使用AWS SAM与FlaskLambda时无法获取请求体的问题

应用代码 #

import json
from flask import request
from flask_lambda import FlaskLambda

app = FlaskLambda(__name__)

@app.route('/submission', methods=['GET', 'POST'])
def send_submissions():

    if request.method == 'GET':
        return (
            json.dumps({
                'name': request.headers['Name']
            }),
            200,
            {'Content-Type': 'application/json'}
        )
    elif request.method == 'POST':
        data = request.get_json()
        return (
            json.dumps({
                'name': request.headers['Name'],
                'data': data
            }),
            200,
            {'Content-Type': 'application/json'}
        )
        
if __name__ == '__main__':
    app.run(debug=True)

出现的错误 #

调用request.get_json()时触发以下异常：

Traceback (most recent call last):
  File "/var/task/flask/app.py", line 2190, in wsgi_app
    response = self.full_dispatch_request()
  File "/var/task/flask/app.py", line 1486, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/var/task/flask/app.py", line 1484, in full_dispatch_request
    rv = self.dispatch_request()
  File "/var/task/flask/app.py", line 1469, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "/var/task/app.py", line 20, in send_submissions
    data = request.get_json()
  File "/var/task/werkzeug/wrappers/request.py", line 611, in get_json
    data = self.get_data(cache=cache)
  File "/var/task/werkzeug/wrappers/request.py", line 423, in get_data
    rv = self.stream.read()
  File "/var/task/werkzeug/wsgi.py", line 829, in readall
    data = self.read(1024 * 64)
  File "/var/task/werkzeug/wsgi.py", line 810, in readinto
    b[:out_size] = data
TypeError: can assign only bytes, buffers, or iterables of ints in range(0, 256)

改用request.get_data()时会抛出类似异常；使用request.form.to_dict()则返回空字典。

测试请求代码 #

headers = {'Name': 'Foo', 'Content-Type': 'application/json'}
r = requests.post(f"{base_url}/submission", data={"message": "Hello, World!"}, headers=headers)

问题分析与解决办法 #

1. 请求参数传递错误 #

你设置了Content-Type: application/json请求头，但用data参数传递字典，requests会默认将字典编码为application/x-www-form-urlencoded格式，导致后端无法识别为JSON数据。

修复请求代码：改用json参数传递数据，requests会自动完成JSON编码并设置正确的Content-Type（手动指定也可）：

headers = {'Name': 'Foo'}
r = requests.post(f"{base_url}/submission", json={"message": "Hello, World!"}, headers=headers)

2. FlaskLambda兼容性问题 #

FlaskLambda项目长期未维护，与新版本Flask（2.x+）、Werkzeug存在兼容性bug，其中就包含请求体流处理异常，导致无法读取请求体。

替代方案：使用aws-wsgi库
aws-wsgi是AWS官方支持的工具，可将Flask应用转换为Lambda兼容的WSGI handler，兼容性更强：

1. 安装依赖：pip install aws-wsgi
2. 修改应用代码：

   import json
   from flask import Flask, request
   import aws_wsgi
   
   app = Flask(__name__)
   
   @app.route('/submission', methods=['GET', 'POST'])
   def send_submissions():
       if request.method == 'GET':
           return json.dumps({'name': request.headers['Name']}), 200, {'Content-Type': 'application/json'}
       elif request.method == 'POST':
           data = request.get_json()
           return json.dumps({'name': request.headers['Name'], 'data': data}), 200, {'Content-Type': 'application/json'}
   
   def handler(event, context):
       return aws_wsgi.response(app, event, context)
   
   if __name__ == '__main__':
       app.run(debug=True)
   

3. 在SAM模板中配置Lambda handler为app.handler（假设代码文件名为app.py）

3. 测试验证 #

• 本地测试可直接使用Flask开发服务器，确认请求能正常获取JSON数据；
• 部署前建议用sam local invoke或sam local start-api测试，排查环境差异问题。

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