PyQGIS: How can I load a specific layer from a file containing multiple

Question

I have a vector file that contains multiple layers, if I open it in QGIS I get the "Select vector layers to add dialog" and I can choose one and it loads fine.

However in a .py script if I load it via

layer = QgsVectorLayer(filename, "mylayer", "ogr")

I just get one of the layers (layer 0)

How do I specify which layer I want to load? Or if it IS actually loading all of them, how do I change to the one I want?

My full script

import os
import sys
import qgis
from qgis.core import *
from qgis.gui import *
from PyQt4.QtCore import *
from PyQt4.QtGui import *
import ogr

qgs = QgsApplication([], False)
QgsApplication.setPrefixPath( r"C:\Program Files\QGIS 2.18\apps\qgis-ltr", True )
QgsApplication.initQgis()


filename = "C:/datafiles/sourcefile"
if os.path.exists(filename):
    print('found it')


layer = QgsVectorLayer(filename, "mylayer", "ogr")
if layer.isValid(): 
    print("Layer was loaded successfully!")
else:  
    print("Layer failed to load!")
    sys.exit()

print (layer.type())
print (layer.name())
print (layer.id()) 

print('finished')
qgs.exitQgis()

Answer 1

...and then five minutes later I stumble across the answer here iface.addVectorLayer on a gml file causes "Select vector layers to add" dialog (pyqgis)

You add

|layername=NameOfTheLayerYouWantToLoad

to the file path, giving:

filename = "C:/datafiles/sourcefile|layername=NameOfTheLayerYouWantToLoad"

Answer 2

Welcome to GIS.SE!

If you know the sublayer name already:

layer_path = '/path/to/base_layer.gpx' # GPX is just an example
sublayer_name = 'tracks'
uri = "{}|layername={}".format(layer_path, sublayer_name)
sublayer = QgsVectorLayer(uri, 'layer name', 'ogr')

If you want to know sublayer names given a layer (I'm not sure if there is a better way, though):

if len(layer.dataProvider().subLayers()) > 1:
    for subLayer in layer.dataProvider().subLayers():
        print(subLayer.split("!!::!!")[1])

Which will print something like:

sublayer_name_0
sublayer_name_1
sublayer_name_2
sublayer_name_3

Extracted from an array like this:

['0!!::!!sublayer_name_0!!::!!2!!::!!MultiLineString!!::!!geom', 
 '1!!::!!sublayer_name_1!!::!!4!!::!!MultiPolygon!!::!!geom', 
 '2!!::!!sublayer_name_2!!::!!4!!::!!LineString!!::!!geom', 
 '3!!::!!sublayer_name_3!!::!!12!!::!!LineString!!::!!geom']