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.
The arcpy.mapping module 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 as is provided by the Add Data button in ArcMap, which positions a layer in the data frame based on geometry type and layer weight rules.
When adding a layer to a map document, the layer must reference an existing layer found in a layer file on disk, the same map document and data frame, the same map document with a different data frame, or a completely separate map...