In a larger project, I am catching specific exceptions, adding some details to the message and then re-raising the same exception. Here is the basic idea of what I'm doing.
#!/usr/bin/env ruby
begin
puts 12 / 0
rescue ZeroDivisionError => e
raise e.exception("new message #{e.message}")
end
When I execute temp.rb from the command line, two exceptions are printed:
% ./temp.rb
./temp.rb:4:in `/': new message divided by 0 (ZeroDivisionError)
from ./temp.rb:4:in `<main>'
./temp.rb:4:in `/': divided by 0 (ZeroDivisionError)
from ./temp.rb:4:in `<main>'
I'm assuming that there is a stack or a list of exceptions somewhere and the exit process of Ruby prints out the entire list or stack of exceptions. I can see great benefits to this. But in my case, I'd like to get rid of the original exception from the list or stack. Is there a way to do this?
The "second" (original) exception is the cause of the rescued exception which is references by your new exception as it is created with Exception#exception.
Starting with Ruby 3.1, irb prints the details of the cause in addition to the actual exception to aid in debugging. Previous Ruby versions have ignored the cause here.
Thus, you can use other means to handle your exceptions rather than using Ruby's default handler of printing the details of an otherwise unhandled exception, e.g. by adding an explicit exception handler. Alternatively, you can also explicitly set the cause as you are creating your new exception:
begin
puts 12 / 0
rescue ZeroDivisionError => e
raise e.class, "new message #{e.message}", cause: nil
end
May be something like this to extend Exception with custom method
Exception.class_eval do
def add_message(msg)
mod =
Module.new do
define_method :to_s do
"#{super()} #{msg}"
end
end
extend mod
end
end
And then
begin
puts 12 / 0
rescue ZeroDivisionError => e
raise e.add_message("(even don't try it)")
end
While debugging a really strange issue with ActionMailer, I came to realize I didn't know how to access an object that was creating the exception. Not the exception, but the object itself.
begin
AppMailer.send_invoice(hostel_resident).deliver_later
flash[:success] = "Your invoice was sent successfully!"
rescue => msg
# display the system generated error message
flash[:error] = "#{msg}"
end
NoMethodError: undefined method `disposition_type' for #<Mail::UnstructuredField:0x009g71c2a68258>
This code works great to catch any exceptions and print the message.
However, how do I get ahold of the <Mail::UnstructuredField:0x009g71c2a68258> object? I'd like to be able to play around with this guy, read messages inside it, and just generally have access to it.
This has to be possible, but inspect doesn't help, cause is no use and backtrace just shows you where it happened. I need that object though, the receiver of the nonexistent method.
Thanks!
actionmailer (4.2.4)
mail (2.6.3)
Seems like you're using mail gem. This is a known issue which is already reported in the GitHub. See #851.
Try using different version of the gem, something in 2.6 series.
This seems to work, using receiver on NameError (which NoMethodError is a child of)
obj = Object.new
puts obj.to_s
begin
obj.do_something
rescue NoMethodError => e
puts e.message
puts e.receiver
end
# #<Object:0x007fa5ac84da88>
# undefined method `do_something' for #<Object:0x007fa5ac84da88>
# #<Object:0x007fa5ac84da88>
This seems to require ruby >= 2.3, to do this for < 2.3, AFAIK you have to do something like this (not tested in older rubies, but should work):
class MyNoMethodError < NoMethodError
attr_accessor :my_receiver
end
obj = Object.new
puts obj.to_s
begin
begin
obj.do_something
rescue NoMethodError => e
# rescue the exception and wrap it in the method that caused it, using `self` instead of `obj`
error = MyNoMethodError.new(e)
error.my_receiver = obj
raise error
end
rescue MyNoMethodError => c
puts c.inspect # custom exception stuff
puts c.cause.inspect # original exception stuff
puts c.my_receiver
end
# #<Object:0x007f884e846d58>
# #<MyNoMethodError: undefined method `do_something' for #<Object:0x007f884e846d58>>
# #<NoMethodError: undefined method `do_something' for #<Object:0x007f884e846d58>>
# #<Object:0x007f884e846d58>
I'm using a Gem called bindata which has the following code block:
def assign(val)
raise ArgumentError, "can't set a nil value for #{debug_name}" if val.nil?
