将 rvest 与 drake 一起使用：外部指针无效错误

Question

当我第一次运行下面的代码时，一切正常。但是当我在 html_file %>%... 命令中更改某些内容时，例如评论 tolower()，我收到以下错误：

Error: target title failed.
diagnose(title)error$message:
  external pointer is not valid
diagnose(title)error$calls:
   1. └─html_file %>% html_nodes("h2") %>% html_text()

代码：

library(rvest)
library(drake)

some_string <- '
  <div class="main">
      <h2>A</h2>
      <div class="route">X</div>
  </div> 
'

html_file <- read_html(some_string)
title <- html_file %>% 
  html_nodes("h2") %>% 
  html_text()

plan <- drake_plan(
  html_file = read_html(some_string),
  title = html_file %>% 
    html_nodes("h2") %>% 
    html_text() %>% 
    tolower()
)

make(plan)

我找到了两个可能的解决方案，但我对它们不感兴趣。
1. 将 drake_plan 中的两个步骤合二为一。
2. 按照建议使用 xml2::write_html() 和 xml2::read_html() here.
有没有更好的方法来解决它？ P.S。问题已经讨论 , Rstudio forum, and on github。

Answer 1

默认情况下，drake 将目标保存为 RDS 文件（其他选项 here). So https://github.com/tidyverse/rvest/issues/181#issuecomment-395064636, which you brought up, is exactly the problem. I like (1) because text is compatible with RDS. Speaking broadly, it is up to the user to choose good targets compatible with drake's data storage system. See https://books.ropensci.org/drake/plans.html#how-to-choose-good-targets for a discussion and links to similar issues. But you want to go with (2), you could return the file path to your HTML file from within a dynamic file。

将 rvest 与 drake 一起使用：外部指针无效错误

Using rvest with drake: external pointer is not valid error

rvest

drake-r-package