SRP: The StrictKeyAccess extension will raise an error whenever a key is accessed that does not exist in the hash.

In Python a "Hash" is called a "Dictionary", and ...

> "It is an error to extract a value using a non-existent key."

See: https://docs.python.org/2/tutorial/datastructures.html#dictionaries

EXAMPLE:

    class StrictHash < Hash
      include Hashie::Extensions::StrictKeyAccess
    end
    >> hash = StrictHash[foo: "bar"]
    => {:foo=>"bar"}
    >> hash[:foo]
    => "bar"
    >> hash[:cow]
      KeyError: key not found: :cow

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## Next Release
 
 * Your contribution here.
+* [#314](https://github.com/intridea/hashie/pull/314): Added a `StrictKeyAccess` extension that will raise an error whenever a key is accessed that does not exist in the hash - [@pboling](https://github.com/pboling).
 * [#304](https://github.com/intridea/hashie/pull/304): Ensured compatibility of `Hash` extensions with singleton objects - [@regexident](https://github.com/regexident).
 * [#306](https://github.com/intridea/hashie/pull/306): Added `Hashie::Extensions::Dash::Coercion` - [@marshall-lee](https://github.com/marshall-lee).
 * [#310](https://github.com/intridea/hashie/pull/310): Fixed `Hashie::Extensions::SafeAssignment` bug with private methods - [@marshall-lee](https://github.com/marshall-lee).

