Making SQL join query for NULL in search cursor in ArcPy?

Question

I am trying to learn how to do some error catching/avoiding in ArcPy. Here is a simplified version of what I would like to do. I would like to take some points, and then check to see if there is a line in a line feature class with a matching ID to the point, and the same thing to match each point to a polygon. If each point doesn't have a matching line and polygon, I want to skip that point and move on to the next. Can someone tell me what I'm doing wrong? Here is an example script showing how I am trying to do this:

# Import arcpy module
import sys, os, time, arcpy, smtplib
from arcpy import env
from arcpy.sa import *

env.overwriteOutput = True

env.workspace = r"D:\WorkingDirectory\Geoprocessing\Work.gdb"
PointFC = r"D:\WorkingDirectory\Geoprocessing\Work.gdb\points"
LineFC = r"D:\WorkingDirectory\Geoprocessing\Work.gdb\lines"
PolygonFC = r"D:\WorkingDirectory\Geoprocessing\Work.gdb\points"

Line_idField = arcpy.AddFieldDelimiters(LineFC, line_id)
Poly_idField = arcpy.AddFieldDelimiters(PolygonFC, poly_id)


try:
    rows = arcpy.SearchCursor(PointFC)
    for row in rows:
        Pt_id = row.pt_id
        PTpoly_id = row.poly_id
        PTline_id = row.line_id

        #This section if for skipping null values to help prevent crashes
        arcpy.MakeFeatureLayer_management (PolygonFC,  "Polylyr")
        arcpy.AddJoin_management(row, "poly_id","Polylyr", "poly_id","KEEP_COMMON")
        #This is where I get an error - it doesn't like the IS
        PolyIDnull = Poly_idField IS NULL
        arcpy.AddMessage('\nThis site does not have a polygon : ' + str(row.pt_id))
        if PolyIDnull:
            continue

        arcpy.RemoveJoin_manaement(row, "Polylyr")

        arcpy.MakeFeatureLayer_management (LineFC,  "Linelyr")
        arcpy.AddJoin_management(row, "line_id","Lineylyr", "line_id","KEEP_COMMON")
        LineIDnull = Line_idField IS NULL
        arcpy.AddMessage('\nThis site does not have a line : ' + str(row.pt_id))
        if LineIDnull:
            continue
        arcpy.RemoveJoin_manaement(row, "Linelyr")


except Exception, e:
import traceback
map(arcpy.AddError, traceback.format_exc().split("\n"))
arcpy.AddError(str(e))`

Answer 1

I see two things straight away that are going to make your interpreter spit grits at you:

# This is where I get an error - it doesn't like the IS
        PolyIDnull = Poly_idField IS NULL

You need a test here, not a statement. What you did is not a test for a value, but actually setting the value. Test like so:

# I'm unsure of this. NULL in Python is None, but a null value may actually be an
# empty string in this case (if PTpoly_id = "") .
if (PTpoly_id = None):
    arcpy.AddMessage('\nThis site does not have a polygon : ' + str(row.pt_id))

The other problem I already addressed in the above code. IF statements should (usually) be followed by a colon and a line break, with the conditional statements indented inside of it.

After taking a careful look at your intended process and the code, I must admit that I am baffled as to why you are going about it as you are. Given what you wish to do, I would recommend doing two inner joins (keep only matching records)- once from the point file to the line layer, and again from that resulting file to the polygon layer. That should give you only the points which appear in both. However, if you only wish to see which appear in one OR the other, simply do two left joins (keeping all records) instead. What you have here is quite a confusing workflow that I cannot help but feel is unnecessary.