How to efficiently get the mean of the elements in two list of lists in Python

You can do it in O(n) (single pass over each list) by converting 1 to a dict, then per item in the 2nd list access that dict (in O(1)), like this:

mylist1 = [["lemon", 0.1], ["egg", 0.1], ["muffin", 0.3], ["chocolate", 0.5]]
mylist2 = [["chocolate", 0.5], ["milk", 0.2], ["carrot", 0.8], ["egg", 0.8]]

l1_as_dict = dict(mylist1)

myoutput = []
for item,price2 in mylist2:
    if item in l1_as_dict:
        price1 = l1_as_dict[item]
        myoutput.append([item, (price1+price2)/2])

print(myoutput)

Output:

[['chocolate', 0.5], ['egg', 0.45]]