下面是一个示例代码，演示如何按某个键名分组计数对象数组中的值：

from collections import defaultdict

# 定义一个对象数组
objects = [
    {"name": "apple", "color": "red"},
    {"name": "banana", "color": "yellow"},
    {"name": "cherry", "color": "red"},
    {"name": "date", "color": "brown"},
    {"name": "elderberry", "color": "purple"},
    {"name": "fig", "color": "purple"},
    {"name": "grape", "color": "purple"},
    {"name": "apple", "color": "red"},
    {"name": "banana", "color": "yellow"},
    {"name": "apple", "color": "red"},
]

# 使用 defaultdict 创建一个空字典，值的类型为整数
count_by_color = defaultdict(int)

# 遍历对象数组，按颜色进行计数
for obj in objects:
    color = obj["color"]
    count_by_color[color] += 1

# 打印计数结果
for color, count in count_by_color.items():
    print(f"{color}: {count}")

输出结果：

red: 4
yellow: 2
brown: 1
purple: 3

这里使用了 defaultdict 来创建一个空字典，并将默认值设置为整数类型。然后，遍历对象数组，获取每个对象的颜色属性，并将计数器加1。最后，打印计数结果。