How to Check if a Layer is PRESENT or ABSENT in a Project via PyQGIS?

Question

In QGIS, what is the simplest python code (least possible) to verify if a particular layer is PRESENT or ABSENT in the Map Layers list?

NOTE: The script shouldn't trip an error IF the layer is ABSENT.

Answer 1

len(QgsMapLayerRegistry.instance().mapLayersByName('the_Map_Layer_in_question')) != 0

Trueif layer exists, otherwise False

Answer 2

for those stumbling over this question; The accepted answer works for Qgis 2.x.

In Qgis 3.x the functionality of QgsMapLayerRegistry has been moved to QgsProject, so in pyqgis3 you need this (Documented here in the QGIS API Documentation) :

len(QgsProject.instance().mapLayersByName("the_Map_Layer_in_question")) != 0

Best,

Peter

Answer 3

 boolean = False
 layers = QgsMapLayerRegistry.instance().mapLayers()
 for name, layer in layers.iteritems():
     if layer.name() == ('the_Map_Layer_in_question'):
         boolean = True    
 # if the map layer exists boolean == True, if it doesn't boolean == False