update

code/pyqgis/save_attributes_console.py

@@ -16,15 +16,6 @@
 output_name = 'output.csv'
 output_path = os.path.join(data_dir, output_name)
 
-# We will use QgsVectorFileWriter.create() method
-# It takes the following parameters
-# fileName: Path to the file
-# fields: Fields to write
-# geometryType: geometry type of output file
-# srs: CRS of the output file
-# transformContext: Datum transformation settings
-# options: Save Options such as format, encoding etc.
-
 # Define the options for saving the layer
 save_options = QgsVectorFileWriter.SaveVectorOptions()
 save_options.driverName = 'CSV'
@@ -40,7 +31,7 @@
     fields=layer.fields(),
     geometryType=QgsWkbTypes.NoGeometry,
     srs=layer.crs(),
-    transformContext=transform_context,
+    transformContext=QgsProject.instance().transformContext(),
     options=save_options)
 
 # Check if we were able to create the writer

code/pyqgis/save_attributes_processing.py

@@ -2,54 +2,63 @@
 from qgis.core import (QgsProcessing,
                        QgsProcessingAlgorithm,
                        QgsProcessingParameterFeatureSource,
+                       QgsProcessingParameterEnum,
                        QgsProcessingParameterFileDestination,
-                       QgsWkbTypes)
+                       QgsVectorFileWriter,
+                       QgsWkbTypes,
+                       QgsProject)
 
 
 class SaveAttributesAlgorithm(QgsProcessingAlgorithm):
     """Saves the attributes of a vector layer to a CSV file."""
-    OUTPUT = 'OUTPUT'
-    INPUT = 'INPUT'
 
     def initAlgorithm(self, config=None):
         self.addParameter(
             QgsProcessingParameterFeatureSource(
-                self.INPUT,
-                self.tr('Input layer'),
+                'INPUT',
+                'Input layer',
                 [QgsProcessing.TypeVectorAnyGeometry]
             )
         )
 
         # We add a file output of type CSV.
         self.addParameter(
             QgsProcessingParameterFileDestination(
-                self.OUTPUT,
-                self.tr('Output File'),
+                'OUTPUT',
+                'Output File',
                 'CSV files (*.csv)',
             )
         )
 
     def processAlgorithm(self, parameters, context, feedback):
-        source = self.parameterAsSource(
+        layer = self.parameterAsVectorLayer(
             parameters,
-            self.INPUT,
+            'INPUT',
             context)
-
-        sink, output = self.parameterAsSink(
-            parameters,
-            self.OUTPUT,
-            context,
-            source.fields(),
-            QgsWkbTypes.NoGeometry,
-            source.sourceCrs()
-            )
 
-        fieldnames = [field.name() for field in source.fields()]
+        output = self.parameterAsFileOutput(
+            parameters,
+            'OUTPUT',
+            context)
 
         # Compute the number of steps to display within the progress bar and
         # get features from source
-        total = 100.0 / source.featureCount() if source.featureCount() else 0
-        features = source.getFeatures()
+        total = 100.0 / layer.featureCount() if layer.featureCount() else 0
+        features = layer.getFeatures()
+
+        # Define the options for saving the layer
+        save_options = QgsVectorFileWriter.SaveVectorOptions()
+        save_options.driverName = 'CSV'
+        save_options.fileEncoding = 'UTF-8'
+
+        # Create the writer
+        writer = QgsVectorFileWriter.create(
+            fileName=output,
+            fields=layer.fields(),
+            geometryType=QgsWkbTypes.NoGeometry,
+            srs=layer.crs(),
+            transformContext=QgsProject.instance().transformContext(),
+            options=save_options)
 
 
         for current, f in enumerate(features):
@@ -58,12 +67,12 @@ def processAlgorithm(self, parameters, context, feedback):
                 break
 
