Is there a way to access method arguments in Ruby?

New to Ruby and ROR and loving it each day, so here is my question since I have not idea how to google it (and I have tried :) )

we have method

def foo(first_name, last_name, age, sex, is_plumber)
    # some code
    # error happens here
    logger.error "Method has failed, here are all method arguments #{SOMETHING}"    
end

So what I am looking for way to get all arguments passed to method, without listing each one. Since this is Ruby I assume there is a way :) if it was java I would just list them :)

Output would be:

Method has failed, here are all method arguments {"Mario", "Super", 40, true, true}

In Ruby 1.9.2 and later you can use the parameters method on a method to get the list of parameters for that method. This will return a list of pairs indicating the name of the parameter and whether it is required.

e.g.

If you do

def foo(x, y)
end

then

method(:foo).parameters # => [[:req, :x], [:req, :y]]

You can use the special variable __method__ to get the name of the current method. So within a method the names of its parameters can be obtained via

args = method(__method__).parameters.map { |arg| arg[1].to_s }

You could then display the name and value of each parameter with

logger.error "Method failed with " + args.map { |arg| "#{arg} = #{eval arg}" }.join(', ')

---

Note: since this answer was originally written, in current versions of Ruby eval can no longer be called with a symbol. To address this, an explicit to_s has been added when building the list of parameter names i.e. parameters.map { |arg| arg[1].to_s }

Since Ruby 2.1 you can use binding.local_variable_get to read value of any local variable, including method parameters (arguments). Thanks to that you can improve the accepted answer to avoid evil eval.

def foo(x, y)
  method(__method__).parameters.map do |_, name|
    binding.local_variable_get(name)
  end
end

foo(1, 2)  # => 1, 2

One way to handle this is:

def foo(*args)
    first_name, last_name, age, sex, is_plumber = *args
    # some code
    # error happens here
    logger.error "Method has failed, here are all method arguments #{args.inspect}"    
end

This is an interesting question. Maybe using local_variables? But there must be a way other than using eval. I'm looking in Kernel doc

class Test
  def method(first, last)
    local_variables.each do |var|
      puts eval var.to_s
    end
  end
end

Test.new().method("aaa", 1) # outputs "aaa", 1

This may be helpful...

  def foo(x, y)
    args(binding)
  end

  def args(callers_binding)
    callers_name = caller[0][/`.*'/][1..-2]
    parameters = method(callers_name).parameters
    parameters.map { |_, arg_name|
      callers_binding.local_variable_get(arg_name)
    }    
  end

If you need arguments as a Hash, and you don't want to pollute method's body with tricky extraction of parameters, use this:

def mymethod(firstarg, kw_arg1:, kw_arg2: :default)
  args = MethodArguments.(binding) # All arguments are in `args` hash now
  ...
end

Just add this class to your project:

class MethodArguments
  def self.call(ext_binding)
    raise ArgumentError, "Binding expected, #{ext_binding.class.name} given" unless ext_binding.is_a?(Binding)
    method_name = ext_binding.eval("__method__")
    ext_binding.receiver.method(method_name).parameters.map do |_, name|
      [name, ext_binding.local_variable_get(name)]
    end.to_h
  end
end

You can define a constant such as:

ARGS_TO_HASH = "method(__method__).parameters.map { |arg| arg[1].to_s }.map { |arg| { arg.to_sym => eval(arg) } }.reduce Hash.new, :merge"

And use it in your code like:

args = eval(ARGS_TO_HASH)
another_method_that_takes_the_same_arguments(**args)

Before I go further, you're passing too many arguments into foo. It looks like all of those arguments are attributes on a Model, correct? You should really be passing the object itself. End of speech.

You could use a "splat" argument. It shoves everything into an array. It would look like:

def foo(*bar)
  ...
  log.error "Error with arguments #{bar.joins(', ')}"
end

If you would change the method signature, you can do something like this:

def foo(*args)
  # some code
  # error happens here
  logger.error "Method has failed, here are all method arguments #{args}"    
end

Or:

def foo(opts={})
  # some code
  # error happens here
  logger.error "Method has failed, here are all method arguments #{opts.values}"    
end

In this case, interpolated args or opts.values will be an array, but you can join if on comma. Cheers

It seems like what this question is trying to accomplish could be done with a gem I just released, https://github.com/ericbeland/exception_details. It will list local variables and vlaues (and instance variables) from rescued exceptions. Might be worth a look...

If the function is inside some class then you can do something like this:

class Car
  def drive(speed)
  end
end

car = Car.new
method = car.method(:drive)

p method.parameters #=> [[:req, :speed]]