understanding Hash#fetch

example.rb

#!/usr/bin/env ruby
def param_method
  puts 'I was called both times'
  'result'
end

def block_method
  puts 'I was called only once'
  'result'
end

args = {a: 'result'}

args.fetch(:a, param_method)
args.fetch(:b, param_method)

args.fetch(:a) { block_method }
args.fetch(:b) { block_method }