Ruby – Classes and Modules

This document expands on Ruby's features, focusing on classes and modules.

Official Documentation: https://ruby-doc.org/

Table of Contents

Click to expand contents

• Getting Started
• Introduction
• Classes
  • Define a class
  • Instances
  • Attributes
  • Method Access Control
  • Initialize Method
  • Challenge: Dice
• Class Attributes and Methods
  • Class Methods
  • Class Attributes
  • Class read/write methods
  • Challenge: Radio
• Inheritance
  • Class Inheritance
  • Override and Extend
  • Access the superclass
  • Challenge: Secure Radio
• Dates and Times
  • Time
  • Date
  • DateTime
  • Challenge: Birth Date Analysis
• Modules
  • Namespacing
  • Mixins
  • Load, Require and Include
  • Challenge: To-do List
• Exceptions
  • Handle Exceptions
  • Handle Specific Exceptions
  • Exception Methods
  • Raise Exceptions
  • Custom Exceptions
  • Challenge: Custom Exceptions
• Further Resources

Getting Started

This repository follows Kevin Skoglund's Ruby Classes and Modules course and makes use of concepts mentioned on the Ruby Fundamentals repo.

To get started, Ruby needs to be installed on the machine:

ruby -v 

Should return something similar to: ruby 2.3.7p456 (2018-03-28 revision 63024) [universal.x86_64-darwin18]

If the ruby command is not found, install Ruby.

Introduction

Ruby is an Object Oriented Programming (OOP) language and custom classes and modules can be defined to improve how code is organized with the aim to make it easier to maintain and add new features.

Classes provide templates for creating objects or instances of a class that have unique attributes and behavior and can interact with each other in complex ways.

Classes

An important concept of OOP is likely to be classes. This section will look at defining custom classes.

A class defines code grouped by a common theme or purpose that provides a template of the state and behavior of objects created from it.

Objects that exist in Ruby have classes as well, such as 'Integer', 'String', 'Symbol', the 'NilClass' and many more.

Ruby can be asked what class an object belongs to by using the class method:

# classes/define-class.rb
word = 'Ruby'
word.class # String

[1..5].class # Array

There are built-in Ruby classes, however custom ones can be defined.

Define a class

Classes are defined in a block wrapped by the class keyword, followed by the name of the class, and end.

# classes/define-class.rb
class ShoppingCart
  #...
end

Classes are typically defined within the following guidelines:

• Named using singular nouns.
• Capitalized and camel cased names. (e.g, "ShoppingCart")
• Define each class in a separate file
• Grouped class files in a directory

Classes can be redefined and modified without error which is not common is most programming languages.

Classes can have attributes which hold the current state of an object and methods which define behavior of the object, similar to custom methods.

Additionally, the Ruby path does not need to be added to class files, only to the main script file:

#! /usr/bin/env ruby

Instances

An instance is a single object that is created from a class definition. For example, assigning a String to a variable is creating a new instance of the String class:

# classes/instances.rb
food = "pasta" # Instance of String
food = String.new # Also an instance of a string
array = Array.new # Instance of an array
hash = Hash.new # Instance of a hash

Objects can be instantiated from custom classes as well:

# classes/instances.rb
class Person
end

person_1 = Person.new
person_2 = Person.new

Each instance is based on its respective class, however they might hold different values. For example, a String class can hold any combination of characters, "Ruby" or 7u3y. Both are strings but have different values.

This can be observed by checking the object_id of both instances of Person. Each value will be different, meaning that each one is taking a different part of memory to run.

person_1.object_id # 70142496862940
person_2.object_id # 70142496862780

A custom method can be added to a class and used on any instance:

# classes/instances.rb
class Person
  def greet
    "Hello!"
  end
end

person_3 = Person.new
puts person_3.greet # Hello!

Additionally, it is more common for classes to be defined on a single file. These are then required into other scripts, it could even be a main script file. Usually the main script file will be named init.rb and other files might be imported:

# ruby_project/init.rb
require_relative 'classes/person'
person = Person.new
puts person.greet # Hello!

The example above shows the file where the Person class is defined being imported into a main script file. Then a Person can be instantiated and methods associated with that object can be used.

Attributes

Attributes are values that persist in the instance of a class and are what makes an instance unique. For instance, a Person might be named Salia and another named Joe, this value would live in an attribute, part of that instance.

Attributes can be seen as properties of an object and in some programming languages that is how they are called.

It might be easier to think of it as properties in some cases, as for instance a car could be green. Green is a property of a car but it could be any other color and different cars will have other colors.

A house might have two rooms and one bathroom but others might have a different number of rooms and bathrooms. These are all properties a house can have but in Ruby they are called attributes.

Attributes will be stored inside instance variables and are set using variable scope indicators.

# classes/attributes.rb
class Animal
  def make_noise
    @noise
  end

  def set_noise
    @noise = 'Oink!'
  end
end

In the example above the instance variable @noise is used in two methods, one to set it and another to return it.

# classes/attributes.rb
pig = Animal.new
pig.set_noise
puts pig.make_noise

When an instance of Animal is created, the value of @noise persists in the instance. It can be accessed in both methods and this is an important difference from local variables in terms of scope. An instance of a class always has access to its instance variables. From anywhere inside its class definition it can pull values from instance variables.

