Trying to extract a list of Unique Values from a field using python

Question

I have a number of columns in a number of tables withinh a FGDB where I need to extract the unique values for each column.

For Example: the values may be [1,2,2,2,3,4], and I am trying to return [1,2,3,4]

I could do this job a number of other ways in ARCGIS but I am trying to extend myself.

I have found a piece of python on the web that I think will do the job but I am struggling to get it to run (I keep getting an invalid syntax error as I keep getting the syntax error in line 3) this will no doubt be a really simple user error.

Code Snippet below

import arcpy

def unique_values(r'N:\GISProjects\Landuse\Plant_Biosecurity_Project\ArcGIS_Online.gdb\Holdings_Property_Merge' , 'LU_ALUMMaj'):
with arcpy.da.SearchCursor(table, [field]) as cursor:
    return sorted({row[0] for row in cursor})

This is the Error Message I get from sublime text:

 File "C:\Users\hawkinle\Desktop\STDTAS\Unique_Data.py", line 3
def unique_values(r'N:\GISProjects\Steve_Eastwood_Landuse\Plant_Biosecurity_Project\ArcGIS_Online.gdb\Holdings_Property_Merge' , 'LU_ALUMMaj'):
                                                                                                                             ^
SyntaxError: invalid syntax
[Finished in 0.1s with exit code 1]

Updates from Original Question

I have now updated my code with the answer provided by below but am recieving a secondary error.

New Code Snippet:

import arcpy

def unique_values(table , field):
    with arcpy.da.SearchCursor(table, [field]) as cursor:
        return sorted({row[0] for row in cursor})

myValues = unique_values(r'N:\\GISProjects\\Landuse\\Plant_Biosecurity_Project\\ArcGIS_Online.gdb\\Holdings_Property_Merge' , 'LU_ALUMMaj')

print (myValues)

I am getting a new error message related to a runtime error

Traceback (most recent call last):
File "C:\Users\hawkinle\Desktop\STDTAS\Unique_Data.py", line 7, in <module>
myValues = unique_values(r'N:\\GISProjects\\Steve_Eastwood_Landuse \Plant_Biosecurity_Project\\ArcGIS_Online.gdb\\Holdings_Property_Merge' , 'LU_ALUMMaj')
 File "C:\Users\hawkinle\Desktop\STDTAS\Unique_Data.py", line 4, in unique_values
with arcpy.da.SearchCursor(table, [field]) as cursor:
RuntimeError: cannot open 'N:\\GISProjects\\Steve_Eastwood_Landuse\\Plant_Biosecurity_Project\\ArcGIS_Online.gdb\\Holdings_Property_Merge'

[Finished in 8.0s with exit code 1]

I assume from the reading I have done that this relates to setting the env.workspace ?

Answer 1

You've pretty much got it, you just need to specify the name of your parameters table and field in your function definition, and then pass those values when you call the function. Also watch your indentation, as it's vital for Python.

def unique_values(table , field):
    with arcpy.da.SearchCursor(table, [field]) as cursor:
        return sorted({row[0] for row in cursor})

myValues = unique_values(r'N:\GISProjects\Landuse\Plant_Biosecurity_Project\ArcGIS_Online.gdb\Holdings_Property_Merge' , 'LU_ALUMMaj')

print (myValues)

Basically this is saying that when you call the function unique_values() you'll pass values to two parameters, one called table, the other called field. These are then used in your function. When you call the function, in the line

myValues = unique_values(r'N:\GISProjects\Landuse\Plant_Biosecurity_Project\ArcGIS_Online.gdb\Holdings_Property_Merge' , 'LU_ALUMMaj')  

you are passing the values to these parameters.

This is the same as declaring your parameters separately and passing them to the cursor directly:

table = r'N:\GISProjects\Landuse\Plant_Biosecurity_Project\ArcGIS_Online.gdb\Holdings_Property_Merge'
field = 'LU_ALUMMaj'

with arcpy.da.SearchCursor(table, [field]) as cursor:
    myValues = sorted({row[0] for row in cursor})

print myValues

Answer 2

I would advise using Python's built-in set() function along with a SearchCursor as a generator expression to find the unique values. You'll find this approach extremely efficient with large or small datasets:

import arcpy

fc = r'C:\path\to\your.gdb\featureclass'

unique_values = set(row[0] for row in arcpy.da.SearchCursor(fc, "some_field"))

Answer 3

The following approach was published on https://arcpy.wordpress.com/2012/02/01/create-a-list-of-unique-field-values/ It is using arcpy and numpy and has a smaller memory footprint than the SearchCursor approach.

import arcpy
import numpy

def unique_values(table , field):
    data = arcpy.da.TableToNumPyArray(table, [field])
    return numpy.unique(data[field]).tolist()

myValues = unique_values(r'N:\GISProjects\Landuse\Plant_Biosecurity_Project\ArcGIS_Online.gdb\Holdings_Property_Merge' , 'LU_ALUMMaj')

print (myValues)

Answer 4

Using arcpy.Frequency_analysis() quickly puts all the unique values from a field into a new table, which you can then use for cursor operations. One single command to do what all these other solutions do and faster and easier. As a bonus, you also get a count of how many times each value appears.

Answer 5

open Python window in Arcgis Pro. make sure you have layer in ToC and paste this print(*sorted(list(set(row[0] for row in arcpy.da.SearchCursor("layer name", "Column Name")))), sep="\n")

Source: https://www.youtube.com/watch?v=9Ey9NUNaP7k&t=3s