使用 R 提取 html 文本 - 无法访问某些节点

Question

我有大量可在线获取的取水许可证，我想从中提取一些数据。例如

url <- "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1"

我根本不懂 html，但在 google 和朋友的帮助下一直在努力。我可以使用 xpath 或 css 选择器毫无问题地访问某些节点，例如访问标题：

library(rvest)
url %>% 
read_html() %>% 
html_nodes(xpath = '//*[@id="main"]/div/h1') %>%
html_text()
[1] "Details for CRC000002.1"

或者使用css选择器：

url %>% 
read_html() %>% 
html_nodes(css = "#main") %>% 
html_nodes(css = "div") %>% 
html_nodes(css = "h1") %>% 
html_text()
[1] "Details for CRC000002.1"

到目前为止，一切都很好，但我真正想要的信息被埋得更深了，我似乎无法得到它。例如，客户端名称字段（在本例中为“Killermont Station Limited”）具有以下 xpath：

clientxpath <- '//*[@id="main"]/div/div[1]/div/table/tbody/tr[1]/td[2]'
url %>% 
read_html() %>% 
html_nodes(xpath = clientxpath) %>%
html_text()
character(0)

css 选择器变得相当复杂，但我得到了相同的结果。 html_nodes() 的帮助文件说：

# XPath selectors ---------------------------------------------
# chaining with XPath is a little trickier - you may need to vary
# the prefix you're using - // always selects from the root noot
# regardless of where you currently are in the doc

但它没有给我任何关于在 xpath 中使用替代前缀的线索（如果我知道 html 可能很明显）。

我的朋友指出，有些文档是用 javascript (ajax) 编写的，这也可能是问题的一部分。也就是说，我试图在上面找到的位显示在 html 中，但它位于一个名为 'div.ajax-block' 的节点内。

css selectors: #main > div > div.ajax-block > div > table > tbody > tr:nth-child(1) > td:nth-child(4)

有人可以帮忙吗？谢谢！

Answer 1

当涉及到抓取时，大多数（如果不是全部）SO R 贡献者默认在简短的“答案”中“使用重量级第三方依赖项”，这非常令人不安。 99% 的时间你不需要 Selenium。你只需要锻炼一下灰色的小细胞。

首先，页面异步加载内容的一个重要线索是出现的等待微调器。第二个在您的 sn-p 中，其中 div 实际上包含选择器名称的一部分，其中包含 ajax。 XHR 请求正在发挥作用的迹象。

如果您在浏览器中打开开发者工具并重新加载页面，然后转到网络，然后您将看到 XHR 选项卡：

页面上的大部分“真实”数据都是动态加载的。我们可以编写模拟浏览器调用的httr 调用。

然而…

我们首先需要对主页进行一次GET 调用，以启动一些 cookie，这些 cookie 将为我们保留，然后找到用于防止滥用网站的每个生成的会话令牌。它是使用 JavaScript 定义的，所以我们将使用 V8 包来评估它。我们可以只使用正则表达式来查找字符串。随心所欲。

library(httr)
library(rvest)
library(dplyr)
library(V8)

ctx <- v8() # we need this to eval some javascript

# Prime Cookies -----------------------------------------------------------

res <- httr::GET("https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1")

httr::cookies(res)
##          domain flag path secure          expiration                 name
## 1 .ecan.govt.nz TRUE    /  FALSE 2019-11-24 11:46:13   visid_incap_927063
## 2 .ecan.govt.nz TRUE    /  FALSE                <NA> incap_ses_148_927063
##                                                              value
## 1 +p8XAM6uReGmEnVIdnaxoxWL+VsAAAAAQUIPAAAAAABjdOjQDbXt7PG3tpBpELha
## 2         nXJSYz8zbCRj8tGhzNANAhaL+VsAAAAA7JyOH7Gu4qeIb6KKk/iSYQ==
pg <- httr::content(res)

html_node(pg, xpath=".//script[contains(., '_monsido')]") %>%
  html_text() %>%
  ctx$eval()
## [1] "2"
monsido_token <- ctx$get("_monsido")[1,2]

