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

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用python的Flask写的服务器，web用jquery的ajax请求时候报这个错。
官方文档给的说明在这里http://flask.pocoo.org/snippets/56/

简单点说就是在启动app的地方加上一个装饰器@crossdomain(origin='*')

from flask import Flask
app = Flask(__name__)

@app.route("/")
@crossdomain(origin='*')
def hello():
    return "Hello World!"

if __name__ == "__main__":
    app.run()

具体的装饰器实现代码也要复制进去

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):
    if methods is not None:
        methods = ', '.join(sorted(x.upper() for x in methods))
    if headers is not None and not isinstance(headers, basestring):
        headers = ', '.join(x.upper() for x in headers)
    if not isinstance(origin, basestring):
        origin = ', '.join(origin)
    if isinstance(max_age, timedelta):
        max_age = max_age.total_seconds()

    def get_methods():
        if methods is not None:
            return methods

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

    def decorator(f):
        def wrapped_function(*args, **kwargs):
            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,POST,PUT,DELETE'
            h['Access-Control-Max-Age'] = str(max_age)
            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

完整代码如下

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):
    if methods is not None:
        methods = ', '.join(sorted(x.upper() for x in methods))
    if headers is not None and not isinstance(headers, basestring):
        headers = ', '.join(x.upper() for x in headers)
    if not isinstance(origin, basestring):
        origin = ', '.join(origin)
    if isinstance(max_age, timedelta):
        max_age = max_age.total_seconds()

    def get_methods():
        if methods is not None:
            return methods

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

    def decorator(f):
        def wrapped_function(*args, **kwargs):
            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)
            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



from flask import Flask
app = Flask(__name__)

@app.route("/")
@crossdomain(origin='*')
def hello():
    return "Hello World!"

if __name__ == "__main__":
    app.run()

上述方法虽然可行，但是不够完善，经昊宇大神点播，采用以下办法更合适

# -*- coding:utf-8 -*-
from flask import Flask, request
import json


def after_request(response):
    response.headers['Access-Control-Allow-Origin'] = '*'
    response.headers['Access-Control-Allow-Methods'] = 'PUT,GET,POST,DELETE'
    response.headers['Access-Control-Allow-Headers'] = 'Content-Type,Authorization'
    return response


def create_app():
    app = Flask(__name__)
    app.after_request(after_request)