关于python：Javascript – 请求的资源上没有’Access-Control-Allow-Origin’标头

Javascript - No 'Access-Control-Allow-Origin' header is present on the requested resource

我需要通过XmlHttpRequest从JavaScript向Python服务器发送数据。因为我使用的是localhost，所以我需要使用CORS。我正在使用Flask框架及其模块flask_cors。

作为JavaScript我有这个：

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var xmlhttp;
if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp = new XMLHttpRequest();
}
else {// code for IE6, IE5
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.open("POST","http://localhost:5000/signin", true);
var params ="email=" + email +"&password=" + password;

xmlhttp.onreadystatechange = function() {//Call a function when the state changes.
if(xmlhttp.readyState == 4 && xmlhttp.status == 200) {
alert(xmlhttp.responseText);
}
}
xmlhttp.send(params);

和Python代码：

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@app.route('/signin', methods=['POST'])
@cross_origin()
def sign_in():
email = cgi.escape(request.values["email"])
password = cgi.escape(request.values["password"])

但是当我执行它时，我得到了这样的信息：

XMLHttpRequest cannot load localhost:5000/signin. No
'Access-Control-Allow-Origin' header is present on the requested
resource. Origin 'null' is therefore not allowed access.

我该如何解决？我知道我需要使用一些"Access-Control-Allow-Origin"标头，但我不知道如何在此代码中实现它。顺便说一下，我需要使用纯JavaScript。

我使用了烧瓶扩展。

使用pip install flask-cors安装

那就简单了

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from flask_cors import CORS
app = Flask(__name__)
CORS(app)

这将允许所有域

相关讨论

我使用这个装饰器让Javascript与Flask一起工作，并在我可接受的方法列表中添加"OPTIONS"。应该在路径装饰器下面使用装饰器，如下所示：

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@app.route('/login', methods=['POST', 'OPTIONS'])
@crossdomain(origin='*')
def login()
...

编辑：
链接似乎被打破了。这是我用的装饰器。

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from datetime import timedelta
from flask import make_response, request, current_app
from functools import update_wrapper

def crossdomain(origin=None, methods=None, headers=None, max_age=21600,
attach_to_all=True, automatic_options=True):
"""Decorator function that allows crossdomain requests.
Courtesy of
https://blog.skyred.fi/articles/better-crossdomain-snippet-for-flask.html
"""
if methods is not None:
methods = ', '.join(sorted(x.upper() for x in methods))
# use str instead of basestring if using Python 3.x
if headers is not None and not isinstance(headers, basestring):
headers = ', '.join(x.upper() for x in headers)
# use str instead of basestring if using Python 3.x
if not isinstance(origin, basestring):
origin = ', '.join(origin)
if isinstance(max_age, timedelta):
max_age = max_age.total_seconds()

def get_methods():
""" Determines which methods are allowed
"""
if methods is not None:
return methods

options_resp = current_app.make_default_options_response()
return options_resp.headers['allow']

def decorator(f):
"""The decorator function
"""
def wrapped_function(*args, **kwargs):
"""Caries out the actual cross domain code
"""
if automatic_options and request.method == 'OPTIONS':
resp = current_app.make_default_options_response()
else:
resp = make_response(f(*args, **kwargs))
if not attach_to_all and request.method != 'OPTIONS':
return resp

h = resp.headers
h['Access-Control-Allow-Origin'] = origin
h['Access-Control-Allow-Methods'] = get_methods()
h['Access-Control-Max-Age'] = str(max_age)
h['Access-Control-Allow-Credentials'] = 'true'
h['Access-Control-Allow-Headers'] = \
"Origin, X-Requested-With, Content-Type, Accept, Authorization"
if headers is not None:
h['Access-Control-Allow-Headers'] = headers
return resp

f.provide_automatic_options = False
return update_wrapper(wrapped_function, f)
return decorator