Returning unique value from table using ArcPy?

Question

Using Arcpy in Python, I would like to return / print '0.15' from the following table:

When I use the following code I get "(u'D', 0.20)" as my print statement in Python:

import arcpy
fc = "C:/Users/.../table_test2"
q = 'C'
with arcpy.da.SearchCursor(fc, ['Index_Score'], where_clause=q) as cursor:
    print row


(u'D', 0.20)

This is not what I am looking for as I am looking for the value '0.15'.

When I tried changing my query statement 'q' to: q = '"SA_ALL" = "C"' I get the error:

" Traceback (most recent call last):
File "<pyshell#143>", line 2, in <module>
for row in cursor:
RuntimeError: An expected Field was not found or could not be retrieved properly.    [table_test2]"

Can someone help me and explain what I am doing wrong? Also what does the 'u' mean in my first returned print statement?

Answer 1

There are a couple of issues that are occurring here. First of all formatting of a query string can be a bit tedious, the delimiter you use for the field name is dependent on the input data type (shapefile, personal geodatabase, file geodatabase). See additional documentation. For a file geodatabase the delimiter is a double quote, but the string delimiter is a single quote, to do this you will need to use escape characters or string formatting.

Try this:

'"SA_ALL" = \'C\''

The second issue is accessing the data element you want out of the cursor object. Add a for loop to iterate the rows returned in your cursor, even though you may only return one row. The returned object will be a list, so index the position of the item in that list. If you are still running into a format issue, use str(row[0]) or float(row[0]) to cast the output into the format you want.

with arcpy.da.SearchCursor(fc, ['Index_Score'], '"SA_ALL" = \'C\'') as cursor:
    for row in cursor:
        print row[0]

Answer 2

You might look into using the AddFieldDelimiters function. It can make writing queries easier.

import arcpy

fc = "C:/Users/.../table_test2"
query = """{0} = '{1}'""".format(arcpy.AddFieldDelimiters(fc, "SA_ALL"), "C")

with arcpy.da.SearchCursor(fc, ("SA_ALL", "Index_Score"), query) as cursor:
    for row in cursor:
        print row[1]

Using just "C" is not a proper query.

Answer 3

This example uses a dictionary comprehension to convert your two fields into a dictionary and then simply prints the value based on your query check["C"]:

import arcpy

fc = r'C:\temp\test.gdb\test'

check = {row[0]: row[1] for row in arcpy.da.SearchCursor(fc, ("SA_ALL", "Index_Score"))}
print check["C"]

To answer the second part of your question, u'' indicates a unicode string data type.