Quick and Simple HTTP Server Using Python

You can use Python as a quick way to host static content. On Windows, there are many options for running Python, I’ve personally used Cygwin and ActivePython.

To use Python as a simple HTTP server just change your working directory to the folder with your static content and type python -m SimpleHTTPServer 8000, everything in the directory will be available at http:/localhost:8000/

Edit: To do this with Python, 3.4.1 (and probably other versions of Python 3), use the http.server module:

python -m http.server 8000

or possibly:

python3 -m http.server 8000

on Windows:

py -m http.server 8000

External API Authentication in Rails using Devise and JWT Tokens


We want to create an API for our Rails application which requires a user to first authenticate with their username and password to verify their identity, but subsequently, we wish to identify the user using a JWT token.

So what is a JWT anyway?

The advantage of using JWTs (or JSON Web Tokens) is that they are an industry standard (RFC 7519) method for representing claims securely between two parties. They are trustworthy because they are digitally signed and secure.JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA or ECDSA. Once the user is logged in, each subsequent request will include the JWT, allowing the user to access routes, services, and resources that are permitted with that token.

A typical JSON Web Token looks something like this (taken from the jwt.io website).

Our solution will focus on an HMAC solution. What’s more, the secret (which is used to encrypt the token) will change each and every time a user authenticates with the server. In this way, if the JWT does get into the wrong hands (you are using SSL, aren’t you?) simply re-authenticating will make the previous secret invalid and the JWT would become useless. However, you should still use safeguards to protect the token as much as possible and do not keep them longer than required.

After the secret is generated a token is generated and returned to the user, the client can send it back whenever accessing a protected resource or route (typically) in the Authorization header using the Bearer schema: Authorization: Bearer <token>

Because they’re small, portable and reliable; JWTs are becoming extremely popular for web-based API authentication and rapidly becoming the industry standard.

But what happens if the JWT is intercepted and stolen?

The short answer is that it’s really bad. But what makes it less bad than a username and password being compromised is that it can be immediately invalidated without requiring (or impacting) on directly on the user. Also, the token itself is only useful to an attacker for a limited time. Once the token expires, it becomes useless.

However, it should be noted that there are circumstances where a stolen JWT can actually be worse. This largely depends on how the token was obtained in the first place. If an attacker has successfully executed a man-in-the-middle attack, the hacker may be able to simply obtain a new token whenever required.

Basically, as always there is no silver bullet when it comes to security concerns, and you should always follow best practices and take your own server security into account. While this has worked safely for some time in my applications; this code has been modified and generalized to make this tutorial easier to understand, it should not be considered a complete and secure implementation for a production environment.

Enough with the disclaimer; get to the solution!

This is actually really easy to setup in Rails with Devise. There are 2 main components. The first is a special API session controller to handle the initial authentication. Since this will not be completed through the standard Rails for and devise controller, we need to make a controller to handle it. I recommending creating a specialized session controller to do this so that the API authentication is structurally separate from the rest of your application so that it can be isolated for security and testing purposes.

module Api
class SessionsController < Devise::SessionsController
skip_before_action :verify_authenticity_token

def create
self.resource = warden.authenticate!(auth_options)
sign_in(resource_name, resource)
respond_to do |format|
format.json {render json: {token: generate_token(self.resource)}}

current_user.update_attributes(shared_secret: nil, token_expires: nil)


JWT.encode(token_payload(resource), resource.shared_secret, 'HS256')

{user: resource.email, exp: 1.week.from_now}

shared_secret: create_secret,
token_expires: 1.week.from_now


It should be pretty self explanatory. It accepts an email and password and performs the same actions as a regular Devise controller. it signs the user in, and then updates the user record with a randomly generated secret and sets an expiry for the secret generation.

You also need to configure a route so you can post the email and password to this controller. Its a little more complicated than usual but not overwhelmiingly so:

namespace :api do
devise_for :users, skip: :all
devise_scope :user do
post 'users', to: 'sessions#create', as: nil

This technique of authenticating the user session can be used so you can determine how long it has been since the user ACTUALLY authenticated. You could write a rake task to automatically invalidate all secrets older than a specified duration. Our example will not go into the expiry of the token, but it should be easy for any experienced Rails dev.

