# calculate field (prefer Python) [closed]

I have an attribute field with string data values (e.g. "P0001","C3492","00123"). I need to replace them so that they're all unique and begin with "W" followed by digits totaling 5 characters.

I thought perhaps ranking may work but not sure how to rank a string. The resulting output will look like ("W0001","W0002","W0003"). I prefer a Python solution in the field calculator but can also use VB.

## closed as off-topic by PolyGeo♦Dec 7 '16 at 0:54

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If I understand your question correctly, I would suggest using a python generator in combination with an `arcpy.da.UpdateCursor` First you would define your generator function. Something like this:

``````def get_unique_identifier(n):
num = 0
for i in xrange(n):
while num < n:
yield 'W{0:04}'.format(num)
num += 1
``````

Then you can use an `update cursor` like this to update the proper field with the value generated by the function/generator above:

``````in_feature_class = r'C:\path_to_data\your_data.shp'
field_to_update = 'the_field'
number_of_records = arcpy.GetCount_management(in_feature_class)
unique_values = get_unique_identifier(number_of_records)
with arcpy.da.UpdateCursor(in_feature_class, field_to_update) as u_cur:
for row in u_cur:
row[0] = unique_values.next()
u_cur.updateRow(row)
``````

This is a quick go and there are certainly things that could be improved upon. If the order in which these fields are updated matters you may want to utilize an `ORDER BY` field in the `sql_clause` when creating your update cursor so that you update the values in the sorted order that you want. However this should get you on your way.

• Instead of a function, you could also do `for i,row in enumerate(u_cur):...u_cur.update('W{0:04}'.format(i))` -- Exact same thing and you can define the start value as well. – Paul Dec 7 '16 at 0:55