QGIS set all layers with custom transparency with python?

Question

I'm exploring QGIS python functions to set all raster layers with a custom transparency, i.e 0,0,0,100 .

Looking at some previous questions. There is a way to loop all the layers, but the aspect to set transparency value on RGB values, I've been having difficulties understanding the documentation.

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

for layer in iface.legendInterface().layers():
   layer.renderer().setOpacity(0.2)

How do I set layer transparency in QGIS 2.0 with Python? The example here uses TransparentSingleValuePixel but i notice there is a TransparentThreevaluePixel

x = QgsRasterTransparency.TransparentSingleValuePixel()
x.pixelValue = 123
x.transparencyPercent = 50
rt.setTransparentSingleValuePixelList( [ x ] )

Answer 1

You want to use a QgsRasterTransparency object then: http://qgis.org/api/2.8/classQgsRasterTransparency.html (add it to the renderer() of the raster layer using setRasterTransparency())

r=iface.mapCanvas().layers()[0]
rt=QgsRasterTransparency()
rt.initializeTransparentPixelList(0,0,0)

QgsMessageLog.logMessage(str(rt.transparentThreeValuePixelList()))
# -> [<qgis._core.TransparentThreeValuePixel object at 0x117dab5a8>]

r.renderer().setRasterTransparency(rt)

voilà: same result as adding them by hand in the layer properties: