scrapy爬虫:scrapy.FormRequest中formdata参数详解

1. 背景

在网页爬取的时候，有时候会使用scrapy.FormRequest向目标网站提交数据（表单提交）。参照scrapy官方文档的标准写法是：

# header信息
unicornHeader = {
  'Host': 'www.example.com',
  'Referer': 'http://www.example.com/',
}

# 表单需要提交的数据
myFormData = {'name': 'John Doe', 'age': '27'}

# 自定义信息，向下层响应(response)传递下去
customerData = {'key1': 'value1', 'key2': 'value2'}

yield scrapy.FormRequest(url = "http://www.example.com/post/action",
             headers = unicornHeader,
             method = 'POST',       # GET or POST
             formdata = myFormData,    # 表单提交的数据
             meta = customerData,    # 自定义，向response传递数据
             callback = self.after_post,
             errback = self.error_handle,
             # 如果需要多次提交表单，且url一样，那么就必须加此参数dont_filter，防止被当成重复网页过滤掉了
             dont_filter = True   
             )

但是，当表单提交数据myFormData 是形如字典内嵌字典的形式，又该如何写？

2. 案例 — 参数为字典

在做亚马逊网站爬取时，当进入商家店铺，爬取店铺内商品列表时，发现采取的方式是ajax请求，返回的是json数据。

请求信息如下：

响应信息如下：

如上图所示，From Data中的数据包含一个字典：

marketplaceID:ATVPDKIKX0DER
seller:A2FE6D62A4WM6Q
productSearchRequestData:{"marketplace":"ATVPDKIKX0DER","seller":"A2FE6D62A4WM6Q","url":"/sp/ajax/products","pageSize":12,"searchKeyword":"","extraRestrictions":{},"pageNumber":"1"}

# formDate 必须构造如下：
myFormData = {
  'marketplaceID' : 'ATVPDKIKX0DER',
  'seller' : 'A2FE6D62A4WM6Q',
  # 注意下面这一行，内部字典是作为一个字符串的形式
  'productSearchRequestData' :'{"marketplace":"ATVPDKIKX0DER","seller":"A2FE6D62A4WM6Q","url":"/sp/ajax/products","pageSize":12,"searchKeyword":"","extraRestrictions":{},"pageNumber":"1"}'
}

在amazon中实际使用的构造方法如下：

def sendRequestForProducts(response):
  ajaxParam = response.meta
  for pageIdx in range(1, ajaxParam['totalPageNum']+1):
    ajaxParam['isFirstAjax'] = False
    ajaxParam['pageNumber'] = pageIdx

    unicornHeader = {
      'Host': 'www.amazon.com',
      'Origin': 'https://www.amazon.com',
      'Referer': ajaxParam['referUrl'],
    }

    '''
    marketplaceID:ATVPDKIKX0DER
    seller:AYZQAQRQKEXRP
    productSearchRequestData:{"marketplace":"ATVPDKIKX0DER","seller":"AYZQAQRQKEXRP","url":"/sp/ajax/products","pageSize":12,"searchKeyword":"","extraRestrictions":{},"pageNumber":1}
    '''

    productSearchRequestData = '{"marketplace": "ATVPDKIKX0DER", "seller": "' + f'{ajaxParam["sellerID"]}' + '","url": "/sp/ajax/products", "pageSize": 12, "searchKeyword": "","extraRestrictions": {}, "pageNumber": "' + str(pageIdx) + '"}'
    formdataProduct = {
      'marketplaceID': ajaxParam['marketplaceID'],
      'seller': ajaxParam['sellerID'],
      'productSearchRequestData': productSearchRequestData
    }
    productAjaxMeta = ajaxParam
    # 请求店铺商品列表
    yield scrapy.FormRequest(
      url = 'https://www.amazon.com/sp/ajax/products',
      headers = unicornHeader,
      formdata = formdataProduct,
      func = 'POST',
      meta = productAjaxMeta,
      callback = self.solderProductAjax,
      errback = self.error, # 处理http error
      dont_filter = True,  # 需要加此参数的
    )

3. 原理分析

举例来说，目前有如下一笔数据：

formdata = {
      'Field': {"pageIdx":99, "size":"10"},
      'func': 'nextPage',
      }

