Getting all results from Selection made by Attribute or Location using ArcPy?

Question

Using arcpy I'm trying to get the results of a selection, or what is currently selected as a list.

Basically I'm attempting to build an addin which on click will either perform an operation where I invoke SelectLayerByAttribute_management (or ByLocation) to select some features, or the user performs a selection ByAttribute (or ByLocation), and while they have their selection hightlighted executes my snippet by pressing a button on the toolbar. In each case I want to get a list of results to perform a specific task on only certain selected features.

I can't find out how to get a list of the currently selected features, or find a listener for selection having been made so I can enable the button, and ensure I can perform my operation on an existing selection.

Answer 1

To iterate over the layers in the map, get a reference to the current map document and list the layers within it:

mxd = arcpy.mapping.MapDocument("CURRENT")
layers = arcpy.mapping.ListLayers(mxd):
for layer in layers:
   # do something with 'layer'

To just get the number of selected features, try leveraging the Describe object's fidSet property:

desc = arcpy.Describe(layer)
sel_count = len(desc.fidSet.split(";"))

You can check this count to determine whether any features are selected, or just do if desc.fidSet: which should accomplish the same.

To get a particular field's values for each selected feature within a layer, use a SearchCursor (either the arcpy.SearchCursor or arcpy.da.SearchCursor variety), which honors the selection on layers. See the other questions linked in the comments above.

Interactive session in ArcMap's Python window to demonstrate how you might do this:

>>> mxd = arcpy.mapping.MapDocument("CURRENT")
>>> layers = arcpy.mapping.ListLayers(mxd)
>>> for layer in layers:
...     desc = arcpy.Describe(layer)
...     if desc.fidSet:
...         print layer.name, "has {0} features selected:".format(len(desc.fidSet.split(';')))
...         for row in arcpy.da.SearchCursor(layer, "*"):
...             print row
...             
atlantic_hurricanes_2000 has 2 features selected:
(220, (-86.60000000039969, 30.500000000299735), datetime.datetime(2000, 9, 22, 0, 0), u'12:00:00', 30.5, -86.6, 1006.0, 35.0, u'tropical storm', u'Helene', datetime.datetime(2000, 9, 22, 12, 0))
(221, (-85.40000000009991, 31.599999999900035), datetime.datetime(2000, 9, 22, 0, 0), u'18:00:00', 31.6, -85.4, 1010.0, 25.0, u'tropical depression', u'Helene', datetime.datetime(2000, 9, 22, 18, 0))
atlantic_hurricanes_2000_lines has 1 features selected:
(8, (-70.38282674742592, 29.09090367649801), 84.23001922499401, u'Helene', 34.76190476190476, None)

Answer 2

I haven't tried this, but you could call ListLayers, then cycle through all possible layers and use the getSelectedExtent method, which returns an Extent object; if the extent isn't null/zero, the layer is selected. Then you'd know which layer the user has selected, and you can use a cursor or CopyFeatures_management on them. See the following links:

Layer (arcpy.mapping)

Extent (arcpy)

Hope this helps.