unless has_parameter?(:value)
raw_val = val.respond_to?(:snapshot) ? val.snapshot : val
#value = begin
raw_val.dup
rescue TypeError
# can't dup Fixnums
raw_val
end
end
end
This code is meant to dup the object sent to the assign function, but a Fixnum cannot be dup'd so the creators put the rescue block in there. I have tested this code multiple times in Rails Console and the rescue works fine. However:
I, [2017-04-08T00:03:47.165387 #27899] INFO -- : *** below you find the most recent exception thrown, this will be likely (but not certainly) the exception that made the application exit abnormally ***
E, [2017-04-08T00:03:47.165450 #27899] ERROR -- : can't dup Fixnum (TypeError)
/home/blueserver/.rvm/gems/ruby-1.9.3-p327-turbo#symphony/gems/bindata-1.4.3/lib/bindata/base_primitive.rb:90:in `dup'
/home/blueserver/.rvm/gems/ruby-1.9.3-p327-turbo#symphony/gems/bindata-1.4.3/lib/bindata/base_primitive.rb:90:in `assign'
(eval):3:in `assign'
It seems like this code is working in catching the error %99.99 of the time, but every once in a while it crashes instead. Any idea why the rescue would randomly fail? My code is splattered with calls to the Bindata gem, so putting a rescue surrounding every call would be a tedious effort.
Thanks!
I have the following code:
rescue Timeout::Error, StandardError => e
puts "Caught exception: #{e.message}".red
log.puts("#{e.backtrace}")
email_ids_all.each do |email_delete|
call= "/api/v2/emails/#{email_delete}/"
......
Before this rescue piece I have defined log and email_ids_all. However, neither of these are recognized within the ruby script. If i do this:
rescue Timeout::Error, StandardError => e
File.open(rescuelogfile, 'w') do |log| #setup log to write response codes.
puts "Caught exception: #{e.message}".red
log.puts("#{e.backtrace}")
email_ids_all.each do |email_delete|
call= "/api/v2/emails/#{email_delete}/"
....
log works fine, which makes sense. It would take a lot of writing to redefine the email_ids_all array and other variables contained inside my rescue block.
Is there anyway to allow variables to be recognized inside the rescue? Basically my code is laid out like this:
begin
#some code
rescue
#above code
end
I am using ruby 1.9.3.
EDIT----
log starts right after my begin statement :
begin
File.open(logfile, 'w') do |log| #setup log to write response codes.
log.puts works throughout the entire code except when an error is thrown, and then it runs the rescue script where log is not available.
The same goes for email_ids_all. There is an API call that generates about 10,000 emails and each of them is added to the array email_ids_all. The script is receiving an error about halfway through generating these emails, and so I need the rescue script to delete all email ids in the email_ids_all array. But for whatever reason, I get the following error:
FS_Test_Env.rb:762:in `block in <main>': undefined local variable or method `email_ids_all' for main:Object (NameError)
from FS_Test_Env.rb:759:in `open'
from FS_Test_Env.rb:759:in `rescue in <main>'
from FS_Test_Env.rb:7:in `<main>'
Any thoughts?
The way you put it, it should work, for example:
irb(main):001:0> begin
irb(main):002:1* x = 1
irb(main):003:1> x / 0
irb(main):004:1> rescue Exception => e
irb(main):005:1> p x
irb(main):006:1> end
1
=> 1
So it looks like the exception is thrown before your variables are defined.
The scope of the block parameter log is limited to that block. This is the whole point of the open with block.
Maybe you want to do:
begin
log = File.open('logfile', 'w')
...
rescue
...
ensure
log.close
end
Note that this does not cover errors when opening the logfile.
Regarding email_ids_all, I guess (!) you have the exception in a statement like:
email_ids_all = ... a long and complex calculation which raises an exception
If yes, the problem is that the assignment happens only after the whole right-hand side is calculated. The var email_ids_all is not yet created when the exception happens.
In order to access the elements created before the exception, you have to keep track of them, e.g.
begin
email_ids = []
10000.times do
email_ids << ... # create element eventually raising an exception
end
rescue
... # treat the already created elements
end
How do I add information to an exception message without changing its class in ruby?
