Python提取JSON中InstanceType并统计数量问题求助

我想用Python从JSON文件里导出InstanceType字段，统计每种实例类型的数量，只需要获取该字段及其出现次数。以下是JSON文件片段：

"InstanceTypes": [
    {
        "InstanceType": "c7gn.xlarge",
        "CurrentGeneration": true,
        "FreeTierEligible": false,
        "SupportedUsageClasses": [
            "on-demand",
            "spot"
        ],
        "SupportedRootDeviceTypes": [
            "ebs"
        ],
        "SupportedVirtualizationTypes": [
            "hvm"
        ],
        "BareMetal": false,
        "Hypervisor": "nitro",
        "ProcessorInfo": {
            "SupportedArchitectures": [
                "arm64"
            ],
            "SustainedClockSpeedInGhz": 2.6
        },
        "VCpuInfo": {
            "DefaultVCpus": 4,
            "DefaultCores": 4,
            "DefaultThreadsPerCore": 1,
            "ValidCores": [
                1,
                2,
                3,
                4
            ],
            "ValidThreadsPerCore": [
                1
            ]
        },
        "MemoryInfo": {
            "SizeInMiB": 8192
        },
        "InstanceStorageSupported": false,
        "EbsInfo": {
            "EbsOptimizedSupport": "default",
            "EncryptionSupport": "supported",
            "EbsOptimizedInfo": {
                "BaselineBandwidthInMbps": 2083,
                "BaselineThroughputInMBps": 260.375,
                "BaselineIops": 8333,
                "MaximumBandwidthInMbps": 10000,
                "MaximumThroughputInMBps": 1250.0,
                "MaximumIops": 40000
            },
            "NvmeSupport": "required"
        },
        "NetworkInfo": {
            "NetworkPerformance": "Up to 40 Gigabit",
            "MaximumNetworkInterfaces": 4,
            "MaximumNetworkCards": 1,
            "DefaultNetworkCardIndex": 0,
            "NetworkCards": [
                {
                    "NetworkCardIndex": 0,
                    "NetworkPerformance": "Up to 40 Gigabit",
                    "MaximumNetworkInterfaces": 4,
                    "BaselineBandwidthInGbps": 12.5,
                    "PeakBandwidthInGbps": 40.0
                }
            ],
            "Ipv4AddressesPerInterface": 15,
            "Ipv6AddressesPerInterface": 15,
            "Ipv6Supported": true,
            "EnaSupport": "required",
            "EfaSupported": false,
            "EncryptionInTransitSupported": true,
            "EnaSrdSupported": false
        },
        "PlacementGroupInfo": {
            "SupportedStrategies": [
                "cluster",
                "partition",
                "spread"
            ]
        },
        "HibernationSupported": false,
        "BurstablePerformanceSupported": false,
        "DedicatedHostsSupported": true,
        "AutoRecoverySupported": true,
        "SupportedBootModes": [
            "uefi"
        ],
        "NitroEnclavesSupport": "unsupported",
        "NitroTpmSupport": "unsupported"
    }
]

我编写了如下Python代码，但执行时提示找不到InstanceType字段，明明JSON结构包含该字段，却无法解决：

import json

def extract_string_from_json(json_file_path, key):
    with open(json_file_path, 'r') as file:
        data = json.load(file)
        # Navigate through the JSON structure to access the specific string
        # For example, if the string is nested within dictionaries and lists:
        string_value = data.get(key, "Key not found")
        return string_value

# Example usage:

json_file_path = r'C:\Users\Path\Documents\ptah\Python\path\data.json'  # Replace 'data.json' with your JSON file path
key_to_extract = 'InstanceType'  # Replace 'specific_key' with the key containing the string

string_value = extract_string_from_json(json_file_path, key_to_extract)
if string_value != "Key not found":
    print("String value:", string_value)
else:
    print(f"Key '{key_to_extract}' not found in JSON file.")

问题根源 #

你的代码直接在JSON顶层对象中查找InstanceType，但实际上这个字段是嵌套在InstanceTypes数组下的每个子对象里的，顶层JSON对象并没有InstanceType这个key，所以会提示找不到。

修正后的代码 #

要遍历InstanceTypes数组，提取每个元素的InstanceType值，再用collections.Counter来统计每种类型的出现次数：

import json
from collections import Counter

def count_instance_types(json_file_path):
    with open(json_file_path, 'r') as file:
        data = json.load(file)
        # 获取InstanceTypes数组，若不存在则返回空列表
        instance_types = data.get("InstanceTypes", [])
        # 提取每个实例的InstanceType字段，过滤缺失该字段的对象
        type_list = [item.get("InstanceType") for item in instance_types if item.get("InstanceType")]
        # 统计次数
        type_counts = Counter(type_list)
        return type_counts

# 使用示例
json_file_path = r'C:\Users\Path\Documents\ptah\Python\path\data.json'
counts = count_instance_types(json_file_path)

# 打印结果
for instance_type, count in counts.items():
    print(f"{instance_type}: {count}")

说明 #

1. 先从JSON中取出InstanceTypes数组，避免因数组不存在导致报错；
2. 遍历数组中的每个对象，提取InstanceType值，过滤掉可能缺失该字段的对象；
3. 用Counter快速统计每种类型的出现次数，最后遍历输出结果。

内容的提问来源于stack exchange，提问作者rodriguezrj