Request 部分源码：

class Request(object_ref):

    def __init__(self, url, callback=None, method='GET', headers=None, body=None, 
                 cookies=None, meta=None, encoding='utf-8', priority=0,
                 dont_filter=False, errback=None):

        self._encoding = encoding  # this one has to be set first
        self.method = str(method).upper()
        self._set_url(url)
        self._set_body(body)
        assert isinstance(priority, int), "Request priority not an integer: %r" % priority
        self.priority = priority

        assert callback or not errback, "Cannot use errback without a callback"
        self.callback = callback
        self.errback = errback

        self.cookies = cookies or {}
        self.headers = Headers(headers or {}, encoding=encoding)
        self.dont_filter = dont_filter

        self._meta = dict(meta) if meta else None

    @property
    def meta(self):
        if self._meta is None:
            self._meta = {}
        return self._meta





    url: 就是需要请求，并进行下一步处理的url

    callback: 指定该请求返回的Response，由那个函数来处理。

    method: 请求一般不需要指定，默认GET方法，可设置为"GET", "POST", "PUT"等，且保证字符串大写

    headers: 请求头

    cookies: cookies,模拟用户登录需要指定用户的cookies,字典dict型

    meta: 比较常用，在不同的请求之间传递数据使用的。字典dict型

      request_with_cookies = Request(
          url="http://www.example.com",
          cookies={'currency': 'USD', 'country': 'UY'},
          meta={'dont_merge_cookies': True}
      )

    encoding: 编码类型,使用默认的 'utf-8' 就行。

    dont_filter: 表明该请求不由调度器过滤。这是当你想使用多次执行相同的请求,忽略重复的过滤器。默认为False。

    errback: 指定错误处理函数



如果希望程序执行一开始就发送POST请求，可以重写Spider类的start_requests(self) 方法，并且不再调用start_urls里的url。

class mySpider(scrapy.Spider):

    name = 'myspider'
    allow_domeas = ['renren.com']
    start_urls = ["http://www.renren.com/PLogin.do"]

    def start_requests(self):
        # FormRequest 是Scrapy发送POST请求的方法
        for url in self.start_urls:

            yield scrapy.FormRequest(
                url = url,
                formdata = {"email" : "mr_mao_hacker@163.com", "password" : "axxxxxxxe"},
                callback = self.parse_page
            )

    def parse_page(self, response):
        """
        请求成的回调函数
        """
        pass

Response
class Response(object_ref):
    def __init__(self, url, status=200, headers=None, body='', flags=None, request=None):
        self.headers = Headers(headers or {})
        self.status = int(status)
        self._set_body(body)
        self._set_url(url)
        self.request = request
        self.flags = [] if flags is None else list(flags)

    @property
    def meta(self):
        try:
            return self.request.meta
        except AttributeError:
            raise AttributeError("Response.meta not available, this response " \
                "is not tied to any request")


    status: 响应码
    body： 响应体
    url：响应url
    self.request (request对象)
    self.headers　(响应头)