Using Select Layer By Attribute in for loop with ArcPy?

Question

I'm trying to iterate through a feature class and select each feature one by one by using the OBJECTID field. When I use a single value, the SelectLayerByAttribute works, but how do I compare the OBJEECTID to a variable that increases in count through the loop. The program is then suppose to create a layer for each feature in the feature class. This is what I have so far:

import arcpy
import os
arcpy.env.workspace = "C:\Users\Daimon Nurse\Desktop\DFMPROJECT\DFMPROJECT.gdb"
arcpy.env.overwriteOutput=True
inlayer = "C:\Users\Daimon Nurse\Desktop\DFMPROJECT\DFMPROJECT.gdb\ZonesPolyline"


with arcpy.da.SearchCursor(inlayer, ("OID@", "SHAPE@AREA")) as cursor:
    i = 1
    for row in cursor:
        print("Feature {0} has an area of {1}".format(row[0], row[1]))
        outlayer = "ZonePolygon"
        arcpy.MakeFeatureLayer_management (inlayer, outlayer)
        arcpy.SelectLayerByAttribute_management (outlayer, "NEW_SELECTION", """           "OBJECTID" = 1 """)
        output = r'C:\Users\Daimon Nurse\Desktop\Grounds Project\DFMGROUNDS.gdb'
        outfile = os.path.join (output, i)
        arcpy.CopyFeatures_management(outlayer, outfile)
        print i
        i = i + 1

Answer 1

Instead of using a where_clause of:

"""           "OBJECTID" = 1 """

I think you should try:

'"OBJECTID" = {0}'.format(i)

Also, instead of:

outfile = os.path.join (output, i)

try:

outfile = os.path.join(output, "fc{0}".format(i))

What I have used in both of the above is Python string formatting. Python strings can be delimited using single or double quotes. I used single in the first so as not to clash with double quotes that indicated the field name. In the second I used double quotes because I think they look better.

I added "fc" to your feature class name because I don't think ArcGIS will like a feature class name that is, or starts with, a number.

Also take a look at:

arcpy.env.workspace = "C:\Users\Daimon Nurse\Desktop\DFMPROJECT\DFMPROJECT.gdb"
arcpy.env.overwriteOutput=True
inlayer = "C:\Users\Daimon Nurse\Desktop\DFMPROJECT\DFMPROJECT.gdb\ZonesPolyline"

which needs to escape the single backslashes in your pathnames e.g.:

arcpy.env.workspace = r"C:\Users\Daimon Nurse\Desktop\DFMPROJECT\DFMPROJECT.gdb"
arcpy.env.overwriteOutput=True
inlayer = r"C:\Users\Daimon Nurse\Desktop\DFMPROJECT\DFMPROJECT.gdb\ZonesPolyline"

Answer 2

I think you should leverage the 'where clause' of the MakeFeatureLayer. Here is an extract of a much larger script that does most of what you want to do:

desc = arcpy.Describe(InFC)
OIDfield = desc.OIDFieldName

with arcpy.da.SearchCursor(InFC,[OIDfield,"SHAPE@AREA"]) as SCur:
    for Ft in SCur:
        arcpy.MakeFeatureLayer_management(InFC,"Layer","%s = %d" % (OIDfield,Ft[0]))
        # do some stuff
        arcpy.Delete_management("Layer") # remove the layer for each iteration

Note: the search cursor uses square brackets on the fields, not parenthesis: SearchCursor(inlayer, ("OID@", "SHAPE@AREA")) is incorrect, SearchCursor(inlayer, ["OID@", "SHAPE@AREA"]) is correct.

I get the OIDFieldName using code (via Describe in Table Properties) so it will work for shapefiles and geodatabase feature classes, OID@ should also work but I can't say for sure - I've always done it that way as I need the field name for the query.

Notes on string formatting:

PolyGeo has shown one sort of format using '"OBJECTID" = {0}'.format(i), I mostly use the % style of formatting '"OBJECTID" = %d' % i. Either way works; I prefer the % formatting as it's more like C and I'm used to reading it that way.