Arcpy getCount_Management doesn't match row count with Cursor

Question

I have some troubles to understand the behaviour of my code below. Obviously, GetCount_Management doesn't match the rows in my layer selection.

This code is part of a long for loop. I have hidden the rest for clarity purposes.

#...

# tmpThi is a in_memory output of CreateThiessenPolygons_analysis
# evtSts is a pandas Serie containing the name of rainfall stations of interest

arcpy.MakeFeatureLayer_management(tmpThi, "thLyr") # creating a layer

evtStsNames = [name for name in evtSts] # creating a list of station of interest

# select the Thiessen polygons where station name 'Nom' is in the list
arcpy.SelectLayerByAttribute_management("thLyr", "NEW_SELECTION", ' "Nom" IN (\'' + '\',\''.join(evtStsNames) + '\')' ) 

rows = arcpy.SearchCursor("thLyr") # Counting selected features with Cursor
rowCount = 0
for row in rows:
    rowCount += 1

print len(evtStsNames), arcpy.GetCount_management("thLyr").getOutput(0), rowCount # This should print the same result

This print a lower GetCount_management as follow:

3 2 3
4 3 4
7 5 7
1 1 1
3 1 3
4 2 4
1 1 1
6 2 6
14 6 14
...

This is quite confusing to me.

It seems that if I store the Thiessen polygons not in "in_memory" getCount match rowCount. I am still trying to understand what really happens

Answer 1

For the searchCursor, the first parameter is a dataset. The selection is thus not taken into account and rowCount will take all features from the feature class.

For CopyFeature() and GetCount(), the first parameter is a layer which will take the selection into account.

If you want to use a query on your feature class inside the cursor, you should write in directly into the cursor :

rows = arcpy.SearchCursor("thLyr", ' "Nom" IN (\'' + '\',\''.join(evtStsNames) + '\')' ) 

by the way, you should consider arcpy.da.SearchCursor instead of arcpy.SearchCursor because it is often more efficient.