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Arcpy.mapping.AddLayer selection Funk (serious ESRI devs only)

Question asked by josepharmbruster0lat on Feb 5, 2018
Latest reply on Feb 15, 2018 by bixb0012
Hey everyone,
It's been ages since i've had to resort to the forums, but the time has come...  This is issue is occurring with:
10.3.0.4322
AND
10.5.1.7333
Problem: When a layer is added to the TOC, just as you see in the snippet below, feature selections will no longer be respected (or at least detected) by geoprocessing tools.
Description: The symptom of the problem is that obtaining the feature selection count of a geoprocessing layer in a python toolbox is inconsistent.  I've pinned it down to some issue with the way Arc (or ArcPy) is handling layers by Name.  Here are some snippets:
# make this an absolute path to some feature class in a geodatabase, you only need a minimum of two features to reproduce the issue, i'm using polygons.

path = #path to your polygon feature class with at least two features in it.
mxd = arcpy.mapping.MapDocument('CURRENT')
new_layer_name = os.path.basename(path)
arcpy.MakeFeatureLayer_management(path, new_layer_name)
new_layer = arcpy.mapping.Layer(new_layer_name)
arcpy.mapping.AddLayer(mxd.activeDataFrame, new_layer_name, 'BOTTOM')
del new_layer
del mxd
Use the code above in a geoprocessing tool to add a layer to the map.

Now, use the code below in a separate geoprocessing tool, to report the number of features selected in the map.
desc = arcpy.Describe(input_feature_class)
if desc.FIDSet:
 messages.addMessage('selection made')
 # you'll never get here with the layer above!!
else:
 messages.addMessage('no selection made')
 # you'll always get here even with a selection :-(
# count will always be the entire feature class count, since no selection was registered
# to the layer....
feature_count = int(arcpy.GetCount_management(input_feature_class).getOutput(0)

Notes:
- If you use the code above to add the feature class to the map.  Remove it from the toc (right click, remove) and add it using arc catalog (drag-drop), the feature selection will STILL not work.  Hence why I suggest it has something to do with the internal name resolution.
- If you start a new ArcMap instance, add the layer by drag-dropping it from Arc Catalog, the selection WILL work.  If you remove this layer and add it using the code above, all subsequent selections will NOT be respected.

I look forward to hearing from ESRI support.
Let me know if you have any questions,
Joseph Armbruster
0Lat Inc, Orlando Florida.

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