Calculating line lengths using PyQGIS

From the QGIS Documents: Geometry Handling, you can use the following code to get the length of any selected line(s):

layer = qgis.utils.iface.activeLayer()
features = layer.selectedFeatures()
for f in features:
    geom = f.geometry()
    print "Length:", geom.length()

Great code, however, this only works for the selected layer and only prints it. With some help from other posts and @Joseph I turned it into a code that adds an attribute to all layers in your project with the length.

from PyQt4.QtCore import QVariant

for layer in QgsMapLayerRegistry.instance().mapLayers().values():
    features = layer.getFeatures()
    for f in features:
    geom = f.geometry()
    leng = geom.length()
    res = layer.dataProvider().addAttributes([QgsField("Length", QVariant.Int)])
    fieldIndex = layer.dataProvider().fieldNameIndex("Length")
    attrFeatMap = {}
    attrMap = {fieldIndex: leng}
    for feature in layer.getFeatures():
        attrFeatMap[] = attrMap