ArcMap

UpdateCursor

Summary

The UpdateCursor function creates a cursor that allows you to update or delete rows on the specified feature class, shapefile, or table. The cursor places a lock on the data that will remain until either the script completes or the update cursor object is deleted.

Discussion

Update cursors can be iterated with a for loop or in a while loop using the cursor's next method to return the next row. When using the next method on a cursor to retrieve all rows in a table containing N rows, the script must make N calls to next. A call to next after the last row in the result set has been retrieved returns None, which is a Python data type that acts here as a placeholder.

Using UpdateCursor with a for loop.

import arcpy

fc = "c:/data/base.gdb/roads"
field1 = "field1"
field2 = "field2"

cursor = arcpy.UpdateCursor(fc)
for row in cursor:
    # field2 will be equal to field1 multiplied by 3.0
    row.setValue(field2, row.getValue(field1) * 3.0)
    cursor.updateRow(row)

Using UpdateCursor with a while loop.

import arcpy

fc = "c:/data/base.gdb/roads"
field1 = "field1"
field2 = "field2"

cursor = arcpy.UpdateCursor(fc)
row = cursor.next()
while row:
    # field2 will be equal to field1 multiplied by 3.0
    row.setValue(field2, row.getValue(field1) * 3.0)
    cursor.updateRow(row)
    row = cursor.next()

Syntax

UpdateCursor (dataset, {where_clause}, {spatial_reference}, {fields}, {sort_fields})
ParameterExplanationData Type
dataset

The feature class, shapefile, or table containing the rows to be updated or deleted.

String
where_clause

An optional expression that limits the rows returned in the cursor. For more information on where clauses and SQL statements, see Building a query expression.

String
spatial_reference

Coordinates are specified in the spatial_reference provided and converted on the fly to the coordinate system of the dataset.

SpatialReference
fields

A semicolon-delimited string of fields to be included in the cursor. By default, all fields are included.

String
sort_fields

The fields used to sort the rows in the cursor. Ascending and descending order for each field is denoted by A for ascending and D descending, using the form "field1 A;field2 B".

String

Return Value

Data TypeExplanation
Cursor

A Cursor object that can distribute Row objects.

Code sample

UpdateCursor example

Update field values in a feature class based on another field's value.

import arcpy

# Create update cursor for feature class
rows = arcpy.UpdateCursor("c:/data/base.gdb/roads")

# Update the field used in buffer so the distance is based on the
# road type. Road type is either 1, 2, 3, or 4. Distance is in meters.
for row in rows:
    # Fields from the table can be dynamically accessed from the
    # row object. Here fields named BUFFER_DISTANCE and ROAD_TYPE
    # are used
    row.setValue("BUFFER_DISTANCE", row.getValue("ROAD_TYPE") * 100)
    rows.updateRow(row)

# Delete cursor and row objects to remove locks on the data.
del row
del rows

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