Add legend with ArcPy over my interpolated map

Question

I have created an interpolated map with arcpy.IDW_ga and I am exporting it to PDF with arcpy.mapping.ExportToPDF. How can I add a legend to the map with ArcPy? In the end, I want to apply the code to create 100 maps, so manually is not a solution. I know I can just go to Insert>> Legend, but I need an automated solution.

arcpy.IDW_ga(in_features = in_features, 
             z_field= z_field, out_ga_layer="", 
             out_raster="Interpolation/year/" + name_save, 
             cell_size="6.46298456489016E-03", 
             power="2", 
             search_neighborhood="NBRTYPE=Standard S_MAJOR=0.336780807796406 S_MINOR=0.336780807796406 ANGLE=0 NBR_MAX=15 NBR_MIN=10 SECTOR_TYPE=ONE_SECTOR", 
             weight_field="")

mxd = arcpy.mapping.MapDocument("CURRENT")
df = arcpy.mapping.ListDataFrames(mxd, "Layers")[0]  
arcpy.mapping.ExportToPDF(mxd,  "C:/Users/"mypdf.pdf")

Answer 1

Create/save the map document with a legend. When you add your layer it will automatically be added to the legend as long as Add a new item to the legend when a new layer is added to the map is selected.

You can 'click this box' via arcpy as well.

legend = arcpy.mapping.ListLayoutElements(mxd, "LEGEND_ELEMENT")[0]
legend.autoAdd = True

Full code to add your raster result:

#get mxd object
mxd = arcpy.mapping.MapDocument (r"path\to\your\mxd.mxd")
#get data frame
df = arcpy.mapping.ListDataFrames (mxd) [0]

#get legend
legend = arcpy.mapping.ListLayoutElements(mxd, "LEGEND_ELEMENT")[0]
#add new items to legend
legend.autoAdd = True

#IDW
arcpy.IDW_ga(in_features = in_features, 
             z_field= z_field, out_ga_layer="", 
             out_raster="Interpolation/year/" + name_save, 
             cell_size="6.46298456489016E-03", 
             power="2", 
             search_neighborhood="NBRTYPE=Standard S_MAJOR=0.336780807796406 S_MINOR=0.336780807796406 ANGLE=0 NBR_MAX=15 NBR_MIN=10 SECTOR_TYPE=ONE_SECTOR", 
             weight_field="")
#create raster layer
rastLyr = arcpy.MakeRasterLayer_management("Interpolation/year/" + name_save, 'rast') [0]
#add raster layer - it will be added to the legend as well
arcpy.mapping.AddLayer (df, rastLyr)