从网页上，可以看到请求数据如下：

Field=%7B%22pageIdx%22%3A99%2C%22size%22%3A%2210%22%7D&func=nextPage

第一种，按照如下方式发出请求，结果如下（正确）：

yield scrapy.FormRequest(
      url = 'https://www.example.com/sp/ajax',
      headers = unicornHeader,
      formdata = {
            'Field': '{"pageIdx":99, "size":"10"}',
            'func': 'nextPage',
            },
      func = 'POST',
      callback = self.handleFunc,
    )
# 请求数据为：Field=%7B%22pageIdx%22%3A99%2C%22size%22%3A%2210%22%7D&func=nextPage

第二种，按照如下方式发出请求，结果如下（错误，无法获取到正确的数据）：

yield scrapy.FormRequest(
      url = 'https://www.example.com/sp/ajax',
      headers = unicornHeader,
      formdata = {
            'Field': {"pageIdx":99, "size":"10"},
            'func': 'nextPage',
            },
      func = 'POST',
      callback = self.handleFunc,
    )
# 经过错误的编码之后，发送的请求为：Field=size&Field=pageIdx&func=nextPage

我们跟踪看一下scrapy中的源码：

# E:/Miniconda/Lib/site-packages/scrapy/http/request/form.py

# FormRequest
class FormRequest(Request):

  def __init__(self, *args, **kwargs):
    formdata = kwargs.pop('formdata', None)
    if formdata and kwargs.get('func') is None:
      kwargs['func'] = 'POST'

    super(FormRequest, self).__init__(*args, **kwargs)

    if formdata:
      items = formdata.items() if isinstance(formdata, dict) else formdata
      querystr = _urlencode(items, self.encoding)
      if self.func == 'POST':
        self.headers.setdefault(b'Content-Type', b'application/x-www-form-urlencoded')
        self._set_body(querystr)
      else:
        self._set_url(self.url + ('&' if '?' in self.url else '?') + querystr)

  # 关键函数 _urlencode
  def _urlencode(seq, enc):
    values = [(to_bytes(k, enc), to_bytes(v, enc))
         for k, vs in seq
         for v in (vs if is_listlike(vs) else [vs])]
    return urlencode(values, doseq=1)

分析过程如下：

# 第一步：items = formdata.items() if isinstance(formdata, dict) else formdata
# 第一步结果：经过items()方法执行后，原始的dict格式变成如下列表形式：
   dict_items([('func', 'nextPage'), ('Field', {'size': '10', 'pageIdx': 99})])

# 第二步：再经过后面的 _urlencode方法将items转换成如下：
  [(b'func', b'nextPage'), (b'Field', b'size'), (b'Field', b'pageIdx')]

# 可以看到就是在调用 _urlencode方法的时候出现了问题，上面的方法执行过后，会使字典形式的数据只保留了keys（value是字典的情况下，只保留了value字典中的key）.

解决方案： 就是将字典当成普通的字符串，然后编码（转换成bytes），进行传输，到达服务器端之后，服务器会反过来进行解码，得到这个字典字符串。然后服务器按照Dict进行解析。

拓展：对于其他特殊类型的数据，都按照这种方式打包成字符串进行传递。

4. 补充1 ——参数类型

formdata的 参数值 必须是unicode , str 或者 bytes object，不能是整数。

案例：

yield FormRequest(
  url = 'https://www.amztracker.com/unicorn.php',
  headers = unicornHeader,
  # formdata 的参数必须是字符串
  formdata={'rank': 10, 'category': productDetailInfo['topCategory']},
  method = 'GET',
  meta = {'productDetailInfo': productDetailInfo},
  callback = self.amztrackerSale,
  errback = self.error, # 本项目中这里触发errback占绝大多数
  dont_filter = True, # 按理来说是不需要加此参数的
)

# 提示如下ERROR：
Traceback (most recent call last):
 File "E:\Miniconda\lib\site-packages\scrapy\utils\defer.py", line 102, in iter_errback
  yield next(it)
 File "E:\Miniconda\lib\site-packages\scrapy\spidermiddlewares\offsite.py", line 29, in process_spider_output
  for x in result:
 File "E:\Miniconda\lib\site-packages\scrapy\spidermiddlewares\referer.py", line 339, in <genexpr>
  return (_set_referer(r) for r in result or ())
 File "E:\Miniconda\lib\site-packages\scrapy\spidermiddlewares\urllength.py", line 37, in <genexpr>
  return (r for r in result or () if _filter(r))
 File "E:\Miniconda\lib\site-packages\scrapy\spidermiddlewares\depth.py", line 58, in <genexpr>
  return (r for r in result or () if _filter(r))
 File "E:\PyCharmCode\categorySelectorAmazon1\categorySelectorAmazon1\spiders\categorySelectorAmazon1Clawer.py", line 224, in parseProductDetail
  dont_filter = True,
 File "E:\Miniconda\lib\site-packages\scrapy\http\request\form.py", line 31, in __init__
  querystr = _urlencode(items, self.encoding)
 File "E:\Miniconda\lib\site-packages\scrapy\http\request\form.py", line 66, in _urlencode
  for k, vs in seq
 File "E:\Miniconda\lib\site-packages\scrapy\http\request\form.py", line 67, in <listcomp>
  for v in (vs if is_listlike(vs) else [vs])]
 File "E:\Miniconda\lib\site-packages\scrapy\utils\python.py", line 117, in to_bytes
  'object, got %s' % type(text).__name__)
TypeError: to_bytes must receive a unicode, str or bytes object, got int

