AWS Lambda运行Python脚本报dbresponse未定义NameError

问题现象 #

• 编写的Python脚本用于更新DynamoDB指定字段值并返回更新结果，在VSCode环境连接AWS服务运行时无报错、执行正常；调整为Lambda适配格式部署后，测试运行触发错误。

本地可正常运行的VSCode代码 #

import boto3
from boto3.dynamodb.conditions import Key, Attr

dynamodb = boto3.resource('dynamodb',region_name='us-east-1')
table = dynamodb.Table('Visitors')

response = table.update_item(
    Key={'ID': 0},
    UpdateExpression="ADD #ct :increment",
    ExpressionAttributeNames={'#ct': 'visitorcount'},
    ExpressionAttributeValues={':increment': 1},
    ReturnValues = "UPDATED_NEW"
)
visitorCount = int(response["Attributes"]["visitorcount"])
print('visitorCount obtained from table = ' + str(visitorCount))

部署到Lambda的问题代码 #

import json
import boto3
from boto3.dynamodb.conditions import Key, Attr

dynamodb = boto3.resource('dynamodb',region_name='us-east-1')
table = dynamodb.Table('Visitors')

def lambda_handler(event, context):
    dbresponse = table.update_item(
        Key={'ID': 0},
        UpdateExpression="ADD #ct :increment",
        ExpressionAttributeNames={'#ct': 'visitorcount'},
        ExpressionAttributeValues={':increment': 1},

    ReturnValues = "UPDATED_NEW"
    )
    
visitorCount = int(dbresponse["Attributes"]["visitorcount"])
print('visitorCount obtained from table = ' + str(visitorCount))

Lambda测试报错信息 #

• 测试事件名称：mycodetest

{
  "errorMessage": "name 'dbresponse' is not defined",
  "errorType": "NameError",
  "requestId": "",
  "stackTrace": [
    "  File \"/var/lang/lib/python3.9/importlib/__init__.py\", line 127, in import_module\n    return _bootstrap._gcd_import(name[level:], package, level)\n",
    "  File \"<frozen importlib._bootstrap>\", line 1030, in _gcd_import\n",
    "  File \"<frozen importlib._bootstrap>\", line 1007, in _find_and_load\n",
    "  File \"<frozen importlib._bootstrap>\", line 986, in _find_and_load_unlocked\n",
    "  File \"<frozen importlib._bootstrap>\", line 680, in _load_unlocked\n",
    "  File \"<frozen importlib._bootstrap_external>\", line 850, in exec_module\n",
    "  File \"<frozen importlib._bootstrap>\", line 228, in _call_with_frames_removed\n",
    "  File \"/var/task/lambda_function.py\", line 18, in <module>\n    visitorCount = int(dbresponse[\"Attributes\"][\"visitorcount\"])\n"
  ]
}

故障根因 #

报错由两个缩进错误直接导致：

1. 变量dbresponse定义在lambda_handler函数内部，属于函数局部变量，但读取dbresponse、计算访客数、打印结果的代码写在了lambda_handler函数外部的全局作用域。Lambda加载代码文件时会先执行全局作用域代码，此时lambda_handler还未被触发，内部的dbresponse变量根本没有被创建，直接触发NameError。
2. update_item方法的ReturnValues参数缩进错误，没有和同属方法入参的Key、UpdateExpression等参数保持同级缩进，会被Python识别为函数外部的代码，触发语法层面的异常。

修复后可正常运行的Lambda代码 #

import json
import boto3
from boto3.dynamodb.conditions import Key, Attr

# DynamoDB资源初始化放在全局作用域是正确写法，Lambda会复用执行环境的已有资源，降低冷启动耗时
dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
table = dynamodb.Table('Visitors')

def lambda_handler(event, context):
    dbresponse = table.update_item(
        Key={'ID': 0},
        UpdateExpression="ADD #ct :increment",
        ExpressionAttributeNames={'#ct': 'visitorcount'},
        ExpressionAttributeValues={':increment': 1},
        ReturnValues="UPDATED_NEW"
    )
    # 所有依赖dbresponse的逻辑全部放在lambda_handler函数内部，保持4空格统一缩进
    visitorCount = int(dbresponse["Attributes"]["visitorcount"])
    print('visitorCount obtained from table = ' + str(visitorCount))
    
    # 符合Lambda规范的HTTP返回格式
    return {
        'statusCode': 200,
        'headers': {
            'Access-Control-Allow-Origin': '*'
        },
        'body': json.dumps({
            'visitorCount': visitorCount
        })
    }

部署前注意项 #

• 给Lambda绑定的执行角色添加DynamoDB表Visitors的dynamodb:UpdateItem权限，否则修复缩进后会触发权限拒绝错误
• Python是严格缩进敏感的语言，代码块内统一使用4空格缩进，不要混用空格和Tab键，避免出现非预期的作用域错误

内容的提问来源于stack exchange，提问作者CurtFarley