Accessing Ruby Method Arguments And Values Dynamically

1 minute read

Here’s the magic spell:

def sample(arg1, arg2='default value2', arg3:, arg4: 'default value4', arg5: 'default value5')
  method(__method__).parameters.each_with_object({}) { |(_, key), obj|
    obj[key] = binding.local_variable_get(key).to_s

result = sample 'value1', 'value2', arg3: 'value3', arg4: 'value4'

p result

This prints out all the arguments and their corresponding values as a hash:

{:arg1=>"value1", :arg2=>"value2", :arg3=>"value3", :arg4=>"value4", :arg5=>"default value5"}

Breakdown and Analyze

__method__ is a special ruby variable that returns the name of the method (as a symbol) it is placed in. So in our case, it returns :sample.

method(__method__) returns a Method object of this sample method.

method(__method__).parameters returns a 2D array of all the arguments. Each inner array has 2 elements: The first is the symbol representing the type of the argument. The 4 values I have seen so far are: :req, :opt, :keyreq, :key.

If you print the value of this array for the above method, you’ll see this:

[[:req, :arg1], [:opt, :arg2], [:keyreq, :arg3], [:key, :arg4], [:key, :arg5]]

We are then using each_with_object to collect the args and their values as an key-value pair of hash. Since the type of the argument, present as the first item in each inner array, is not required for our desired output, we ignore it with _ variable name.

To get the value of any local variable, present anywhere, we can use binding.local_variable(). You just have to pass the variable name as a symbol. You can try to print the value of variable ‘result’ using this technique:

blah= binding.local_variable_get(:result)
p blah
# outputs this:
# {:arg1=>"value1", :arg2=>"value2", :arg3=>"value3", :arg4=>"value4", :arg5=>"default value5"}