# 正确写法：
formdata = {'rank': str(productDetailInfo['topRank']), 'category': productDetailInfo['topCategory']},

原理部分（源代码）：

# 第一阶段: 字典分解为items
    if formdata:
      items = formdata.items() if isinstance(formdata, dict) else formdata
      querystr = _urlencode(items, self.encoding)

# 第二阶段: 对value，调用 to_bytes 编码
def _urlencode(seq, enc):
  values = [(to_bytes(k, enc), to_bytes(v, enc))
       for k, vs in seq
       for v in (vs if is_listlike(vs) else [vs])]
  return urlencode(values, doseq=1)

# 第三阶段: 执行 to_bytes ，参数要求是bytes, str
def to_bytes(text, encoding=None, errors='strict'):
  """Return the binary representation of `text`. If `text`
  is already a bytes object, return it as-is."""
  if isinstance(text, bytes):
    return text
  if not isinstance(text, six.string_types):
    raise TypeError('to_bytes must receive a unicode, str or bytes '
            'object, got %s' % type(text).__name__)

5. 补充2 ——参数为中文

formdata的 参数值 必须是unicode , str 或者 bytes object，不能是整数。

以1688网站搜索产品为案例：

搜索信息如下（搜索关键词为：动漫周边）：

可以看到 动漫周边 == %B6%AF%C2%FE%D6%DC%B1%DF

# scrapy中这个请求的构造如下

# python3 所有的字符串都是unicode
unicornHeaders = {
  ':authority': 's.1688.com',
  'Referer': 'https://www.1688.com/',
}

# python3 所有的字符串都是unicode
# 动漫周边 tobyte为：%B6%AF%C2%FE%D6%DC%B1%DF
formatStr = "动漫周边".encode('gbk')

print(f"formatStr = {formatStr}")
yield FormRequest(
  url = 'https://s.1688.com/selloffer/offer_search.htm',
  headers = unicornHeaders,
  formdata = {'keywords': formatStr, 'n': 'y', 'spm': 'a260k.635.1998096057.d1'},
  method = 'GET',
  meta={},
  callback = self.parseCategoryPage,
  errback = self.error, # 本项目中这里触发errback占绝大多数
  dont_filter = True, # 按理来说是不需要加此参数的
)

# 日志如下：
formatStr = b'\xb6\xaf\xc2\xfe\xd6\xdc\xb1\xdf'
2017-11-16 15:11:02 [scrapy.downloadermiddlewares.redirect] DEBUG: Redirecting (302) to <GET https://sec.1688.com/query.htm?smApp=searchweb2&smPolicy=searchweb2-selloffer-anti_Spider-seo-html-checklogin&smCharset=GBK&smTag=MTE1LjIxNi4xNjAuNDYsLDU5OWQ1NWIyZTk0NDQ1Y2E5ZDAzODRlOGM1MDI2OTZj&smReturn=https%3A%2F%2Fs.1688.com%2Fselloffer%2Foffer_search.htm%3Fkeywords%3D%25B6%25AF%25C2%25FE%25D6%25DC%25B1%25DF%26n%3Dy%26spm%3Da260k.635.1998096057.d1&smSign=05U0%2BJXfKLQmSbsnce55Yw%3D%3D> from <GET https://s.1688.com/selloffer/offer_search.htm?keywords=%B6%AF%C2%FE%D6%DC%B1%DF&n=y&spm=a260k.635.1998096057.d1>
# https://s.1688.com/selloffer/offer_search.htm?keywords=%B6%AF%C2%FE%D6%DC%B1%DF&n=y&spm=a260k.635.1998096057.d1

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