Store contents of two lists as key-value pairs in a dictionary

Question

I have two Python lists. One list, called area , stores the areas of homes. Different homes may have the same area. Another list, called price , stores the corresponding prices of these homes. I want to create a dictionary called priceDict , whose keys are the unique areas of homes and whose values are lists of the prices of homes of the same area. For example, suppose area = [1500, 500, 1500, 2000, 2500, 2000] , price = [30000, 10000, 20000, 40000, 50000, 45000] . I would like to create the dictionary priceDict = {1500: [30000, 20000], 500: [10000], 2000: [40000, 45000], 2500: [50000]} .

I wrote the following Python code:

area = [1500, 500, 1500, 2000, 2500, 2000]
price = [30000, 10000, 20000, 40000, 50000, 45000]
priceDict = dict.fromkeys(area,[])
    
for houseArea in priceDict:
    for areaIndex in range(len(area)):
        if area[areaIndex] == houseArea:
            priceDict[houseArea].append(price[areaIndex])

print(priceDict)

However, when I executed the code, the output I got was:

{1500: [30000, 20000, 10000, 40000, 45000, 50000], 500: [30000, 20000, 10000, 40000, 45000, 50000], 2000: [30000, 20000, 10000, 40000, 45000, 50000], 2500: [30000, 20000, 10000, 40000, 45000, 50000]}

I would like to ask what's wrong with my code and how I can re-write my code so that it performs the intended function.

Answer 1

Your first problem is here:

priceDict = dict.fromkeys(area,[])

This makes every key in priceDict share the same list.

The second problem is you are nesting for-loops rather than using a single for-loop and zipping to pair up your items:

priceDict = {}
for k, v in zip(area, price):
    priceDict.setdefault(k, []).append(v)

Note that priceDict.setdefault(k, []).append(v) is slightly inefficient because it creates an empty list on every iteration. A more efficient way is to use a defaultdict of list .

from collections import defaultdict

priceDict = defaultdict(list)
for k, v in zip(area, price):
    priceDict[k].append(v)