How can I save a column from the attribute table of a vector layer to a variable?

Question

I am trying to write a python plugin for QGIS that can convert three vector layers into an object File for an urban climate model. Unfortunately I am stuck again with a possibly easy question:

How can I save a column from the attribute table of a vector layer to a variable (for later use)?

For example, I have a layer with buildings (build) with the hight of the buildings in field 35. I want to save all the building hights into a variable (theINPolyHField).

I tried the following:

build = canvas.layer(2)
build_prov = build.dataProvider()

theINPolyHField = build_prov.select([35], build.extent(), False, False) # but returns "None"
build.selectedFeatureCount() # returns 0

What am I doing wrong?

Answer 1

According to the PyQGIS Cookbook all four parameters of select() are optional, so try this

provider.select([35])

Answer 2

After looking around for a while i analyzed another script and wrote me the following function:

def extract_attribute(layer, attribute_index): 
# function to extract a list of attributes from a layer
  qgis.mainWindow().statusBar().showMessage("Extracting layer fields")
  feature = QgsFeature()
  layer.dataProvider().select(layer.dataProvider().attributeIndexes())
  layer.dataProvider().rewind()
  attr_list = []
      while layer.dataProvider().nextFeature(feature):
      attributes = feature.attributeMap()
      attr_list.append(attributes[attribute_index])
return attr_list

There may be easier ways, but i think it does what i want… Thanks!

Answer 3

Just a note:

For QGIS 2.0 and higher this is different:

def extract_attribute(layer, attribute_index):
# function to extract a list of attributes from a layer
  qgis.mainWindow().statusBar().showMessage("Extracting layer fields")
  # Create an empty list to append all attributes
  attr_list = []
  # Iterating over the features (rows) in the vector layer 
  for feature in layer.getFeatures():
    # Iterating over all attributes (columns) of a feature (row)
    for attr in feature.attributes():
      attr_list.append(attr[attribute_index])
return attr_list