How to batch clip to single-part shapefile using ArcGIS 9.3 and Python 2.5?

Question

How do I clip a shapefile to a single-part shapefile using ArcGIS 9.3.1 and Python 2.5?

Example: I have a set of lines (lines.shp) I am trying to clip to each record in a shapefile called Grids.shp In grids.shp there is a field called [NAME] with a record for each grid; grid-1, grid-2, ...

The grids represent Map extents, and the lines will be roads/streams/other base layers (could not post base layers publicly so have provided Lines.shp as an example).

I have been trying to use the SearchCursor method but have some trouble defining objects and whatnot as I am new to Python. I got this same process to work in V10 but the V10 syntax is way different/simpler, especially for cursors and uses Python 2.6

EDIT : Clip_wCursor_v2a.py ESRI Forum with python script attached.

# ---------------------------------------------------------------------------
# Clip_wCursor_v2.py
# Created for ArcGIS 9.3.1
#   (clip generated by ArcGIS/ModelBuilder)
# Attempted cursor mods by a newb, Donovan
# ---------------------------------------------------------------------------

# Import system modules
import sys
import string
import os
import arcgisscripting

# Create the Geoprocessor object
gp = arcgisscripting.create(9.3)

# Load required toolboxes...
gp.AddToolbox("C:/Program Files/ArcGIS/ArcToolbox/Toolboxes/Analysis Tools.tbx")

# Designate a workspace
gp.workspace = r"C:\gis\data"

# Assign variables...
clipto = "grids.shp" # grids to clip to
clipit = "lines.shp" # features to be clipped

# Cursor time...
rows = gp.SearchCursor(grids)
row = rows.Next()
feat = row.Shape # used as the clip_features in the Clip_analysis
while row:
    n = str(row.NAME) # assign a variable for the processing message based on a field
    print "clipping to: " +n # tells you what grid was clipped
    # Clip_analysis <in_features> <clip_features> <out_feature_class> {cluster_tolerance}
    # output format, grid_1_c.shp, grid_2_c.shp, etc...
    gp.Clip_analysis(clipit, feat, str(row.NAME)+"_c"+".shp", "") #update str if necessary
    row=rows.next()

# reset the array...
del rows
del gp

print "...fin..."
print "no error checking happened, examine your workspace..."

appreciate all the input,

I could only mark one as the correct answer, and i considered the fact that my original error was because of the lack of a while and improper use of row in the clip function, although the makefeaturelayer method is definitely valid!

Answer 1

Since your cursor isn't iterable, use a while loop instead of a for loop:

rows = gp.SearchCursor(grids)
row = rows.Next()
while row: 
    ...do something with each row...
    row = rows.Next()

Answer 2

For one thing, I am pretty sure that when using a for loop, .next() is not required, since it automatically iterates through the object. In the case of while, however, the .next() method is necessary to continue moving through the rows. I'm not sure if your use of row as a reference to the .next() method is a problem in the iteration you start with the loop, but I would consider changing the row in the loop to something else for readability if nothing else. Also, your clip should be inside the loop if you want it to process for every row. Have you considered simply using Make Feature Layer to create a layer to clip against, using a unique key (like the object id) for layer creation?

Assuming you have 100 grids....

clipby = grids+"_LYR"
for id in xrange(0,101):
    gp.MakeFeatureLayer_management (grids, clipby, " [object_id] ="+id)
    gp.Clip_analysis(lines, clipy, Output, "")
    gp.Delete_management(clipby)  

A bit hackish, maybe, but it's worked for me when I want to avoid cursors (which I almost always do).

Update For your new problem,

Traceback (most recent call last):
  File "C:\gis\script\Clip_wCursor_v2.py", line 26, in <module>
    gp.Clip_analysis(lines, row, str(row.NAME)+"_c"+".shp", "")
RuntimeError: Object: Error in executing tool  

This is because the row object is not a valid feature to clip by (it is a cursor object). If I am reading the post from ESRI correctly, you need to create a reference to the row's shape, which can be done by grid = row.SHAPE. If you change your clip line to

grid = row.SHAPE
gp.Clip_analysis(lines, grid, str(row.NAME)+"_c"+".shp", "")  

Everything should work.

P.S. Create the row.SHAPE reference inside your while loop. It may work outside, but let's play it safe.

Answer 3

Remember that in Python, indentation is everything, including variable scope. So even if the cursor were iterable, gname would only be in scope during the loop (which effectively does nothing), then when you get to the Output = line, Python will complain that gname doesn't exist.

In addition, instead of creating a grid shapefile and using that as a cookie cutter, you could do it in code using, for example, this Python recipe. This would be more useful if you had to cut up vector data on a regular basis.