Why does ListLayoutElements give IndexError: list index out of range?

Question

When trying to create a legend on ArcMap use the following Python/ArcPy code I received an error message:

import arcpy
mxd = arcpy.mapping.MapDocument(r"E:\cities\MyProject.mxd")
df = arcpy.mapping.ListDataFrames(mxd)[0]
lyr1 = arcpy.mapping.Layer(r"E:\cities\cities.shp")
lyr2 = arcpy.mapping.Layer(r"E:\cities\region.shp")
lyr3 = arcpy.mapping.Layer(r"E:\cities\lakes.shp")
lyr4 = arcpy.mapping.Layer(r"E:\cities\rivers.shp")
legend = arcpy.mapping.ListLayoutElements(mxd,"LEGEND_ELEMENT")[0]
legend.autoAdd = True
arcpy.mapping.AddLayer(df, lyr1, "BOTTOM")
arcpy.mapping.AddLayer(df, lyr2, "BOTTOM")
arcpy.mapping.AddLayer(df, lyr3, "BOTTOM")
arcpy.mapping.AddLayer(df, lyr4, "BOTTOM")
legend.adjustColumnCount(2)
mxd.save()

and I get this message :

Traceback (most recent call last):
 File "E:\cities\scripts\Lgend.py", line 8, in <module>
 legend = arcpy.mapping.ListLayoutElements(mxd,"LEGEND_ELEMENT")[0]
IndexError: list index out of range

What does this indicate?

• Nathan W: Do you have a legend in your document?
• user24541: -1 down vote accept no i haven't a legend in the map document ,i want to add it but it dosn't work ,somthing is wrong in the code!!

Answer 1

I think are misreading the API docs (if you have read them):

Returns a Python list of layout elements that exist within a map document (.mxd) layout.

This means if it doesn't exist you can't use this method.

legend = arcpy.mapping.ListLayoutElements(mxd,"LEGEND_ELEMENT") is returning a empty list so doing [0] will give you a index error.

• user24541: sorry but i don't understand what do u mean,can u explain please ?
• Nathan W: It will only give you back what is already in the map document, if it's not there you can't get it back. If you want to create a new one you need to use a different method.
• user24541: like what? can u help me please ?i'm a beginner in python .
• PolyGeo: @user24541 You need to use ArcMap to insert a legend in your layout before arcpy.mapping can access it. It is not possible to create a legend element using ArcPy.

Answer 2

It's not obvious to a python beginner but code like foo.method()[0] is calling foo.method and then it returns a list and [0] is indexing into the list. It is shorthand for this

 legendlist = arcpy.mapping.ListLayoutElements(mxd, "LEGEND_LIST")
 legend = legendlist[0]

If there are no elements of the type LEGEND_ELEMENT then the method ListLayoutElements returns an empty list, so indexing the empty list with [0] fails. The suggested method for python is to use a "try" block so the script would continue even without a legend.

try:
 legend = arcpy.mapping.ListLayoutElements(mxd,"LEGEND_ELEMENT")[0]
 legend.autoAdd = True
 legend.adjustColumnCount = 2
except IndexError:
 print("This map does not have any legends.")
# add your layers here

You could also use an "if" statement to test the returned list (for example "if len(legendlist)...") but all the python "best practices" docs indicate a try block is preferred.