rvest，html_nodes（）錯誤：無法將類型'environment'強制爲類型'list'的向量。失敗RScript，在會話

Question

中工作html_nodes()函數在作爲可執行RScript運行時失敗如下，但在交互式運行時成功。有人知道運行中會有什麼不同嗎？

交互式運行使用新的會話運行，源語句是第一次運行。

$ ./test-pdp.R 
> 
> ################################################################################ 
> # Setup 
> ################################################################################ 
> suppressPackageStartupMessages(library(plyr)) 
> suppressPackageStartupMessages(library(dplyr)) 
> suppressPackageStartupMessages(library(stringr)) 
> suppressPackageStartupMessages(library(rvest)) 
> suppressPackageStartupMessages(library(httr)) 
> 
> 
> read_html("http://google.com") %>% 
+  html_nodes("div") %>% 
+  length() 
Error in as.vector(x, "list") : 
    cannot coerce type 'environment' to vector of type 'list' 
Calls: %>% ... <Anonymous> -> lapply -> as.list -> as.list.default 
Execution halted 

然而，它成功時，作爲source()交互方式運行：

> source("/Users/a6001389/Documents/projects/hottest-deals-page-scrape/src/test-pdp.R", echo=TRUE) 
> #!/usr/bin/RScript 
> options(echo=TRUE) 
> ################################################################################ 
> # Setup 
> ####################################################### .... [TRUNCATED] 
> suppressPackageStartupMessages(library(dplyr)) 
> suppressPackageStartupMessages(library(stringr)) 
> suppressPackageStartupMessages(library(rvest)) 
> suppressPackageStartupMessages(library(httr)) 
> read_html("http://google.com") %>% 
+  html_nodes("div") %>% 
+  length() 
[1] 17 

謝謝 馬特

Answer 1

添加一行：

library(methods) 

每註釋原來的問題由哈德利韋翰沒有解決這個錯誤。爲什麼它解決了錯誤，我不知道。但是我發佈了一個答案，所以這裏有一個很容易引用的解決方案。如果爲什麼這個問題解決了，我會接受這個答案。

添加從下面的評論來自@ mekki - 麥考利成文本這裏，因爲它確實增加了一些清晰：

This thread might shed some light on it. It seems that in some contexts RSCRIPT doesn't load package::methods by default, whereas interactive sessions do load it by default. It seems that the "when" is not clear, but explicitly calling library(methods) for all RSCRIPT executions seems to be the safe bet: can use package interactively, but Rscript gives errors

Answer 2

很可能如何magrittr::%>%運營商工作的副作用。從Magrittr Documentation - Page 8: %>% Pipe：

The magrittr pipe operators use non-standard evaluation. They capture their inputs and examines them to figure out how to proceed. First a function is produced from all of the individual right-hand side expressions, and then the result is obtained by applying this function to the left-hand side. For most purposes, one can disregard the subtle aspects of magrittr's evaluation, but some functions may capture their calling environment, and thus using the operators will not be exactly equivalent to the "standard call" without pipe-operators (Emphasis mine).

因此，嘗試沒有%>%，看它是否是因爲html_nodes被錯誤地捕獲在命令行環境（如您的錯誤信息提示），而在互動環節，它可以抓取會話的環境變量：

google_node <- read_html("http://google.com"); 
div_nodes <- html_nodes(google_node, "div"); 
length(div_nodes); 

當作爲一個可執行的RScript調用時，這樣工作嗎？