如何从需要的“gem_name"中解救出来未安装 gem 时 [英] How to rescue from a require "gem_name" when the gem is not installed

问题描述

我正在编写一个依赖于特定 gem 的库.我需要 gem 并在我的代码中使用它，只要 gem 安装在用户的机器上，一切都很好.但如果不是呢?！

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?!

我认为这很好，因为我可以从 require 命令中解救出来，并在输出中打印一条消息，以通知用户缺少的 gem 并优雅地完成它，但我得到了一个错误！

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!

你能告诉我应该怎么做或者这段代码有什么问题吗:

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

推荐答案

require 如果无法加载所需的库，则会引发 LoadError 异常.但是，您从不从任何地方的 LoadError 中救援，而是从 StandardError 中救援.

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

如果你想从LoadError中解救，你必须这么说:

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

更好的是，确保您确实打印了 正确 缺少的依赖项:

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

(这会重新引发与被救出的完全相同的异常，以防异常实际上是由其他地方的其他缺失库引起的.您不想打印误导性信息，对吧?)

(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?)

根据库的预期目标受众是什么以及他们是否会被转储到控制台的回溯吓跑，您可能希望在任何情况下重新引发异常，而不是仅仅吞下它:

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

或者，您可以跳过 puts 而是引发异常，并将消息设置为您要说的内容:

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

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，但这很容易解决:

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

现在您打印的内容与使用 puts 打印的内容几乎相同，但是您有一个适当的"异常，例如允许更好的调试或允许你的图书馆的消费者来拯救那个异常并以他们的方式处理它，这两者都是不可能的，或者至少很难用你的解决方案来吞下异常.

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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