如何从需求中解救出“gem_name”当宝石没有安装 [英] How to rescue from a require "gem_name" when the gem is not installed

问题描述

我正在写一个依赖于特定宝石的库。我需要宝石，并在我的代码中使用它，只要宝石安装在用户的机器上，所有东西都是疯狂的。但如果它不是？！

我认为这很好，因为我可以从require命令中解救出来，并向输出打印一条消息以通知用户有关缺少的宝石并完成它优雅，但我得到一个错误！

你能告诉我应该怎么做或这段代码有什么问题：

  begin 
需要some_gem
 rescue 
 puts请先安装some_gem！ 
 end 
  

解决方案

如果无法加载所需的库，require 会引发 LoadError 异常。但是，您从 LoadError 无法从任何地方拯救，您可以从 StandardError 中拯救。

如果您想要从 LoadError 进行救援，您必须这样说：

  begin 
 require'some_gem'
 rescue LoadError 
 puts'请先安装some_gem！'
 end 
  

更好的是，确保你正在打印正确的缺失依赖项：

  begin 
 require'some_gem'
 rescue LoadError => e 
 raise e.message =〜/ some_gem / 
 puts'请首先安装some_gem！'
 end 
  

（如果异常实际上是由其他某个其他缺失的库引起的，那么这会重新产生从中解救出的完全相同的异常，您不希望打印误导性信息，对吧？）

根据图书馆的预期目标读者是什么，以及他们是否被回溯到他们的控制台的回溯吓跑，你可能会想要在任何情况下重新抛出异常，而不是仅仅吞下它：

  begin 
 require'some_gem' 
 rescue LoadError =>如果e.message =〜/ some_gem / 
 raise 
 end 
  

'e
puts'请先安装some_gem！

或者，您可以跳过 puts ，而是引发异常并将消息设置为您想说的内容：

  begin 
 require'some_gem'
 rescue LoadError =>如果e.message =〜/ some_gem / 
 raise 
 end 
   e.exception（'请先安装some_gem！ pre> 
 
 
除了现在异常在错误的地方引发，因此具有错误的行号和堆栈跟踪，因此具有误导性，但很容易修复：
  begin 
 require'some_gem'
 rescue LoadError => e 
除非e.message =〜/ some_gem / 
 friendly_ex = e.exception（'请先安装some_gem！'）
 friendly_ex.set_backtrace（e.backtrace）
 raise friendly_ex 
 end 
  
现在，您打印的内容几乎与您使用 puts ，但是你有一个正确的异常，例如允许更好的调试，或者允许库的消费者拯救 / em>异常，并按照他们的方式处理它们，这两者都不可能，或者至少很难用你的解决方案来吞噬异常。
 
I'm writing a library that depend on a specific gem. I require the gem and use it in my code and everything is hunky-dory as long as the gem is installed on the user's machine. but what if it is not?!

I thought it's fine cause I can rescue from the require command and print a message to the output to inform the user about the lacking gem and finish it gracefully but I get an error!

Could you tell me how it should be done or what is wrong with this piece of code: 
begin 
 require "some_gem"
rescue
 puts "please install some_gem first!" 
end

解决方案
require raises a LoadError exception if it can't load the required library. However, you never rescue from LoadError anywhere, you rescue from StandardError.

If you want to rescue from LoadError, you have to say so:
begin 
  require 'some_gem'
rescue LoadError
  puts 'please install some_gem first!'
end
Even better yet, make sure that you are actually printing the correct missing dependency:
begin 
  require 'some_gem'
rescue LoadError => e
  raise unless e.message =~ /some_gem/
  puts 'please install some_gem first!'
end
(This re-raises the exact same exception that was rescued from, in case that the exception was actually caused by some other missing library somewhere else. You wouldn't want to print misleading information, right?)

Depending on what the intended target audience for the library is and whether or not they might be scared away by a backtrace being dumped to their console, you might want to re-raise the exception in any case, instead of just swallowing it:
begin 
  require 'some_gem'
rescue LoadError => e
  puts 'please install some_gem first!' if e.message =~ /some_gem/
  raise
end
Or, you could skip the puts and instead raise an exception with the message set to what you want to say:
begin 
  require 'some_gem'
rescue LoadError => e
  raise e.exception('please install some_gem first!') if e.message =~ /some_gem/
  raise
end
Except now the exception is raised in the wrong place and thus has the wrong line number and stacktrace and thus is misleading, but that is easily fixed:
begin 
  require 'some_gem'
rescue LoadError => e
  raise unless e.message =~ /some_gem/
  friendly_ex = e.exception('please install some_gem first!')
  friendly_ex.set_backtrace(e.backtrace)
  raise friendly_ex
end
Now you print pretty much the same thing that you would have printed with the puts, but you have a "proper" exception that for example allows better debugging or allows a consumer of your library to rescue that exception and handle it their way, both of which would have been impossible or at least hard with your solution that just swallows the exception.

                        
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