How can I set the transparency for all layers within a group of layers?

I assume this will require the use of a FOR loop in the Python Console.

Based on How to set the transparency for multiple layers or add a global transparancy preference?

I tried the following:

for layer in iface.legendInterface().layers():
   layer.renderer().setOpacity(0.5)

But I only get this error:

Traceback (most recent call last):
  File "<input>", line 2, in <module>
AttributeError: 'QgsVectorLayer' object has no attribute 'renderer'

What am I doing wrong?