python 简介需要帮助查找单个字典值 （“model_year”） 的平均值

问：

import csv #Informs Python that we want to use the csv module.

vehicleInventory = {}

with open('cars.csv', newline='') as csvfile:
    filereader = csv.DictReader(csvfile, delimiter = ',', quotechar = '"')
    counter = 0
    for row in filereader:
        counter += 1

        # TASK 1, copying rows in filereader into a dictionary
        vehicleInventory[counter] = row

vehicleInventory[100] = {"model_year": str(2001), "type":"Car", "make":"MINI", "model":"S", "transmission":"6-speed", "primary_color":"Red" }
vehicleInventory[101] = {"model_year": str(2020), "type":"Motorcycle", "make":"Yamaha", "model":"R6", "transmission":"6-speed", "primary_color":"black"}
vehicleInventory[102] = {"model_year": str(2020), "type":"SUV", "make":"Chevrolet", "model":"Suburban", "transmission":"Automatic", "primary_color":"Grey"}

#for key in vehicleInventory:
     #print("Vehicle " + str(key) + " information:")
     #print("MAKE " + vehicleInventory[key]["make"])
     #print("Year: " + vehicleInventory[key]["model_year"])
     #print("MODEL " + vehicleInventory[key]["model"])
     #print( "Color " + vehicleInventory[key]["primary_color"])

这是用于查找值的大部分代码，我需要找到模型年份的平均值。

from functools import reduce
  
def accumulate(x, y): 
    return x + y 
  
def dict_mean(d): 
    values_sum = reduce(accumulate, d.values()) 
    mean = values_sum / len(d) 
    return mean

for key in vehicleInventory:
    d = int(vehicleInventory[key]["model_year"])
    print (str("Mean:", dict_mean(d)))

我尝试了多种方法，这是最新的方法，但由于某种原因，它一直卡在值和上，而不是 int，而它通常不应该在乎。如果有更好的方法，请分享，因为我不知道这是否是远程工作。