这是searchlist（确实是空的）：

httr::VERB(
  verb = "POST", url = "https://www.ecan.govt.nz/data/document-library/searchlist",
  httr::add_headers(
    Referer = "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1",
    `X-Requested-With` = "XMLHttpRequest",
    TE = "Trailers"
  ), httr::set_cookies(
    monsido = monsido_token
  ),
  body = list(
    name = "CRC000002.1",
    pageSize = "999999"
  ),
  encode = "form"
) -> res

httr::content(res)
## NULL ## <<=== this is OK as there is no response

这里是“同意概览”部分：

httr::GET(
  url = "https://www.ecan.govt.nz/data/consent-search/consentoverview/CRC000002.1",
  httr::add_headers(
    Referer = "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1",
    Authority = "www.ecan.govt.nz",
    `X-Requested-With` = "XMLHttpRequest"
  ),
  httr::set_cookies(
    monsido = monsido_token
  )
) -> res

httr::content(res) %>%
  html_table() %>%
  glimpse()
## List of 1
##  $ :'data.frame':    5 obs. of  4 variables:
##   ..$ X1: chr [1:5] "RMA Authorisation Number" "Consent Location" "To" "Commencement Date" ...
##   ..$ X2: chr [1:5] "CRC000002.1" "Manuka Creek, KILLERMONT STATION" "To take water from Manuka Creek at or about map reference NZMS 260 H39:5588-2366 for irrigation of up to 40.8 hectares." "29 Apr 2010" ...
##   ..$ X3: chr [1:5] "Client Name" "State" "To take water from Manuka Creek at or about map reference NZMS 260 H39:5588-2366 for irrigation of up to 40.8 hectares." "29 Apr 2010" ...
##   ..$ X4: chr [1:5] "Killermont Station Limited" "Issued - Active" "To take water from Manuka Creek at or about map reference NZMS 260 H39:5588-2366 for irrigation of up to 40.8 hectares." "29 Apr 2010" ...

以下是“同意条件”：

httr::GET(
  url = "https://www.ecan.govt.nz/data/consent-search/consentconditions/CRC000002.1",
  httr::add_headers(
    Referer = "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1",
    Authority = "www.ecan.govt.nz",
    `X-Requested-With` = "XMLHttpRequest"
  ),
  httr::set_cookies(
    monsido = monsido_token
  )
) -> res

httr::content(res) %>%
  as.character() %>%
  substring(1, 300) %>%
  cat()
## <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
## <html><body><div class="consentDetails">
##     <ul class="unstyled-list">
## <li>
##           
##             
##             <strong class="pull-left">1</strong> <div class="pad-left1">The rate at which wa

这里是“同意相关”：

httr::GET(
  url = "https://www.ecan.govt.nz/data/consent-search/consentrelated/CRC000002.1",
  httr::add_headers(
    Referer = "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1",
    Authority = "www.ecan.govt.nz",
    `X-Requested-With` = "XMLHttpRequest"
  ),
  httr::set_cookies(
    monsido = monsido_token
  )
) -> res

httr::content(res) %>%
  as.character() %>%
  substring(1, 300) %>%
  cat()
## <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
## <html><body>
## <p>There are no related documents.</p>
## 
## 
## 
## 
##   
##     <div class="summary-table-wrapper">
##       <table class="summary-table left">
## <thead><tr>
## <th>Relationship</th>
##           <th>Recor

这是“工作流程：

httr::GET(
  url = "https://www.ecan.govt.nz/data/consent-search/consentworkflow/CRC000002.1",
  httr::add_headers(
    Referer = "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1",
    Authority = "www.ecan.govt.nz",
    `X-Requested-With` = "XMLHttpRequest"
  ),
  httr::set_cookies(
    monsido = monsido_token
  )
) -> res

httr::content(res)
## {xml_document}
## <html>
## [1] <body><p>No workflow</p></body>

以下是“同意流程限制”：

httr::GET(
  url = "https://www.ecan.govt.nz/data/consent-search/consentflowrestrictions/CRC000002.1",
  httr::add_headers(
    Referer = "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1",
    Authority = "www.ecan.govt.nz",
    `X-Requested-With` = "XMLHttpRequest"
  ),
  httr::set_cookies(
    monsido = monsido_token
  )
) -> res

httr::content(res) %>%
  as.character() %>%
  substring(1, 300) %>%
  cat()
## <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
## <html><body><div class="summary-table-wrapper">
##     <table class="summary-table left">
## <thead>
## <th colspan="2">Low Flow Site</th>
##       <th>Todays Flow <span class="lower">(m3/s)</span>
## </th>

您仍然需要解析 HTML，但现在您只需使用普通的 R 包即可完成所有工作。