Next the actual API controller class. This is the meat and potatoes. All your API controllers should inherit from this controller. What this will do is bypass the usual Devise authentication process and instead look at the request header for the Authentication Token. You must supply two values. An API Key that has been uniquely assigned to each user is paired with a valid Authentication Token. This is to help strengthen the JWT Token so that it is not solely responsible for the authentication (both the JWT and the API key must be compromised).

After the API Key and the JWT Authentication Token have been verified, the system will allow the continuation of the child controller action. Notice that the current_user and user_signed_in will be available as normal.

module Api
class ApiController < ApplicationController
skip_before_action :verify_authenticity_token
skip_before_action :authenticate_user!
before_action :authenticate_api_key!
before_action :authenticate_user_from_token!
protect_from_forgery with: :null_session





@resource ||= user_with_key(apikey_from_request).where(email: claims[0]['user']).first
if @resource.nil?
raise Pundit::NotAuthorizedError.new('Unable to deserialize JWT token.')
StandardError => e
Rails.logger.error e
raise Pundit::NotAuthorizedError.new(e)

if apikey_from_request.present?
unless user_with_key(apikey_from_request).present?
raise Pundit::NotAuthorizedError.new('Unable to verify the apii key.')

claims(token = token_from_request, key: shared_key)
JWT.decode(token, key, true)
rescue JWT::DecodeError => e
raise Pundit::NotAuthorizedError.new(e)

jwt_token(user, key: shared_key)
expires = (DateTime.now + 1.day).to_i
JWT.encode({user: user.email, exp: expires}, key, 'HS256')

# Accepts the token either from the header or a query var
# Header authorization must be in the following format
# Authorization: Bearer {yourtokenhere}
auth_header = request.headers['Authorization']
token = auth_header.try(:split, ' ').try(:last)
unless Rails.env.production?
if token.to_s.empty?
token = request.parameters.try(:[], 'token')

# Accepts the ApiKey either from the header or a query var
# Header
ApiKey must be in the following format
ApiKey: {yourkeyhere}
key = request.headers.try(:[], 'ApiKey').try(:split, ' ').try(:last)
if !Rails.env.production? && key.blank?
key = request.parameters.try(:[], 'apikey')

user_secret.tap do |key|
raise Pundit::NotAuthorizedError.new('Unable to verify the secret.') if key.blank?

return if apikey_from_request.nil?

return if apikey_from_request.nil?
User.where(private_key: key).where('private_key_expires > ?', Time.zone.now)

There you go. You now have the basis of a pretty good API Authentication Layer for your Rails app!

A few points of note:

  • This code will allow you (for ease of testing) to supply both the JWT and the API Key as query string parameters when NOT in production mode. However, in production, the request MUST use the correct headers.
  • This code assumes the use of the most excellent Pundit gem and raises a Pundit::NotAuthorizedError if the authentication fails. If you use something else, like CanCan you will need to raise an error appropriate to your application.
  • You may want to expose the current_user and user_signed_in? as controller helpers, if you need to access them in your views, but in order to keep this somewhat minimalistic, I have omitted this.
  • If you want more information about the security implications of using JWTs for Authentication and how to mitigate security risks, this is an excellent resource.
  • If you need help learning more about JWTs or if you would like an online tool to help generate valid JWTs for testing JWT.io is a very neat website.

Automatic Rails Model Notification Concerns

This ActiveRecord Concern is a module I am particularly proud of. It was developed for an application that had the need for an advanced notification system on a whole slew of database changes. Rather than just wire up a basic notification job to each controller action that triggered each model change, I elected to write a model concern that automatically triggered the notification system on different ActiveRecord changes.

The concerns worked amazingly well, and assisted in not only keeping our controllers very light, but also meant that database changes could not escape notifying users of the change.

# This concerns allows you to directly hookup ActiveRecord model changes
# directly into a system-wide notification system using ActiveSupport
# Callbacks. Jobs can be created to reflect the exact work you want done
# when a specific event occurs in the lifecycle of the model you want
# to be notified on.
# ==== Example
#   class MyModel < ApplicationRecord
#     include Notifyable
#     notify :on_create, :handler_job
module Notifyable
  # Make this module a concerns and include the ActiveSupport callbacks module
  extend ActiveSupport::Concern
  include ActiveSupport::Callbacks

  included do
    # Add a has_many model association for the notifications events.
    has_many :notification_events, as: :item

