Introduction to the Update Cursor (arcpy.da) at 10.1

The UpdateCursor creates a read-write access to records returned from a feature class or table.
Returns an iterator of lists. The order of values in the list matches the order of fields specified by the field_names argument.  UpdateCursor objects can be iterated using a for loop and it supports with statements. Using a with statement will ensure that the database locks are removed.


You can use both an insert and update cursor at the same time if an edit session is opened.

Example:

from arcpy import da
fc = r"c:\temp\samples.gdb\nests"
fields = ('BirdPop', 'Rank')

with da.UpdateCursor(fc, fields) as cursor:
    for row in cursor:
        if (row[0] >= 0 and row[0] <= 10):
            row[1] = 1
        elif (row[0] > 10 and row[0] <= 20):
            row[1] = 2
        elif (row[0] > 20 and row[0]<= 30):
            row[1] = 3
        elif (row[0] > 20):
            row[1] = 4
        cursor.updateRow(row)
In this simple example, you see that you can use conditional statements to change values within the 'with' statement.

Enjoy