处理 RSelenium 错误消息

目前正在抓取一个页面，该页面可以具有可变数量的元素，并且有时还使用不同的选择器来格式化相同的数据元素。目前正在尝试使用一些 tryCatch 代码忽略 RSelenium 引发的错误，但当指定的元素不在页面上时仍然会停止：

  result <- tryCatch({
    webElem <- remDr$findElement('xpath', "//tr[(((count(preceding-sibling::*) + 1) = 9) and parent::*)]//span[(((count(preceding-sibling::*) + 1) = 1) and parent::*)]")
  }, warning = function(cond) {
    rank1_US <- NA
  }, 
  error = function(cond) {
    rank1_US <- NA
  }, finally = {
    rank1_US <- webElem$getElementText() %>% unlist(.) %>% ifelse(length(.) == 0, NA, .)
  })

当页面上找不到特定元素时，会出现错误：

Selenium message:Unable to locate element: //tr[(((count(preceding-sibling::*) + 1) = 9) and parent::*)]//span[(((count(preceding-sibling::*) + 1) = 1) and parent::*)]
For documentation on this error, please visit: http://seleniumhq.org/exceptions/no_such_element.html
Build info: version: '3.0.1', revision: '1969d75', time: '2016-10-18 09:48:19 -0700'
System info: host: '462a81a34fb2', ip: '172.17.0.2', os.name: 'Linux', os.arch: 'amd64', os.version: '4.4.39-boot2docker', java.version: '1.8.0_111'
Driver info: org.openqa.selenium.firefox.FirefoxDriver
Capabilities [{rotatable=false, raisesAccessibilityExceptions=false, firefoxOptions={args=[], profile=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... <truncated>
Session ID: c7f538b9-b516-411d-ace9-a980353ae442
*** Element info: {Using=xpath, value=//tr[(((count(preceding-sibling::*) + 1) = 9) and parent::*)]//span[(((count(preceding-sibling::*) + 1) = 1) and parent::*)]}

有人有办法处理错误吗？

你可以尝试这样的事情：

result <- tryCatch({
    suppressMessages({
      webElem <- remDr$findElement('xpath', "//tr[(((count(preceding-sibling::*) + 1) = 9) and parent::*)]//span[(((count(preceding-sibling::*) + 1) = 1) and parent::*)]")
      rank1_US <- webElem$getElementText() %>% unlist(.) %>% ifelse(length(.) == 0, NA, .)
      rank1_US
    })
  }, 
  error = function(e) {
    NA_character_
  }
)