    # This code opens up the parent class and generates several methods
    # directly into it providing the core foundation of the
    # Notifyable concerns. It declares the available callbacks
    # and runs the associated callback when the custom defined
    # calback is triggered.
    %w(on_initialize on_update on_save on_create on_commit on_destroy on_find).each {|name|
      module_eval <<-RUBY, __FILE__, __LINE__ + 1
        define_callbacks :#{name}
        #{name.gsub('on', 'after')} :notify_#{name}
        def notify_#{name.to_s}
          run_callbacks :#{name} do
            @invocation = :#{name}

    # Opens up the calling class so methods can be redefined on
    # the current object. We need to add the +notify+ method so
    # that we can define what callbacks should be watched to
    # trigger notifications.
    # FIXME: Class variables for handlers are bugged if
    # FIXME: different models use different handlers. I
    # FIXME: would love to refactor this so that you can 
    # FIXME: provide a &block instead of just a handler
    # FIXME: name/symbol.
    # Callbacks are always appended *after* the source event
    # declared; so that +:on_save+ will actually declare itself
    # as a +:after_save+ on the parent ActiveRecord class.
    class << self
      def notify(name, *handlers)
        @@handlers ||= HashWithIndifferentAccess.new
        @@handlers.store(name, handlers)
        set_callback(name, :after, :handler_callback)



  # Execute the callback for each handler invocation. It
  # is expected that there will be a corresponding ActiveJob
  # to handle the notification within a +Notification+
  # namespace with the same name as the invocation class,
  # followed by the handler name.
  # Example:
  #   +Notification::MyClassHandlerJob+
  def handler_callback
    @@handlers[@invocation].each do |handler|
      eval "Notification::#{self.class.name}#{handler.to_s.classify}.perform_later(self, '#{@invocation}')"


As directed in the comments of the concerns, the only thing needed to make this work is a method call in your model telling the module when the handler should be notified of the change, and what trigger should it attach. For example, lets assume you have a TodoItem model:

class TodoItem < ApplicationRecord
# Include the Notifyable concerns
include Notifyable

# Instruct Notifyable on which callback and handler should be used.
notify :on_create, :handler_job

Lastly, as you can see from the concerns, you now need to create an ActiveJob class called: Notification::TodoItemHandlerJob which will be enqueued whenever a ‘TodoItem’ database record is created. This job can do whatever you need to do in order to notify the relevant stakeholders of a new TodoItem record.

Whats more, this will be done asynchronously to the main thread of your application which should make your application more performant.

There are a few improvements I would eventually like to make to this:

  1. I’d like to package it as a gem and monkey patch it to the abstract ApplicationRecord class so that the concerns is automatically included on all models and that the include statement is not required.
  2. I’d like to be able to pass a &block to #notify instead of the handler job symbol because then you could eliminate that disgusting eval in the protected #handler_callback
  3. There is a bug in this concerns regarding the class variables the concerns. Because the class variables are stored at the class level, and not the ‘consuming’ class using the concerns, the handlers were being overwritten between models (which means that each model most use the same handler symbol). Fortunately, in the project that uses this, the conformity of all the notification handler jobs using the suffix ‘HandlerJob’ was deemed preferred anyway so it was not seen as a big problem. Alas, I’d very much like to fix it one day.

Social Network Aggregate API Factory Design Pattern in Ruby

I worked on a project which allowed users to authenticate using oauth with several well known social media platforms. After users had linked all their social media presences; we wanted to import the posts for each user from each platform. This is a (redacted) sample of how I accomplished this:

First, I have a neat little module that allows me to encapsulate a list of handler objects and a notification method that can trigger the correct handler based on how the handler has subscribed itself in the factory. This is a little confusing, but basically, each provider class will initialise itself into the factory object, having subscribed itself to a particularly type of social network which it is able to process (handle). I simply abstracted this code because I thought it might be very handy for other projects which follow a similar pattern to this:

module EventDispatcher
def setup_listeners
@event_dispatcher_listeners = {}

def subscribe(event, &callback)
(@event_dispatcher_listeners[event] ||= []) << callback

def notify(event, *args)
if @event_dispatcher_listeners[event]
@event_dispatcher_listeners[event].each do |m|
m.call(*args) if m.respond_to? :call