             # Add a feature in the sink
-            sink.addFeature(f)
+            writer.addFeature(f)
 
             # Update the progress bar
             feedback.setProgress(int(current * total))
 
-        return {self.OUTPUT: output}
+        return {'OUTPUT': output}
 
     def name(self):
         return 'save_attributes'

code/pyqgis/solutions/exercise8.py

@@ -0,0 +1,64 @@
+import os
+import time
+
+data_dir = os.path.join(os.path.expanduser('~'), 'Downloads', 'pyqgis_masterclass')
+
+layer = iface.activeLayer()
+
+# Check if a layer is selected
+if not layer:
+    iface.messageBar().pushMessage('Please select a layer',  level=Qgis.Critical) 
+# Check if the selected layer is a vector layer
+if layer.type() != QgsMapLayer.VectorLayer:
+    iface.messageBar().pushMessage('Please select a vector layer',  level=Qgis.Critical)
+
+# Define parameters for QgsVectorFileWriter
+output_name = 'output.csv'
+output_path = os.path.join(data_dir, output_name)
+
+# We will use QgsVectorFileWriter.create() method
+# It takes the following parameters
+# fileName: Path to the file
+# fields: Fields to write
+# geometryType: geometry type of output file
+# srs: CRS of the output file
+# transformContext: Datum transformation settings
+# options: Save Options such as format, encoding etc.
+
+# Define the options for saving the layer
+save_options = QgsVectorFileWriter.SaveVectorOptions()
+save_options.driverName = 'CSV'
+save_options.fileEncoding = 'UTF-8'
+# We can also add some format-specific layer options 
+# These come from GDAL/OGR
+# https://gdal.org/drivers/vector/csv.html
+save_options.layerOptions = ['SEPARATOR=COMMA']
+
+# Write the file
+writer = QgsVectorFileWriter.create(
+    fileName=output_path,
+    fields=layer.fields(),
+    geometryType=QgsWkbTypes.NoGeometry,
+    srs=layer.crs(),
+    transformContext=transform_context,
+    options=save_options)
+
+# Check if we were able to create the writer
+if writer.hasError() != QgsVectorFileWriter.NoError:
+    iface.messageBar().pushMessage(
+        'Error:', writer.errorMessage, level=Qgis.Critical)
+
+selected_count = layer.selectedFeatureCount()
+if selected_count == 0:
+    iface.messageBar().pushMessage(
+        'Error:', 'No selected features', level=Qgis.Critical)
+else:
+    for f in layer.selectedFeatures():
+        writer.addFeature(f)
+
+    # delete the writer to flush features to disk
+    del writer
+    iface.messageBar().pushMessage(
+    'Success:',
+    f'Output file with {selected_count} features written at {output_path}',
+    level=Qgis.Success)

docs/code/pyqgis/save_attributes_console.py

@@ -16,15 +16,6 @@
 output_name = 'output.csv'
 output_path = os.path.join(data_dir, output_name)
 
-# We will use QgsVectorFileWriter.create() method
-# It takes the following parameters
-# fileName: Path to the file
-# fields: Fields to write
-# geometryType: geometry type of output file
-# srs: CRS of the output file
-# transformContext: Datum transformation settings
-# options: Save Options such as format, encoding etc.
-
 # Define the options for saving the layer
 save_options = QgsVectorFileWriter.SaveVectorOptions()
 save_options.driverName = 'CSV'
@@ -40,7 +31,7 @@
     fields=layer.fields(),
     geometryType=QgsWkbTypes.NoGeometry,
     srs=layer.crs(),
-    transformContext=transform_context,
+    transformContext=QgsProject.instance().transformContext(),
     options=save_options)
 
 # Check if we were able to create the writer

docs/code/pyqgis/save_attributes_processing.py

@@ -2,54 +2,63 @@
 from qgis.core import (QgsProcessing,
                        QgsProcessingAlgorithm,
                        QgsProcessingParameterFeatureSource,
+                       QgsProcessingParameterEnum,
                        QgsProcessingParameterFileDestination,
-                       QgsWkbTypes)
+                       QgsVectorFileWriter,
+                       QgsWkbTypes,
+                       QgsProject)
 
 
 class SaveAttributesAlgorithm(QgsProcessingAlgorithm):
     """Saves the attributes of a vector layer to a CSV file."""
-    OUTPUT = 'OUTPUT'
-    INPUT = 'INPUT'
 
     def initAlgorithm(self, config=None):
         self.addParameter(
             QgsProcessingParameterFeatureSource(
-                self.INPUT,
-                self.tr('Input layer'),
+                'INPUT',
+                'Input layer',
                 [QgsProcessing.TypeVectorAnyGeometry]
             )
         )
 
