在 Ruby 中捕获异常后重新引发(相同的异常)

我试图通过捕捉异常来提高我的 Ruby 技能。我想知道，当有多个方法调用时，是否经常重新引发同样的异常。那么，下面的代码有意义吗？是否可以重新引发相同类型的异常，或者我不应该在 process 方法上捕获它？

class Logo
def process
begin
@processed_logo = LogoProcessor::create_image(self.src)
rescue CustomException
raise CustomException
end
end
end


module LogoProcessor
def self.create_image
raise CustomException if some_condition
end
end
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最佳答案

Sometimes we just want to know an error happened, without having to actually handle the error.

It is often the case that the one responsible for handling errors is user of the object: the caller. What if we are interested in the error, but don't want to assume that responsibility? We rescue the error, do whatever we need to do and then propagate the signal up the stack as if nothing had happened.

For example, what if we wanted to log the error message and then let the caller deal with it?

begin
this_will_fail!
rescue Failure => error
log.error error.message
raise
end

Calling raise without any arguments will raise the last error. In our case, we are re-raising error.

In the example you presented in your question, re-raising the error is simply not necessary. You could simply let it propagate up the stack naturally. The only difference in your example is you're creating a new error object and raising it instead of re-raising the last one.

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This will raise the same type of error as the original, but you can customize the message.

rescue StandardError => e
raise e.class, "Message: #{e.message}"
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A slightly better way to do the same thing as FreePender is to use the exception method from the Exception class, which is the ancestor class to any error classes, like StandardError, so that the method is available to any error classes.

Here the method's documentation that you can find on ApiDock:

With no argument, or if the argument is the same as the receiver, return the receiver. Otherwise, create a new exception object of the same class as the receiver, but with a message equal to string.to_str.

Now let's see how it works:

begin
this_will_fail!
rescue Failure => error
raise error.exception("Message: #{error.message}")
end
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I had the same question as in the comment thread here, i.e. What if the line before (re)raise fails?

My understanding was limited by the missing knowledge that the global variable of $! is "kinda garbage collected" // "scoped to its functional context", which the below example demonstrates:

def func
begin
raise StandardError, 'func!'
rescue StandardError => err
puts "$! = #{$!.inspect}"
end
end


begin
raise StandardError, 'oh no!'
rescue StandardError => err
func
puts "$! = #{$!.inspect}"
raise
end

The output of the above is:

$! = #<StandardError: func!>
$! = #<StandardError: oh no!>
StandardError: oh no!
from (pry):47:in `__pry__'

This behavior is different than how Python's (re)raise works.

The documentation for Exception states:

When an exception has been raised but not yet handled (in rescue, ensure, at_exit and END blocks), two global variables are set:

  • $! contains the current exception.
  • $@ contains its backtrace.

So these variables aren't true global variables, they are only defined inside the block that's handling the error.

begin
raise
rescue
p $!  # StandardError
end


p $!    # nil
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Adding to above answers here:

  • In some applications you may need to log the error twice.
  • For example exception need to be notified to monitoring tools like Nagios/Newrelic/Cloudwatch.
  • At the same time you may have your own kibana backed summary logging tool, internally for your reference.

In those cases you might want to log & handle the errors multiple times.

Example:

begin
begin
nil.to_sym
rescue => e
puts "inner block error message: #{e.message}"
puts "inner block backtrace: #{e.backtrace.join("\n")}"
raise e
end
rescue => e
puts "outer block error message: #{e.message}"
puts "outer block backtrace: #{e.backtrace.join("\n")}"
end

I am using puts here, for your the ease of verifying this code in rails console, in actual production you may need to use rails logger


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