Getting layer extent in PyQGIS?

The answer is almost completely contained in a post I recently wrote.

The extent is returned as a QgsRectangle() object with the following code:

layer = iface.activeLayer() # load the layer as you want
ext = layer.extent()

For getting the coordinates of the vertices for the current layer, you may run this code:

xmin = ext.xMinimum()
xmax = ext.xMaximum()
ymin = ext.yMinimum()
ymax = ext.yMaximum()
coords = "%f,%f,%f,%f" %(xmin, xmax, ymin, ymax) # this is a string that stores the coordinates

Finally, you may run the following code for using the r.neighbors module from the Python Console:

processing.runalg("grass7:r.neighbors",layer,0,3,False,False,"",coords,0,None)

you were on the right track you just need to go a step further. See the QGIS documentation for QgsRectangle

Basically, you do:

• get the current Layer

  layer = iface.activeLayer()
  

• get the extent which is a QgsRectangle object

  ex = layer.extent()
  

• and there extract the Values with:

  xmax = ex.xMaximum()
  ymax = ex.yMaximum()
  xmin = ex.xMinimum()
  ymin = ex.yMinimum()