The approach I'm currently using is
strings.each_with_index do |string, i|
begin
do_risky_operation(string)
rescue
raise $!.class, "Problem with string number #{i}: #{$!}"
end
end
Ideally, I would also like to preserve the backtrace.
Is there a better way?
To reraise the exception and modify the message, while preserving the exception class and its backtrace, simply do:
strings.each_with_index do |string, i|
begin
do_risky_operation(string)
rescue Exception => e
raise $!, "Problem with string number #{i}: #{$!}", $!.backtrace
end
end
Which will yield:
# RuntimeError: Problem with string number 0: Original error message here
# backtrace...
It's not much better, but you can just reraise the exception with a new message:
raise $!, "Problem with string number #{i}: #{$!}"
You can also get a modified exception object yourself with the exception method:
new_exception = $!.exception "Problem with string number #{i}: #{$!}"
raise new_exception
I realize I'm 6 years late to this party, but...I thought I understood Ruby error handling until this week and ran across this question. While the answers are useful, there is non-obvious (and undocumented) behavior that may be useful to future readers of this thread. All code was run under ruby v2.3.1.
#Andrew Grimm asks
How do I add information to an exception message without changing its class in ruby?
and then provides sample code:
raise $!.class, "Problem with string number #{i}: #{$!}"
I think it is critical to point out that this does NOT add information to the original error instance object, but instead raises a NEW error object with the same class.
#BoosterStage says
To reraise the exception and modify the message...
but again, the provided code
raise $!, "Problem with string number #{i}: #{$!}", $!.backtrace
will raise a new instance of whatever error class is referenced by $!, but it will not be the exact same instance as $!.
The difference between #Andrew Grimm's code and #BoosterStage's example is the fact that the first argument to #raise in the first case is a Class, whereas in the second case it is an instance of some (presumably) StandardError. The difference matters because the documentation for Kernel#raise says:
With a single String argument, raises a RuntimeError with the string as a message. Otherwise, the first parameter should be the name of an Exception class (or an object that returns an Exception object when sent an exception message).
If only one argument is given and it is an error object instance, that object will be raised IF that object's #exception method inherits or implements the default behavior defined in Exception#exception(string):
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.
As many would guess:
catch StandardError => e
raise $!
raises the same error referenced by $!, the same as simply calling:
catch StandardError => e
raise
but probably not for the reasons one might think. In this case, the call to raise is NOT just raising the object in $!...it raises the result of $!.exception(nil), which in this case happens to be $!.
To clarify this behavior, consider this toy code:
class TestError < StandardError
def initialize(message=nil)
puts 'initialize'
super
end
def exception(message=nil)
puts 'exception'
return self if message.nil? || message == self
super
end
end
Running it (this is the same as #Andrew Grimm's sample which I quoted above):
2.3.1 :071 > begin ; raise TestError, 'message' ; rescue => e ; puts e ; end
results in:
initialize
message
So a TestError was initialized, rescued, and had its message printed. So far so good. A second test (analogous to #BoosterStage's sample quoted above):
2.3.1 :073 > begin ; raise TestError.new('foo'), 'bar' ; rescue => e ; puts e ; end
The somewhat surprising results:
initialize
exception
bar
So a TestError was initialized with 'foo', but then #raise has called #exception on the first argument (an instance of TestError) and passed in the message of 'bar' to create a second instance of TestError, which is what ultimately gets raised.
TIL.
Also, like #Sim, I am very concerned about preserving any original backtrace context, but instead of implementing a custom error handler like his raise_with_new_message, Ruby's Exception#cause has my back: whenever I want to catch an error, wrap it in a domain-specific error and then raise that error, I still have the original backtrace available via #cause on the domain-specific error being raised.
The point of all this is that--like #Andrew Grimm--I want to raise errors with more context; specifically, I want to only raise domain-specific errors from certain points in my app that can have many network-related failure modes. Then my error reporting can be made to handle the domain errors at the top level of my app and I have all the context I need for logging/reporting by calling #cause recursively until I get to the "root cause".
