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Message: [QUOTE="Jerry Hill, post: 3897212"] Normally you wouldn't mutate the items in a list with a list comprehension. Instead, you would use a for loop, like this: for idx,item in enumerate(my_list_of_dicts): item['position'] = idx Is there a particular reason you want to do this with a list comprehension? Using a list comp means you're going to create an extra copy of your 10 million item list, just so you can add a key to each member, then (probably) throw away the original list. It doesn't seem like the right tool for the job here. [/QUOTE]