[QGIS-Developer] How to retrieve all "virtual fields" of a layer?

[Stefan Keller]

Hi,

In a Python plugin I want to retrieve information about all
attributes/fields of a layer
In the cookbook [1] it's described how to retrieve thems
  for field in layer.pendingFields():
      print field.name(), field.typeName()

=> But how do I get all "virtual fields" programmatically?

:Stefan

[1] http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/vector.html#retrieving-information-about-attributes