pyqgis - How to merge layers into a single QgsVectorLayer?

I have a set of different layers (and their respective shapefiles) with information of electric lines. All the layers have the same Fields in their Attribute Tables, but contain information about different features. I want to merge only the layers that I choose and save them into a new QgsVectorLayer (Or something similar).

To get all of the layers, I'm using this:

    lyrs = self.iface.legendInterface().layers()

And I want to get something like:

    new_layer=lyrs[1]+lyrs[3]
    new_layer.getFeatures() #Do something else from here, but is irrelevant to the question

(Obviously that last part won't work because "TypeError: unsupported operand type(s) for +: 'QgsVectorLayer' and 'QgsVectorLayer'", but that is the idea of what I need to get)

Some details:

1: I would prefer a method that does not create a new shapefile like "qgis:mergevectorlayers" (The method will be called multiple times, and it would create a lot of unnecesary shapefiles)

Answer

I guess a Memory Provider would suite your needs as

It does not store data on disk, allowing developers to use it as a fast backend for some temporary layers.

Based on your descriptions and the linked example from the cookbook I created some code. It is not fully tested and maybe you will have to tweak it a little:

lyrs = self.iface.legendInterface().layers()


fields = lyrs[1].pendingFields()

vl = QgsVectorLayer("LineString", "temporary_lines", "memory")
pr = vl.dataProvider()

for f in fields:
  pr.addAttributes([f])

vl.updateFields()


feats1 = lyr[1].getFeatures()
for feature in feats1:
  pr.addFeatures([feature])

feats3 = lyr[3].getFeatures()
for feature in feats3:
  pr.addFeatures([feature])

vl.updateExtents()


# Do something with it