  %f" %x
    print "Y Coord :  %f" %y
    print

After running the code at Python Console of QGIS I got:

X Coord :  390945.280000
Y Coord :  4451149.380000

X Coord :  439639.970000
Y Coord :  4455290.050000

X Coord :  366311.250000
Y Coord :  4433366.270000

for IDs 1, 11 and 21; as expected.

Afterward, I modified only one record in attributes table (MARKER_COL field); as it can be observed at following image:

for running following version of your code:

import processing

point = '/home/zeito/pyqgis_data/random_points.shp'

layer = processing.getObject(point)
#nbr = QgsExpression( u"ID LIKE '%1'" )
nbr2 = QgsExpression( u"MARKER_COL LIKE '%-ue'" )

itnbr = layer.getFeatures( QgsFeatureRequest( nbr2 ) )

for i in itnbr:
    x = i.geometry().asPoint().x()
    y = i.geometry().asPoint().y()
    print "X Coord :  %f" %x
    print "Y Coord :  %f" %y
    print

Result was:

X Coord :  390945.280000
Y Coord :  4451149.380000

for 'bl-ue' record (MARKER_COL field); again as expected.