         # We add a file output of type CSV.
         self.addParameter(
             QgsProcessingParameterFileDestination(
-                self.OUTPUT,
-                self.tr('Output File'),
+                'OUTPUT',
+                'Output File',
                 'CSV files (*.csv)',
             )
         )
 
     def processAlgorithm(self, parameters, context, feedback):
-        source = self.parameterAsSource(
+        layer = self.parameterAsVectorLayer(
             parameters,
-            self.INPUT,
+            'INPUT',
             context)
-
-        sink, output = self.parameterAsSink(
-            parameters,
-            self.OUTPUT,
-            context,
-            source.fields(),
-            QgsWkbTypes.NoGeometry,
-            source.sourceCrs()
-            )
 
-        fieldnames = [field.name() for field in source.fields()]
+        output = self.parameterAsFileOutput(
+            parameters,
+            'OUTPUT',
+            context)
 
         # Compute the number of steps to display within the progress bar and
         # get features from source
-        total = 100.0 / source.featureCount() if source.featureCount() else 0
-        features = source.getFeatures()
+        total = 100.0 / layer.featureCount() if layer.featureCount() else 0
+        features = layer.getFeatures()
+
+        # Define the options for saving the layer
+        save_options = QgsVectorFileWriter.SaveVectorOptions()
+        save_options.driverName = 'CSV'
+        save_options.fileEncoding = 'UTF-8'
+
+        # Create the writer
+        writer = QgsVectorFileWriter.create(
+            fileName=output,
+            fields=layer.fields(),
+            geometryType=QgsWkbTypes.NoGeometry,
+            srs=layer.crs(),
+            transformContext=QgsProject.instance().transformContext(),
+            options=save_options)
 
 
         for current, f in enumerate(features):
@@ -58,12 +67,12 @@ def processAlgorithm(self, parameters, context, feedback):
                 break
 
             # Add a feature in the sink
-            sink.addFeature(f)
+            writer.addFeature(f)
 
             # Update the progress bar
             feedback.setProgress(int(current * total))
 
-        return {self.OUTPUT: output}
+        return {'OUTPUT': output}
 
     def name(self):
         return 'save_attributes'

docs/code/pyqgis/solutions/exercise10.py

@@ -0,0 +1,35 @@
+layer_id = '[%@layer_id%]'
+fid = [% $id %]
+
+layer = QgsProject.instance().mapLayer(layer_id)
+
+def get_neighbors(fid):
+    f = layer.getFeature(fid)
+    # Use list comprehension to get all intersecting features
+    # You may also use touches() if your data is topologically correct
+    # Supply the bounding box to getFeatures() to use Spatial Index
+    neighbors = [
+        c.id()
+        for c in layer.getFeatures(f.geometry().boundingBox())
+        if c.geometry().intersects(f.geometry()) and c.id() != f.id()
+    ]
+    return neighbors
+
+
+first_degree_neighbors = get_neighbors(fid)
+
+second_degree_neighbors = set()
+
+for n in first_degree_neighbors:
+    neighbors = get_neighbors(n)
+    second_degree_neighbors.update(neighbors)
+
+# Remove all first-degree neighbors from the set
+second_degree_neighbors = second_degree_neighbors.difference(
+    set(first_degree_neighbors))
+
+# Remove the feature itself from the set if it exists
+second_degree_neighbors.discard(fid)
+
+# Apply the selection
+layer.selectByIds(list(second_degree_neighbors), QgsVectorLayer.AddToSelection)