I use something like this:
class BaseDomainError < StandardError
attr_reader :extra
def initialize(message = nil, extra = nil)
super(message)
#extra = extra
end
end
class ServerDomainError < BaseDomainError; end
Then if I am using something like Faraday to make calls to a remote REST service, I can wrap all possible errors into a domain-specific error and pass in extra info (which I believe is the original question of this thread):
class ServiceX
def initialize(foo)
#foo = foo
end
def get_data(args)
begin
# This method is not defined and calling it will raise an error
make_network_call_to_service_x(args)
rescue StandardError => e
raise ServerDomainError.new('error calling service x', binding)
end
end
end
Yeah, that's right: I literally just realized I can set the extra info to the current binding to grab all local vars defined at the time the ServerDomainError is instantiated/raised. This test code:
begin
ServiceX.new(:bar).get_data(a: 1, b: 2)
rescue
puts $!.extra.receiver
puts $!.extra.local_variables.join(', ')
puts $!.extra.local_variable_get(:args)
puts $!.extra.local_variable_get(:e)
puts eval('self.instance_variables', $!.extra)
puts eval('self.instance_variable_get(:#foo)', $!.extra)
end
will output:
#<ServiceX:0x00007f9b10c9ef48>
args, e
{:a=>1, :b=>2}
undefined method `make_network_call_to_service_x' for #<ServiceX:0x00007f9b10c9ef48 #foo=:bar>
#foo
bar
Now a Rails controller calling ServiceX doesn't particularly need to know that ServiceX is using Faraday (or gRPC, or anything else), it just makes the call and handles BaseDomainError. Again: for logging purposes, a single handler at the top level can recursively log all the #causes of any caught errors, and for any BaseDomainError instances in the error chain it can also log the extra values, potentially including the local variables pulled from the encapsulated binding(s).
I hope this tour has been as useful for others as it was for me. I learned a lot.
UPDATE: Skiptrace looks like it adds the bindings to Ruby errors.
Also, see this other post for info about how the implementation of Exception#exception will clone the object (copying instance variables).
Here's another way:
class Exception
def with_extra_message extra
exception "#{message} - #{extra}"
end
end
begin
1/0
rescue => e
raise e.with_extra_message "you fool"
end
# raises an exception "ZeroDivisionError: divided by 0 - you fool" with original backtrace
(revised to use the exception method internally, thanks #Chuck)
My approach would be to extend the rescued error with an anonymous module that extends the error's message method:
def make_extended_message(msg)
Module.new do
##msg = msg
def message
super + ##msg
end
end
end
begin
begin
raise "this is a test"
rescue
raise($!.extend(make_extended_message(" that has been extended")))
end
rescue
puts $! # just says "this is a test"
puts $!.message # says extended message
end
That way, you don't clobber any other information in the exception (i.e. its backtrace).
I put my vote that Ryan Heneise's answer should be the accepted one.
This is a common problem in complex applications and preserving the original backtrace is often critical so much so that we have a utility method in our ErrorHandling helper module for this.
One of the problems we discovered was that sometimes trying to generate more meaningful messages when a system is in a messed up state would result in exceptions being generated inside the exception handler itself which led us to harden our utility function as follows:
def raise_with_new_message(*args)
ex = args.first.kind_of?(Exception) ? args.shift : $!
msg = begin
sprintf args.shift, *args
rescue Exception => e
"internal error modifying exception message for #{ex}: #{e}"
end
raise ex, msg, ex.backtrace
end
When things go well
begin
1/0
rescue => e
raise_with_new_message "error dividing %d by %d: %s", 1, 0, e
end
you get a nicely modified message
ZeroDivisionError: error dividing 1 by 0: divided by 0
from (irb):19:in `/'
from (irb):19
from /Users/sim/.rvm/rubies/ruby-2.0.0-p247/bin/irb:16:in `<main>'
When things go badly
begin
1/0
rescue => e
# Oops, not passing enough arguments here...
raise_with_new_message "error dividing %d by %d: %s", e
end
you still don't lose track of the big picture
ZeroDivisionError: internal error modifying exception message for divided by 0: can't convert ZeroDivisionError into Integer
from (irb):25:in `/'
from (irb):25
from /Users/sim/.rvm/rubies/ruby-2.0.0-p247/bin/irb:16:in `<main>'
Here's what I ended up doing:
Exception.class_eval do
def prepend_message(message)
mod = Module.new do
define_method :to_s do
message + super()
end
end
self.extend mod
end
def append_message(message)
mod = Module.new do
define_method :to_s do
super() + message
end
end
self.extend mod
end
end
Examples:
strings = %w[a b c]
strings.each_with_index do |string, i|
begin
do_risky_operation(string)
rescue
raise $!.prepend_message "Problem with string number #{i}:"
end
end
=> NoMethodError: Problem with string number 0:undefined method `do_risky_operation' for main:Object
and:
pry(main)> exception = 0/0 rescue $!
