Monkey patches Hash#fetch to also show the entire hash in the error
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This gem monkey patches Hash#fetch so that KeyErrors show the contents of the hash in which the key wasn't found.


We are taught to use Hash#fetch instead of Hash#[], so that we don't get unexpected nils floating around in our code. Hash#fetch will raise an exception when the key isn't found, right where you tried to access it. Rather fail early then have a nil pop up somewhere else!

But there is one thing that annoys me. Say you're writing this code:

def hello(options)
  name = options.fetch(:name)
  puts "Hello, #{name}!"

Now, you're calling this method, passing in some params that you got from somewhere else:

# => key not found: :name (KeyError)

Blast! Now I have to see why params doesn't have the key :name in it. Did I misspel it somewhere? Was the name parameter a String instead of a Symbol?

What shall I do: put a puts in the code? Should I raise params.inspect, should I launch a debugger, or should I stare at the code long enough to see the mistake?

All these options suck. Just have verbose_hash_fetch loaded, and you'll see the contents of the hash appear in the error itself!

require 'verbose_hash_fetch'

# => key not found: :name in Hash: {:naem=>"Avdi"} (KeyError)

No more guessing, no more retrying. See it when it happens!


Add this line to your application's Gemfile:

gem 'verbose_hash_fetch'

And then execute:

$ bundle

Or install it yourself as:

$ gem install verbose_hash_fetch


If you have verbose_hash_fetch in your Gemfile, and you're using Rails, then you don't need to do anything.

If you need to load it manually:

require 'verbose_hash_fetch'


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request


I was complaining on Twitter, so Roel van Dijk made it and put it in a Gist. I just converted it into a gem.