README.md

@@ -392,6 +392,25 @@ books.deep_locate -> (key, value, object) { key == :pages && value <= 120 }
 # => [{:title=>"Ruby for beginners", :pages=>120}, {:title=>"CSS for intermediates", :pages=>80}]
 ```
 
+## StrictKeyAccess
+
+This extension can be mixed in to allow a Hash to raise an error when attempting to extract a value using a non-existent key.
+    
+### Example:
+    
+```ruby
+class StrictKeyAccessHash < Hash
+  include Hashie::Extensions::StrictKeyAccess
+end
+
+>> hash = StrictKeyAccessHash[foo: "bar"]
+=> {:foo=>"bar"}
+>> hash[:foo]
+=> "bar"
+>> hash[:cow]
+  KeyError: key not found: :cow
+```
+
 ## Mash
 
 Mash is an extended Hash that gives simple pseudo-object functionality that can be built from hashes and easily extended. It is intended to give the user easier access to the objects within the Mash through a property-like syntax, while still retaining all Hash functionality.

lib/hashie.rb

@@ -26,6 +26,7 @@ module Hashie
     autoload :PrettyInspect,     'hashie/extensions/pretty_inspect'
     autoload :KeyConversion,     'hashie/extensions/key_conversion'
     autoload :MethodAccessWithOverride, 'hashie/extensions/method_access'
+    autoload :StrictKeyAccess, 'hashie/extensions/strict_key_access'
 
     module Parsers
       autoload :YamlErbParser, 'hashie/extensions/parsers/yaml_erb_parser'

lib/hashie/extensions/strict_key_access.rb

@@ -0,0 +1,71 @@
+module Hashie
+  module Extensions
+    # SRP: This extension will fail an error whenever a key is accessed that does not exist in the hash.
+    #
+    #   EXAMPLE:
+    #
+    #     class StrictKeyAccessHash < Hash
+    #       include Hashie::Extensions::StrictKeyAccess
+    #     end
+    #
+    #     >> hash = StrictKeyAccessHash[foo: "bar"]
+    #     => {:foo=>"bar"}
+    #     >> hash[:foo]
+    #     => "bar"
+    #     >> hash[:cow]
+    #       KeyError: key not found: :cow
+    #
+    # NOTE: For googlers coming from Python to Ruby, this extension makes a Hash behave like a "Dictionary".
+    #
+    module StrictKeyAccess
+      class DefaultError < StandardError
+        def initialize(msg = 'Setting or using a default with Hashie::Extensions::StrictKeyAccess does not make sense', *args)
+          super
+        end
+      end
+
+      # NOTE: This extension would break the default behavior of Hash initialization:
+      #
+      #     >> a = StrictKeyAccessHash.new(a: :b)
+      #     => {}
+      #     >> a[:a]
+      #       KeyError: key not found: :a
+      #
+      # Includes the Hashie::Extensions::MergeInitializer extension to get around that problem.
+      # Also note that defaults still don't make any sense with a StrictKeyAccess.
+      def self.included(base)
+        # Can only include into classes with a hash initializer
+        base.send(:include, Hashie::Extensions::MergeInitializer)
+      end
+
+      def [](key)
+        fetch(key)
+      end
+
+      def default(_ = nil)
+        fail DefaultError
+      end
+
+      def default=(_)
+        fail DefaultError
+      end
+
+      def default_proc
+        fail DefaultError
+      end
+
+      def default_proc=(_)
+        fail DefaultError
+      end
+
+      def key(value)
+        result = super
+        if result.nil? && (!key?(result) || self[result] != value)
+          fail KeyError, "key not found with value of #{value.inspect}"
+        else
+          result
+        end
+      end
+    end
+  end
+end

spec/hashie/extensions/strict_key_access_spec.rb

@@ -0,0 +1,99 @@
+require 'spec_helper'
+
+describe Hashie::Extensions::StrictKeyAccess do
+  class StrictKeyAccessHash < Hash
+    include Hashie::Extensions::StrictKeyAccess
+  end
+
+  shared_examples_for 'StrictKeyAccess with valid key' do |options = {}|
+    before { pending_for(options[:pending]) } if options[:pending]
+    context 'access' do
+      it('returns value') do
+        expect(instance[valid_key]).to eq valid_value
+      end
+    end
+    context 'lookup' do
+      it('returns key') do
+        expect(instance.key(valid_value)).to eq valid_key
+      end
+    end
+  end
+  shared_examples_for 'StrictKeyAccess with invalid key' do |options = {}|
+    before { pending_for(options[:pending]) } if options[:pending]
+    context 'access' do
+      it('raises an error') do
+        # Formatting of the error message varies on Rubinius and ruby-head
+        expect { instance[invalid_key] }.to raise_error KeyError
+      end
+    end
+    context 'lookup' do
+      it('raises an error') do
+        # Formatting of the error message does not vary here because raised by StrictKeyAccess
+        expect { instance.key(invalid_value) }.to raise_error KeyError,
+                                                              %(key not found with value of #{invalid_value.inspect})
+      end
+    end
+  end
+  shared_examples_for 'StrictKeyAccess with exploding defaults' do
+    context '#default' do
+      it 'raises an error' do
+        expect { instance.default(invalid_key) }.to raise_error Hashie::Extensions::StrictKeyAccess::DefaultError,
+                                                                'Setting or using a default with Hashie::Extensions::StrictKeyAccess does not make sense'
+      end
+    end
+    context '#default=' do
+      it 'raises an error' do
+        expect { instance.default = invalid_key }.to raise_error Hashie::Extensions::StrictKeyAccess::DefaultError,
+                                                                 'Setting or using a default with Hashie::Extensions::StrictKeyAccess does not make sense'
+      end
+    end
+    context '#default_proc' do
+      it 'raises an error' do
+        expect { instance.default_proc }.to raise_error Hashie::Extensions::StrictKeyAccess::DefaultError,
+                                                        'Setting or using a default with Hashie::Extensions::StrictKeyAccess does not make sense'
+      end
+    end
+    context '#default_proc=' do
+      it 'raises an error' do
+        expect { instance.default_proc = proc {} }.to raise_error Hashie::Extensions::StrictKeyAccess::DefaultError,
+                                                                  'Setting or using a default with Hashie::Extensions::StrictKeyAccess does not make sense'
+      end
+    end
+  end
+
+  let(:klass) { StrictKeyAccessHash }
+  let(:instance) { StrictKeyAccessHash.new(*args) }
+  let(:args) do
+    [
+      { valid_key => valid_value }
+    ]
+  end
+  let(:valid_key) { :abc }
+  let(:valid_value) { 'def' }
+  let(:invalid_key) { :mega }
+  let(:invalid_value) { 'death' }
+
+  context '.new' do
+    it_behaves_like 'StrictKeyAccess with valid key'
+    it_behaves_like 'StrictKeyAccess with invalid key'
+    it_behaves_like 'StrictKeyAccess with exploding defaults'
+  end
+
+  context '[]' do
+    let(:instance) { StrictKeyAccessHash[*args] }
+    it_behaves_like 'StrictKeyAccess with valid key', pending: { engine: 'rbx' }
+    it_behaves_like 'StrictKeyAccess with invalid key', pending: { engine: 'rbx' }
+    it_behaves_like 'StrictKeyAccess with exploding defaults'
+  end
+
+  context 'with default' do
+    let(:args) do
+      [
+        { valid_key => valid_value }, invalid_value
+      ]
+    end
+    it_behaves_like 'StrictKeyAccess with valid key'
+    it_behaves_like 'StrictKeyAccess with invalid key'
+    it_behaves_like 'StrictKeyAccess with exploding defaults'
+  end
+end