Update field of attribute table of a shape in selected rows with arcpy

Question

I read the post Accessing attribute table within shapefile and replace values?, but I don't know how to include a "Where clause".

I want to update the attribute table of a shapefile. In SQL the Code looks like this:

alter table point_shape
 add buffer number;
update point_shape
 set buffer = a/2
 where a >0; 
alter table line_shape
 add buffer number;
update line_shape
 set buffer = a/2
 where a >0; 
alter table point_shape
 add buffer number;
update point_shape
 set buffer = sqrt(b)/acos(-1)
 where b is not null and b >0;
alter table line_shape
 add buffer number;
update line_shape
 set buffer = b/shape_len
 where b is not null and a = 0 and b>0;

I translated the first update statement:

cur = arcpy.UpdateCursor(point_shape)
for row in cur:
 row.setValue('buffer', row.getValue('a') / 2)
 cur.updateRow(row)

But where can I include the where statement?

• Midavalo: What version of ArcGIS are you running?

Answer 1

Use the newer data-access cursor, they are faster. For example the UpdateCursor:

UpdateCursor establishes read-write access to records returned from a feature class or table

And an if statement:

import arcpy
point_shape=r'C:\path\shapefile.shp'
with arcpy.da.UpdateCursor(point_shape,['buffer','a']) as cursor:
 for row in cursor:
 if row[1]>0: #row[1] = the 'a' field
 row[0]=row[1]/2 #row[0] = the 'buffer' field
 cursor.updateRow(row) #update the 'buffer' field with new value