Populating attributes while creating polygons in for-loop from CSV using PyQGIS

Question

I have a working script that builds rectangles from a CSV using @Kadir Sahbaz's answer and @Ben W's answer

I'm wanting to pass non-geometry attributes as well. This runs and creates the fields but with NULL values:

srcFile = "D:\\Data\\filename.csv"

layer = QgsVectorLayer("Polygon?crs=EPSG:2193", "Polygons", "memory")
QgsProject.instance().addMapLayer(layer)

provider = layer.dataProvider()
provider.addAttributes([QgsField('ViewName',QVariant.String, '', 50, 0),
                        QgsField('FigName',QVariant.String, '', 10, 0)])
layer.updateFields()

with open(srcFile,'r') as infile:
    rect = infile.readlines()
    
for b in rect[1:]:
    parts = b.split(',')

    #### Coordinates coming from CSV ####
    xMin = float(parts[2])
    yMin = float(parts[3])
    xMax = float(parts[4])
    yMax = float(parts[5])

    rect = QgsRectangle(xMin, yMin, xMax, yMax)
    polygon = QgsGeometry.fromRect(rect)
    
    feature = QgsFeature()
    feature.setGeometry(polygon)

    provider.addFeatures([feature]) 
 
    fields = QgsFields()
    feature.setFields(fields)
    feature['ViewName'] = parts[0]
    feature['FigName'] = parts[-2]

    provider.addAttributes(fields)
    layer.updateFields()

I'm still getting NULL values. FWIW...if I run @Ben W's code, change the with statement to:

with edit(layer):
    print(parts[0],parts[-2])
    provider.addFeatures([feature])
    t = provider.changeAttributeValues({feature.id(): {f1: parts[0], f2: parts[-2]}})
    print(t)

The values from the CSV are appearing and .changeAttributeValues appears to running properly:

Answer 1

You set feature fields to empty QgsFields.

fields = QgsFields() # fields is empty
feature.setFields(fields) # you set the empty fields

Instead, pass layer.fields() to QgsFeature.

feature = QgsFeature(layer.fields())

You also get NULL values, because you add the feature to the layer (provider.addFeatures([feature])) before you populate the fields.

Use this script:

srcFile = "D:\\Data\\filename.csv"

layer = QgsVectorLayer("Polygon?crs=EPSG:2193", "Polygons", "memory")
QgsProject.instance().addMapLayer(layer)

provider = layer.dataProvider()
provider.addAttributes([QgsField('ViewName',QVariant.String, '', 50, 0),
                        QgsField('FigName',QVariant.String, '', 10, 0)])
layer.updateFields()

with open(srcFile,'r') as infile:
    rows = infile.readlines()

for r in rows:
    parts = r.split(',')

    xMin = float(parts[2])
    yMin = float(parts[3])
    xMax = float(parts[4])
    yMax = float(parts[5])

    rect = QgsRectangle(xMin, yMin, xMax, yMax)
    polygon = QgsGeometry.fromRect(rect)
    
    feature = QgsFeature(layer.fields())
    feature.setGeometry(polygon)
    feature['ViewName'] = parts[0]
    feature['FigName'] = parts[-2]
    provider.addFeatures([feature]) 

Sample CSV:

1,0, 1804512.8556, 5453390.58088, 1813512.8556, 5461390.58088, yedi1, sekiz1, dokuz1
2,0, 1814512.8556, 5473390.58088, 1823512.8556, 5471390.58088, yedi2, sekiz2, dokuz2

Result:

Answer 2

Personally, I prefer to use the changeAttributeValues() method of QgsVectorDataProvider (see API docs here) as I find it more reliable. This method takes an attributes map which is basically a dictionary where the key is a feature id and the value is a second dictionary comprising field indexes as keys and the new attributes as values.

I have tested this starting with a csv file with the following structure:

After running the following code:

srcFile = 'D:\\Data\\filename.csv'

layer = QgsVectorLayer("Polygon?crs=EPSG:2193", "Polygons", "memory")
QgsProject.instance().addMapLayer(layer)
provider = layer.dataProvider()
provider.addAttributes([QgsField('ViewName',QVariant.String, '', 50, 0),
                        QgsField('FigName',QVariant.String, '', 10, 0)])
layer.updateFields()
f1 = layer.fields().lookupField('ViewName')
f2 = layer.fields().lookupField('FigName')
fid = 1

with open(srcFile,'r') as infile:
    csv_lines = infile.readlines()[1:]

for line in csv_lines:
    line = line.split(',')
    #### Coordinates coming from CSV ####
    xMin = float(line[2])
    yMin = float(line[3])
    xMax = float(line[4])
    yMax = float(line[5])

    rect = QgsRectangle(xMin, yMin, xMax, yMax)
    polygon = QgsGeometry.fromRect(rect)
    
    feature = QgsFeature()
    feature.setGeometry(polygon)
    
    with edit(layer):
        provider.addFeatures([feature])
        atts = {f1: line[0], f2: line[-2]}
        provider.changeAttributeValues({fid: atts})
        fid += 1

The resulting layer and attribute table looks like this:

Answer 3

I have had this problem too. You have:

vl = QgsVectorLayer("Polygon?crs=EPSG:2193", "Polygons", "memory")

Maybe add the attribute definitions into the uri.

uri = 'LineString?crs= EPSG:2193&field=id:integer&field=name:String(40)&field=age:double'
layer = QgsVectorLayer(uri, "Polygons", "memory")
pr = vl.dataProvider()
pr.addFeatures(features)
pr.updateExtents()
vl.updateExtents()

The features array would contain features that have both geometry and attributes defined.

In a loop:

feature = QgsFeature()
ID = 1
name = 'Dog'
age = 4.36
g = QgsGeometry.fromPolylineXY(points)
a = [int(ID), name, age]
feature.setGeometry(g)
feature.setAttributes(a)
features.add(feature)

• This way works reliably for me.
• if you get the uri syntax wrong, you usually get NULL attribute values
• Every time I have a problem, it has always been with the uri syntax not matching the real attributes (make sure an int is actually an int, as shown above)