Hi Manny,
My explanation was a little short, sorry for that... and creating these type of "one-liners" doesn't allow for easy interpretation.
Let's break it down into pieces:
# let's say we have a list with duplicate values
list1 = [1, 3, 5, 7, 3, 2, 5, 6, 8, 7, 5, 4, 3, 2, 6, 2]
# the set() is used to convert it to a set of unique values
set1 = set(list1)
print set1
# returns: set([1, 2, 3, 4, 5, 6, 7, 8])
# if you want to convert it to a list again (to loop over it)
list2 = list(set1)
print list2
# returns [1, 2, 3, 4, 5, 6, 7, 8]
A set is very powerful to get a unique list of values and combining sets allows you to obtain values that are in one list but not in the other and info like this. More on this in my post: Some Python Snippets (scroll down to comparing lists)
To create the list some list comprehensions are used (check out this document I posted: Using Python List Comprehensions ).
Let's break down the code:
unique_list = list(set("{0}_{1}".format(r[0], r[1]) for r in arcpy.da.SearchCursor(FC_or_TBL, (fld_name1, fld_name2, fld_search), where)))
The set() and list() at the beginning of the line have been explained above.
Within the brackets of the set() command a lot is going on. Let's not start from left to right, but with the SearchCursor. It takes a table or featureclass, a tuple (or list) of field names and in this case a where clause. This is pretty standard and I don't think needs much explanation. Normally when you use a search cursor you obtain a cursor object which you use to loop through each row (or feature) in the cursor. Next you probably access the fields in each row and do something with it like this:
with arcpy.da.SearchCursor(FC_or_TBL, (fld_name1, fld_name2, fld_search), where) as curs:
for row in curs:
val_fld1 = row[0]
# etc
In case of the list comprehension this is done on one line. It retrieves each row "r" in the SeachCursor and returns this:
"{0}_{1}".format(r[0], r[1])
string.format() allows you to create a string and pass in variables. It is a pretty need way to combine values and string in a certain format
r[0] refers to the value of fld_name1
r[1] refers to the value of fld_name2
{0} refers to the first parameter specified in the format command
{1} refers to the second parameter specified in the format command
... so it the value for fld_name1 is "abc" and the value in fld_name2 would be 1, this would result in the string "abc_1".
I used something similar for creating the where clause:
where = "{0} = '{1}'".format(arcpy.AddFieldDelimiters(FC_or_TBL, fld_search), search_value)
It starts with a string "{0} = '{1}'". The {0} is replaced with the first argument of the format function:
arcpy.AddFieldDelimiters(FC_or_TBL, fld_search)
This will put quotes or brackets or nothing around the field name based on the type of workspace (that can be useful if you don't know if your input is a shapefiles, personal, file or enterprise geodatabase...
The {1} is replace with the value in the variable search_value. Please note that this where clause will fail if your field is not of type TEXT (string). It is possible to extend this functionality and detect the columns type, but for simplicity reasons (read I was to lazy) I did not do that.
If you have any other doubt, please let me know...
Kind regards,
Xander