=> #<ZeroDivisionError: divided by 0>
pry(main)> exception = exception.append_message('. With additional info!')
=> #<ZeroDivisionError: divided by 0. With additional info!>
pry(main)> exception.message
=> "divided by 0. With additional info!"
pry(main)> exception.to_s
=> "divided by 0. With additional info!"
pry(main)> exception.inspect
=> "#<ZeroDivisionError: divided by 0. With additional info!>"
This is similar to Mark Rushakoff's answer but:
Overrides to_s instead of message since by default message is defined as simply to_s (at least in Ruby 2.0 and 2.2 where I tested it)
Calls extend for you instead of making the caller do that extra step.
Uses define_method and a closure so that the local variable message can be referenced. When I tried using a class variable ##message, it warned, "warning: class variable access from toplevel" (See this question: "Since you're not creating a class with the class keyword, your class variable is being set on Object, not [your anonymous module]")
Features:
Easy to use
Reuses the same object (instead of creating a new instance of the class), so things like object identity, class, and backtrace are preserved
to_s, message, and inspect all respond appropriately
Can be used with an exception that is already stored in a variable; doesn't require you to re-raise anything (like the solution that involved passing the backtrace to raise: raise $!, …, $!.backtrace). This was important to me since the exception was passed in to my logging method, not something I had rescued myself.
Most of these answers are incredibly convoluted. Maybe they were necessary in Ruby 1.8 or whatever, but in modern versions* this is totally straightforward and intuitive. Just rescue => e, append to e.message, and raise.
begin
raise 'oops'
rescue => e
e.message << 'y daisy'
raise
end
Traceback (most recent call last):
4: from /Users/david/.rvm/rubies/ruby-2.7.2/bin/irb:23:in `<main>'
3: from /Users/david/.rvm/rubies/ruby-2.7.2/bin/irb:23:in `load'
2: from /Users/david/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/irb-1.2.6/exe/irb:11:in `<top (required)>'
1: from (irb):2
RuntimeError (oopsy daisy)
* I've only tested with 2.7.2 and 3.1.2, but I assume everything in between is covered, and probably some earlier versions of 2.x as well.
Another approach would be to add context (extra information) about the exception as a hash instead of as a string.
Check out this pull request where I proposed adding a few new methods to make it really easy to add extra context information to exceptions, like this:
begin
…
User.find_each do |user|
reraise_with_context(user: user) do
send_reminder_email(user)
end
end
…
rescue
# $!.context[:user], etc. is available here
report_error $!, $!.context
end
or even:
User.find_each.reraise_with_context do |user|
send_reminder_email(user)
end
The nice thing about this approach is that it lets you add extra information in a very concise way. And it doesn't even require you to define new exception classes inside which to wrap the original exceptions.
As much as I like #Lemon Cat's answer for many reasons, and it's certainly appropriate for some cases, I feel like if what you are actually trying to do is attach additional information about the original exception, it seems preferable to just attach it directly to that exception it pertains to rather than inventing a new wrapper exception (and adding another layer of indirection).
Another example:
class ServiceX
def get_data(args)
reraise_with_context(StandardError, binding: binding, service: self.class, callee: __callee__) do
# This method is not defined and calling it will raise an error
make_network_call_to_service_x(args)
end
end
end
The downside of this approach is that you have to update your error handling to actually use the information that may be available in exception.context. But you would have to do that anyway in order to recursively call cause to get to the root excetion.
It's possible to use :cause key to prevent message duplication
The cause of the generated exception (accessible via Exception#cause) is automatically set to the "current" exception ($!), if any. An alternative value, either an Exception object or nil, can be specified via the :cause argument.
begin
do_risky_operation
rescue => e
raise e.class, "#{e.message} (some extra message)", e.backtrace, cause: nil
end