Displaying buffer in new layer using PyQGIS

Question

I'm trying to create a new layer, in which the buffers are displayed. I tried this code, but it does not work. Where is the error?

layer = iface.activeLayer()
buff_layer = QgsVectorLayer("Polygon?crs=epsg:4326","buffer", "memory")
bufferDist = 10

for feat in layer.getFeatures():
  geom = feat.geometry()
  buff = geom.buffer(bufferDist,5)
  feat.setGeometry(buff)
  buff_layer.addFeature(feat)   

QgsProject.instance().addMapLayers([buff_layer])

Answer 1

Firstly, for every feature, you have to create a new QgsFeature and set its geometry and then add it to the layer.

Solution 1: Edit Session

layer = iface.activeLayer()
buff_layer = QgsVectorLayer("Polygon?crs=epsg:4326", "buffer", "memory")
bufferDist = 10

# start edit session
buff_layer.startEditing()
for feat in layer.getFeatures():
  geom = feat.geometry()
  buff = geom.buffer(bufferDist, 5)

  # create a new empty feature
  f = QgsFeature()
  f.setGeometry(buff)
  buff_layer.addFeature(f)

# commit changes to the layer
buff_layer.commitChanges()

QgsProject.instance().addMapLayer(buff_layer)

Solution 2: Using Data Provider

layer = iface.activeLayer()
buff_layer = QgsVectorLayer("Polygon?crs=epsg:4326", "buffer", "memory")
bufferDist = 10

for feat in layer.getFeatures():
  geom = feat.geometry()
  buff = geom.buffer(bufferDist, 5)

  # create a new empty feature
  f = QgsFeature()
  f.setGeometry(buff)

  ### 
  buff_layer.dataProvider().addFeatures([f])
  ###

QgsProject.instance().addMapLayer(buff_layer)