There will be many
situations where you will need to add a layer to a map document. The mapping module provides this functionality through the
AddLayer() function. In this recipe, you will learn how to add a layer to a map document using this function.
arcpy.mapping provides the ability to add layers or group layers into an existing map document file. You can take advantage of the ArcMap "auto-arrange" functionality, which automatically places a layer in the data frame for visibility. This is essentially the same functionality provided by the Add Data button in ArcMap, which positions a layer in the data frame based on geometry type and layer weight rules.
Layers can't be added to a layer file ...