Next we need to develop the Factory object to encapsulate our handler objects. It contains all the configuration attributes of our social network platform API keys and secrets, etc. It instantiates with a hash which has a static method #load to read a specified file (or by default a file in /config/social_network_configuration.json) which returns an instance of itself with the contents of the configuration file passed into the constructor:

require File.expand_path('../../event_dispatcher', __FILE__)

module SocialNetworking
class SocialNetworkFactory
include EventDispatcher
attr_reader :configs

def initialize(data)
@configs = {}
data.each {|n, o| @configs.store n.downcase.to_param.to_sym, o}

process(network, user)
notify(network, user)

# Reads client configuration from a file and returns an instance of the factory
# @param [String] filename
# Path to file to load
# @return [
# Social network factory with API configuration
def self
.load(filename = nil)
if filename && File.directory?(filename)
search_path = File.expand_path(filename)
filename = nil
filename == nil
search_path ||= File.expand_path("#{Rails.root}/config")
if File.exist?(File.join(search_path, 'social_network_configuration.json'))
filename = File.join(search_path, 'social_network_configuration.json')
elsif search_path == '/' || search_path =~ /[a-zA-Z]:[\/\\]/
raise ArgumentError,
'No ../config/social_network_configuration.json filename supplied ' +
'and/or could not be found in search path.'
search_path = File.expand_path(File.join(search_path, '..'))
data = File.open(filename, 'r') {|file| MultiJson.load(file.read)}
return self.new(data)

The configuration file (/config/social_network_configuration.json) looks something like:

"Facebook": {
"oauth_access_token": "...",
"expires": ""
"SoundCloud": {
"client_id": "..."
"Twitter": {
"access_token": "...",
"access_token_secret": "...",
"consumer_key": "...",
"consumer_secret": "..."
"YouKu": {
"client_id": "..."
"YouTube": {
"dev_key": "..."
"Weibo": {
"app_id": "..."

The last part is to create a different handler object for each social network (as each social network has its own specific API for interfacing with the platform. Its pretty basic:

module SocialNetworking
module Providers
class NetworkNameProvider

def initialize(factory)
# you can access the configurations through the factory
@app_id = factory.configs[:network_name]['app_id']

# instruct the factory that this provider handles the
# 'network_name' social network oauth. The factory will
# publish the users authorization object to this handler.
factory.subscribe(:network_name) do |auth|

# Do stuff ...



So an example of a Weibo Provider class might look something like this:

require File.expand_path('../../../../lib/net_utilities', __FILE__)
require 'base62'
require 'httpi'

module SocialNetworking
module Providers
class WeiboProvider
include NetUtilities

def initialize(factory)
@token = get_token
@app_id = factory.configs[:weibo]['app_id']

factory.subscribe(:weibo) do |auth|
Rails.logger.info " Checking Weibo user '#{auth.api_id}'"

@token = auth.token unless auth.token.nil? # || auth.token_expires < DateTime.now
request = HTTPI::Request.new 'https://api.weibo.com/2/statuses/user_timeline.json'
request.query = {source: @app_id, access_token: @token, screen_name: auth.api_id}
response = HTTPI.get request
if response.code == 200
result = MultiJson.load(response.body)
weibos = result['statuses']

weibos.each {|post|

... do something with the post

auth.checked_at = DateTime.now
rescue Exception => e
Rails.logger.warn " Exception caught: #{e.message}"
@token = get_token


auth = Authorization.where(provider: 'weibo').where('token_expires < ?', DateTime.now).shuffle.first
auth = Authorization.where(provider: 'weibo').order(:token_expires).reverse_order.first if auth.nil?
raise 'Cannot locate viable Weibo authorization token' if auth.nil?

id.to_s[0..-15].to_i.base62_encode.swapcase + id.to_s[-14..-8].to_i.base62_encode.swapcase + id.to_s[-7..-1].to_i.base62_encode.swapcase


Of course there are a lot of opportunities too refactor and make the providers better. For example a serious argument could be made that the API handshake should be abstracted to a seperate class to be consumed by the provider rather than the provider doing all the API lifting itself (violates the single-responsibility principal) – but I include it inline to give better idea on how this factory works without getting too abstracted.

The last piece of this puzzle is putting it all together. There are a lot of different ways you could consume this factory; but in this example I am going to do it as a rake task that can be regularly scheduled via a cron task.

