使用R提取HTML文本-无法访问某些节点
Tim*_*imM 3 html r web-scraping rvest
我有大量在线取水许可证，我想从中提取一些数据。例如
url <- "https://www.ecan.govt.nz/data/consent-search/consentdetails/CRC000002.1"
我一点都不懂HTML，但是一直在Google和一位朋友的帮助下插手。我可以使用xpath或css选择器到达某些节点，而不会出现任何问题，例如，获得标题：
library(rvest)
url %>%
read_html() %>%
html_nodes(xpath = '//*[@id="main"]/div/h1') %>%
html_text()
[1] "Details for CRC000002.1"
或使用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，还算不错，但是我真正想要的信息被更深入地埋了，我似乎无法理解。例如，客户名称字段（在这种情况下为“ 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，但是我得到了相同的结果。html_nodes（）的帮助文件显示：
# 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
但这并没有为我提供任何在xpath中使用替代前缀的线索（如果我知道html的话，可能会很明显）。
我的朋友指出，某些文档使用的是javascript（ajax），这也可能是问题的一部分。就是说，我试图到达上面的位出现在html中，但它位于名为“ 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！
非常令人不安的是，大多数（即使不是全部）SO R贡献者都默认在刮除“答案”时“使用重量级的第三方依赖关系”。99％的时间您不需要硒。您只需要锻炼灰色小细胞。
首先，大的线索，在页面加载内容异步是等待微调出现。第二个在您的代码段中，其中div实际上包含选择器名称的一部分ajax。XHR请求正在运行中。
如果在浏览器中打开“开发人员工具”并重新加载页面，请依次转到“网络”和“ XHR”选项卡，您将看到：
页面上的大多数“真实”数据都是动态加载的。我们可以编写httr模仿浏览器调用的调用。
但是 ……
我们首先需要打一个GET主页来填充一些cookie，这些cookie将留给我们使用，然后找到每个生成的会话令牌，用于防止站点的滥用。它是使用JavaScript定义的，因此我们将使用该V8包对其进行评估。我们可以只使用正则表达式来查找字符串。做你喜欢的事。
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（实际上是空的）：
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
这是“同意概述”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" ...
这是“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>
这是“同意流动限制”：
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，但是现在您只需使用普通的R包就可以完成所有操作。
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