I am writing a script that looks through my inventory, compares it with a master list of all possible inventory items, and tells me what items I am missing. My goal is a .csv file where the first column contains a unique key integer and then the remaining several columns would have data related to that key. For example, a three row snippet of my end-goal .csv file might look like this: 100001,apple,fruit,medium,12,red 100002,carrot,vegetable,medium,10,orange 100005,radish,vegetable,small,10,red The data for this is being drawn from a couple sources. 1st, a query to an API server gives me a list of keys for items that are in inventory. 2nd, I read in a .csv file into a dict that matches keys with item name for all possible keys. A snippet of the first 5 rows of this .csv file might look like this: 100001,apple 100002,carrot 100003,pear 100004,banana 100005,radish Note how any key in my list of inventory will be found in this two column .csv file that gives all keys and their corresponding item name and this list minus my inventory on hand yields what I'm looking for (which is the inventory I need to get). So far I can get a .csv file that contains just the keys and item names for the items that I don't have in inventory. Give a list of inventory on hand like this: 100003,100004 A snippet of my resulting .csv file looks like this: 100001,apple 100002,carrot 100005,radish This means that I have pear and banana in inventory (so they are not in this .csv file.) To get this I have a function to get an item name when given an item id that looks like this: def getNames(id_to_name, ids): return [id_to_name[id] for id in ids] Then a function which gives a list of keys as integers from my inventory server API call that returns a list and I've run this function like this: invlist = ServerApiCallFunction(AppropriateInfo) A third function takes this invlist as its input and returns a dict of keys (the item id) and names for the items I don't have. It also writes the information of this dict to a .csv file. I am using the set1 - set2 method to do this. It looks like this: def InventoryNumbers(inventory): with open(csvfile,'w') as c: c.write('InvName' + ',InvID' + '\n') missinginvnames = [] with open("KeyAndItemNameTwoColumns.csv","rb") as fp: reader = csv.reader(fp, skipinitialspace=True) fp.readline() # skip header invidsandnames = {int(id): str.upper(name) for id, name in reader} invids = set(invidsandnames.keys()) invnames = set(invidsandnames.values()) invonhandset = set(inventory) missinginvidsset = invids - invonhandset missinginvids = list(missinginvidsset) missinginvnames = getNames(invidsandnames, missinginvids) missinginvnameswithids = dict(zip(missinginvnames, missinginvids)) print missinginvnameswithids with open(csvfile,'a') as c: for invname, invid in missinginvnameswithids.iteritems(): c.write(invname + ',' + str(invid) + '\n') return missinginvnameswithids Which I then call like this: InventoryNumbers(invlist) With that explanation, now on to my question here. I want to expand the data in this output .csv file by adding in additional columns. The data for this would be drawn from another .csv file, a snippet of which would look like this: 100001,fruit,medium,12,red 100002,vegetable,medium,10,orange 100003,fruit,medium,14,green 100004,fruit,medium,12,yellow 100005,vegetable,small,10,red Note how this does not contain the item name (so I have to pull that from a different .csv file that just has the two columns of key and item name) but it does use the same keys. I am looking for a way to bring in this extra information so that my final .csv file will not just tell me the keys (which are item ids) and item names for the items I don't have in stock but it will also have columns for type, size, number, and color. One option I've looked at is the defaultdict piece from collections, but I'm not sure if this is the best way to go about what I want to do. If I did use this method I'm not sure exactly how I'd call it to achieve my desired result. If some other method would be easier I'm certainly willing to try that, too. How can I take my dict of keys and corresponding item names for items that I don't have in inventory and add to it this extra information in such a way that I could output it all to a .csv file? EDIT: As I typed this up it occurred to me that I might make things easier on myself by creating a new single .csv file that would have date in the form key,item name,type,size,number,color (basically just copying in the column for item name into the .csv that already has the other information for each key.) This way I would only need to draw from one .csv file rather than from two. Even if I did this, though, how would I go about making my desired .csv file based on only those keys for items not in inventory?