Dir["#{File.dirname(__FILE__)}/../social_networking/**/*.rb"].each {|f| load(f)}

namespace :social_media do
'Perform a complete import of social media posts of all users'
task import: :environment do
factory = SocialNetworking::Atlas::SocialNetworkFactory.load
# Instantiate each of your providers here with the factory object.
SocialNetworking::Atlas::Providers::NetworkNameProvider.new factory
SocialNetworking::Atlas::Providers::WeiboProvider.new factory

# Execute the Oauth authorizations in a random order.
Authorization.where(muted: false).shuffle.each do |auth|
factory.process(auth.provider.to_sym, auth)

I wouldn’t do this in production though, as you may encounter problems if the task gets triggered when the previous iteration is still running. Additionally, I would recommend leveraging ActiveJob to run each handler which would give massive benefits to execution concurrency and feedback on job successes/failures.

Also, you could get really clever and loop over each file in the /providers directory and include and instantiate them all at once, but I have chosen to explicitly declare it in this example.

As you can see this is a nice little pattern which uses some pseudo-event subscription and processing to allow you to import from multiple APIs and maintaining separation of responsibilities. As we loop over each authorization record, this pattern will automatically hand the auth record to the correct handler. You can also chop and change which providers are registered; as any authorization record that doesn’t have a registered handler for its type, will simply be ignored. This means that if the Weibo API changes and we need to fix our handler; it is trivial to remove the handler from production by commenting it out, and all our other handlers will continue to function like nothing ever happened.

This code was written many years ago; and should work on Ruby versions even as old as 1.8. There are probably many opportunities too refactor and enhance this code substantially using a more recent Ruby SDK. Examples of possible enhancements would be allowing the providers to subscribe to the factory using a &block instead of a symbol and allowing the factory to pass a block into to #process method to give access for additional processing to be executed in the context of the provider; but abstracted from it.

Nevertheless, I hope that this pattern proves useful to anyone needing a design pattern to have a handler automatically selected to process some work without complicated selection logic.

Skipping an ActiveRecord Callback Programatically

I am a massive fan of ActiveSupport callbacks and use them frequently. This allows me to chain behaviours together; essentially using data storage as an event based system to enforce business logic. An example of this, is to use after_create_commit callbacks to automatically trigger an email that needs to be sent; such as welcome user email, to automatically generate some accounting record, or an admin email notification.

This does have some drawbacks however. It means that you really need to have a good grasp of the domain logic; and means that it becomes critically important to choose the correct way to update the record (update_column vs update_attribute) lest you fail to trigger important business logic, or trigger them when you shouldn’t. But generally, when used appropriately I find them invaluable. But sometimes you might find yourself in a situation where you need to run some code (such as in a rake task) where you cannot influence the method used to update the database, but the callbacks must not be executed.

As I said, if you have direct control over the ActiveRecord relation, then its easy:

@object.update_column(:the_attribute, 'value')

These will update the database, but skip validations and callbacks.

But if the updates are being triggered by another class/code outside of the scope or control of where you are, this wont work. Perhaps you care calling a method of a related class, and that method specifies .update_attribute and you cannot change it. What then?

Fortunately, there is a solution.

Lets say you have a class definition:

class User < ActiveRecord::Base
after_save :my_method

There are 2 ways you can save the object without triggering the callback.

Method #1:


But thats super gross! Using .send is a code smell if ever there was one.

Method #2:

User.skip_callback(:save, :after, :my_method)

This is much more civilised. What’s more, after you’ve run your rspec, mock, rake task whatever: you can reset the callback with: User.set_callback(:save, :after, :my_method).

Old School Cache Invalidation in a world of Rails 5.2 Recyclable Cache Keys

One of the hallmark features of Rails 5.2 was the introduction of recyclable cache keys.

Once the new cache API was integrated into Basecamp, DHH had this to say about it:

We went from only being able to keep 18 hours of caching to, I believe, 3 weeks. It was the single biggest performance boost that Basecamp 3 has ever seen.

Put simply, the previous cache key generation (unless you over-wrote it) used the updated_at timestamp to differentiate the specific version fo the object. So for example [class]/[id]-[timestamp], like users/5-20110218104500 or thing/15-20110218104500, which is what Active Record in Rails used to do by default when you call #cache_key.

With Rails 5.2 this no longer works as you expect. Instead, it’s simply [class]/[id] -the timestamp is dropped.

The basic idea behind it is that the cache object can be updated directly; and thus cache keys can be reused, dramatically lowering the quantity of cache garbage and increasing the amount of useful objects stored in the cache; which should let you see a significant performance hit by increasing your successful cache hits.

