Enumeration of a cursor

blake.terhune

Xander, I recently unlocked the mysteries of enumeration​ and noticed your cnt variable does something similar. Because I'm still learning, I thought something like this would work instead:

with arcpy.da.SearchCursor(fc_house, flds_house) as curs_in:
    for cnt, row_in in enumerate([i for i in curs_in], start=1):
        curs_in.reset()  ## Return cursor back to the first row after enumeration
        if cnt % 25 == 0:
            print "Processing connection: {0}".format(cnt)

        pnt1 = row_in[0]
        parcel_id = row_in[1]
        date_sale = row_in[2]
        fire_oid = row_in[3]
        # Continue processing rows in cursor...

After some tinkering, I realized that when you enumerate a cursor like this, it goes through all the rows. The problem is that the cursor object ends with no more rows and you have to call reset() on the cursor to start it back at the first row again. Since there is still more processing to be done, you'll essentially be iterating over all rows in the cursor twice with enumeration instead of once with your original counter variable. Do you think the extra time it takes to create the enumeration of the cursor is worth it in a case like this? Maybe only for cursors with a small number of rows?

JoshuaBixby

The enumerate function isn't exhausting the cursor, at least not the first time, the list comprehension is doing it and requiring the cursor to be reset.  Try restructuring your code as follows:

with arcpy.da.SearchCursor(fc_house, flds_house) as curs_in:
   for cnt, row_in in enumerate(curs_in, start=1):
       if cnt % 25 == 0:
           print "Processing connection: {0}".format(cnt)
       pnt1 = row_in[0]
       parcel_id = row_in[1]
       date_sale = row_in[2]
       fire_oid = row_in[3]
       # Continue processing rows in cursor...

The enumerate function operates against "a sequence, an iterator, or some other object which supports iteration."  The data access cursors support iteration, which is also what allows us to use them with a for statement.

Whereas generator expressions are evaluated lazily, comprehensions are not, which means your list comprehension needs to fully iterate over the cursor before returning any and all values in a list.  It is for this reason you are having to reset your cursor before doing any processing.  Since cursors already support iteration, just pass them directly to enumerate and don't bother building a list of the entire cursor.