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I'm working on a model that will create different buffers for different selections. So I'm using Make Feature Layer and adding an expression to select a subset of features and then using the buffer tool on that subset. But there may be times when a selection doesn't return any features, in which case I wouldn't want to run the buffer. Is there a way in ModelBuilder to test whether or not any features are selected and only run the subsequent step if there are?

Alternatively I could export to a Python script and handle this testing there but I'm not quite sure how to accomplish this in Python either.

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You'd use an iterator if you've already got the expressions to feed in... In combination with that, you need logic Get Count, see this below - ironically in the very 1st picture it shows the very ModelBuilder logic you want to apply before a buffer execution:

Get Count (Data Management) Desktop » Geoprocessing » Tool reference » Data Management toolbox http://resources.arcgis.com/en/help/main/10.2/index.html#//0017000000n7000000

From the model perspective, this can be illustrated very simply. This is a graphic of a working model employing the Get Count precondition technique:

enter image description here

PARCELS is a parcel polygon layer in my open ArcMap document with only 2 tools employed (for simplicity), Get Count and Buffer, similar to the example from the webhelp link above. The reason I simplified the example with this model is to demonstrate the interaction more directly with the PARCELS layer which I've applied a definition query to - then I made 2 model executions, one with the def qry purposely made to return nothing (count = 0) and the other to return 1 feature (which is then essentially a pass filter to go on to the next process, Buffer).

Notice the dotted line representing the 'connector' between Row Count and Buffer - this denotes the Precondition. One way to apply this precondition is to right-click on Buffer (in this case), select properties, click on the Preconditions tab, and select Row Count (toggle checkmark) for the Precondition Variable like so:

enter image description here

(Alternately, you could use the connector tool to connect Row Count to Buffer and in the pop-up select Precondition.)

As for the 2 demos, a def query set to return None (no features) and another to return 1 feature, this is the dialog on None (a false condition):

enter image description here

...and, of course, altering the def query to return something (1 feature, in this case), this is what I call the 'pass logic', allowing Buffer to execute (a true condition):

enter image description here

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I use this function in python to test if a layer has a selection:

def hasSelection(lyr):
    ''' Check whether specified layer has a selection. '''
    import arcpy
    desc = arcpy.Describe(lyr)
    if len(desc.FIDSet) == 0:
        # Layer has no selection
        return False
    else:
        # Layer has a selection
        return True

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