The only drawback is that the cache needs to be updated using the new cache API – if you don’t do this, you will suddenly see that your cached objects no longer ‘automatically’ expire.

Whilst I do recommend upgrading your codebase to use the new cache API, you can disable it and return to the older #cache_key style (using update_at timestamp) by simply adding this to your config/environments environment file:

config.active_record.cache_versioning = false

This should get you working in the old way – I will write a post on how to maximise use of the new Rails cache API in a future post.

Decorating Devise (current_user) with Draper

Devise is a wonderful and amazing gem to handle user authentication and provides a number of extremely useful utility helpers throughout your Rails application. Draper is an equally amazing gem, which provides a framework for using the decorator pattern in your Rails applications.

The model that encapsulates your User is usually one of the objects most in need of the decorator pattern, however, the current_user object provided by Devise returns the undecorated version of the user model. However, a simple method placed in your base ApplicationController class can solve this more elegantly that constantly calling current_user.decorate throughout your code:

def current_user   
UserDecorator.decorate(super) unless super.nil?

How to Log Which Line Initiated a Query in Rails

Create a file (something like) config/initializers/active_record_log_subscriber.rb

…and paste this into its contents:

module LogQuerySource
  def debug(*args, &block)
    return unless super

    backtrace = Rails.backtrace_cleaner.clean caller

    relevant_caller_line = backtrace.detect do |caller_line|

    if relevant_caller_line
      logger.debug("  ↳ #{ relevant_caller_line.sub("#{ Rails.root }/", '') }")
ActiveRecord::LogSubscriber.send :prepend, LogQuerySource

Now, whenever a line of code initiates a SQL query the line will be logged.  This greatly improves the ease of tracking down inefficiencies such as like N + 1 queries.

If you would like to learn more about exactly how this mix-in works and how the original author went about figuring out how to add it, you can read more about it here.


Commenting and Documentation in Ruby and Rails

It’s always been frustrating to me that Ruby and Rails don’t really have strong templated standards on how to best document your code. There are several options available and it can all be very confusing and unclear.

There is one template I’ve found works best and its super simple:

# Brief explanation of what the code does.
# * *Args* :
# - ++ ->
# * *Returns* :
# -
# * *Raises* :
# - ++ ->
def my_method


While there are many other perfectly good documentation libraries this has the benefits of being simple to remember, very easy to read in code, and compiles out super nicely using

rake doc:app

Example (strongly based off this post):

# This is an example method commented the way I 
# like. It sums the three arguments and returns 
# that value.
# Lorem ipsum dolor sit amet, consectetur adipising,
# do eiusmod tempor incididunt ut labore et dolore. 
# Ut enim ad minim veniam, quis nostrud exercit
# laboris nisi ut aliquip ex ea commodo consequat.
# * *Args* :
#   - +apples+ -> the number of apples
#   - +oranges+ -> the number of oranges
#   - +pears+ -> the number of pears
# * *Returns* :
#   - the total number of fruit as an integer
# * *Raises* :
#   - +ArgumentError+ -> if any value is nil or negative
def sum_fruit(apples, oranges, pears)

Will render into something like this:

Other than this, I tend to try and follow the general conventions as outlined by the Rails contribution guidelines on commenting.

NotImplementedError Must Have Battered Wife Syndrome

I’ve been doing something very wrong my whole life.  I think it’s time to confess and seek help.

You see, like so many developers I see the NotImplementedError class in various languages and think, to my self “I haven’t had a chance to fully implement this class, so in my methods that my colleagues call that I didn’t finish yet, I will raise that exception to let them know.”

I am ashamed of myself.  And it’s time to get help.

You see the actual purpose of the NotImplementedError (as defined by the Ruby docs) is:

Raised when a feature is not implemented on the current platform. For example, methods depending on the fsync or fork system calls may raise this exception if the underlying operating system or Ruby runtime does not support them.

ERMERGERD!  I am a terrible person. Abusing this error class just because  my sub classes cannot meet the contractual obligation of the super class.

And I know I am not alone.

So whats the best practice, then?

Well, in Ruby’s case it turns out the answer is super simple; don’t raise an error at all! Instead, document the expectation and if a subclass fails to meet it a NameError, or often its subclass NoMethodError, will be raised automatically.