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Flask-Cache 报错:KeyError: 'cache'

下面的代码:

from flask import Flask
from flask_cache import Cache

app = Flask(__name__)

cache = Cache(config={'CACHE_TYPE': 'simple'})


@app.route('/test_cache')
@cache.cached(timeout=5)
def hello_world():
    print('if cache,the second request do not print')
    return 'Hello, World!'

运行:flask run,然后请求:curl http://127.0.0.1:5000/test_cache,有报错:

Exception possibly due to cache backend.
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/flask_cache/__init__.py", line 289, in decorated_function
    rv = self.cache.get(cache_key)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/flask_cache/__init__.py", line 192, in cache
    return app.extensions['cache'][self]
KeyError: 'cache'

请问,是什么原因?