Take values from one field, and concatenate into string field if IDs are the same.

[Server]

Hello, 

Fairly new to python. By no means a beginner, but I still have trouble with logic.

I'm currently trying to create a script which takes an ID field - if the IDs match up, then take the value from a name field, and concatenate together into a list, and fill out a concatenated list field (see table)

I know i will have to use a update_cursor and a counter etc. the logic for whatever reason is escaping me on how to assemble the python. 

ID	Name	Name_cancat
33	hello	Hello, hi, bye
33	hi	Hello, hi, bye
33	bye	Hello, hi, bye
44	red	Red, tan, blue
44	tan	Red, tan, blue
44	blue	Red, tan, blue
88	dog	Dog, cat
88	cat	Dog, cat

I know it will be something like this (obviously missing a ton of code/logic)

with arcpy.da.UpdateCursor("FC", ['ID', 'name', 'name_concat']) as cursor:
       myList = []
       for row in cursor:

              myList.append(row[1])

       row[2] = myList

       cursor.updateRow(row)


   any help pointing me to a resource would be greatly appreciated!

[DonMorrison1]

 I would make 2 passes through the table.

On pass one use a search cursor to create a python dict with each entry having the ID as the key and the list of string to be concatenated as the value.

On the second pass use an update cursor to  populate the name_cancat field in each record by using the value of the ID field to get the concatenated string from the dict.

[curtvprice]

How about the Merge tool with a field map with a "," delimiter to combine the values, then a Join Field (Maybe faster for a large dataset.)

[JohannesLindner · Accepted solution]

Python:

# use a SearchCursor to get ids and names
ids_and_names = [row for row in arcpy.da.SearchCursor("FC", ["ID", "name"])]

# use an UpdateCursor to update the concatenated names
with arcpy.da.UpdateCursor("FC", ["ID", "name_concat"]) as cursor:
    for row in cursor:
        names = [name for id, name in ids_and_names if id == row[0]]
        name_concat = ", ".join(names)
        cursor.updateRow([row[0], name_concat])

You could also use the field calulator with Arcade:

// filter the dataset by the current id
var id = $feature.ID
var features_with_this_id = Filter($featureset, "ID = @ID")
// extract names as array
var names = []
for(var f in features_with_this_id) {
    Push(names, f.name)
}
// return the concatenated array
return Concatenate(names, ", ")

[Server]

this is exactly what I needed! Thank you!