Read/Write Methods

In some cases instance variables might require to be accessed outside an instance. It was already established this is not possible but since methods are accessible outside an instance, these can be used to get access to an instance variable.

These methods are called reader/writer methods, also known as getter/setter methods in other programming languages. These are methods that allow reading or writing to an attribute.

A more traditional way of naming these methods would be using the get and set keywords to name them, with set taking an argument:

# classes/attributes.rb
class Animal
  def get_noise
    @noise
  end

  def set_noise(value)
    @noise = value
  end
end

However, Ruby has a convention for setting these methods which aid their use when an object is instantiated:

# classes/attributes.rb
class Animal
  def noise
    @noise
  end

  def noise=(value)
    @noise = value
  end
end

This means that when, for instance, pig.noise is called it will return the value for @noise. However, when pig.noise('Oooooink!') is called it won't set the value for @noise in that instance since that is not the name of the method defined, noise= is:

# classes/attributes.rb
pig.noise=('Oink, oink, oink!')
pig.noise # Oink, oink, oink!

This syntax is not the most common but it conveys that noise= is the name of the method. Defining a method with that name allows Ruby's sugar syntax to be used and call it in a way that it seems the instance variable value is being set instead:

# classes/attributes.rb
pig.noise = 'Oink, oink!'
pig.noise # Oink, oink!

Ruby is aware that it is a method despite being called this way. If one of these methods is left out it would create a case where an instance variable is either read only or write only.

Attribute Methods

Attribute methods are sometimes called attr_*, with * being a placeholder for the three methods currently available:

• attr_reader
• attr_writer
• attr_accessor

Attribute start with attr_ and they are referenced as attribute writer, attribute accessor even if not all of it is typed out.

This adds another way of making attributes available outside an instance. For example, using attr_reader :noise replaces the method defined earlier to access it:

# Using attribute reader is the same as the method below.
attr_reader :noise
# Replaced by attr_reader
def noise
  @noise
end

The writer method could be replaced with attr_writer and would have the same effect:

# Using attribute reader is the same as the method below.
attr_writer :noise

# Replaced by attr_writer
def noise=(value)
  @noise = value
end

The third method (attr_accessor) can be used when both read/write is needed. This allows an attribute to be both read and set in one line:

# Using attribute reader is the same as the method below.
attr_accessor :noise
# Replaced by attr_accessor
def noise
  @noise
end
# Replaced by attr_accessor
def noise=(value)
  @noise = value
end

The attr_accessor method is commonly used in classes, multiple attributes can be added in one go:

# classes/attributes.rb
attr_accessor :noise, :age

These can be used interchangeably with methods since there might be cases where it could make sense to have a custom read/write method.

For example, in the class Radio below, @volume is readable and a custom method is defined to set it. This allows to control how the value is received.

However, the instance variable is being changed directly in crank_it_up and being accessed directly in volume_status instead of using attr_reader.

class Radio
  # Volume attribute can be read.
  attr_reader :volume
  
  # Write method is custom and checks if the value is as expected before setting it.
  def volume=(value)
    return if value < 1 || value > 10
    @volume = value
  end

  # Bypasses the writter method and sets volume to a value it wouldn't normally accept.
  def crank_it_up
    @volume = 11
  end

  def volume_status
    "Current volume #{@volume}"
  end

The example above is valid, however the convention is to use the self keyword when setting an attribute (self.volume) and omit it when volume is being read:

  #...
  def crank_it_up
    self.volume = 11
  end

  def volume_status
    "Current volume #{volume}"
  end
  #...

When setting a value, self is used to make it clear to Ruby that the value is being set to an instance variable rather than a local one. When a value is being read it is convention to omit it but it would be valid if self was present there.

Using self boils down to a few points:

• Use self to reference the current instance from inside the class definition.
• Add self when calling writer methods (self.first_name=)
  • Helps Ruby understand whether it is calling a set method.
• Omit self when calling any other method (first_name)
• When in doubt, including self is always going to be the safest choice.

Method Access Control

Methods are the primary interfaces to a class, they can be used to set values and execute other actions. These interfaces should only be exposed when necessary, not all methods will require to run outside its class definition.

This is where access control comes in, restricts access to methods from outside an instance. It defines which methods are callable and which ones aren't.

There are three levels of method access control:

Access control level	Description
public	Anyone can access the method (default)
protected	Can only be called by instances of the class and its subclasses
private	Can only be called by instances of a class

The focus of this section will be on public and private access as protected is better covered when class inheritance in introduced.

One method can be made private by adding the keyword above them to a class. It only needs to be added once and will make any methods below restricted to run inside the class only:

# classes/method_access_control.rb
class Person
  attr_accessor :first_name, :last_name
  
  def full_name
    "#{first_name} #{last_name}"
  end  

  def initial_and_last_name
    "#{get_initial(first_name)} #{last_name}"
  end  

  private
    def get_initial(name)
      name.chars.first + '.'
    end
end

ree = Person.new
ree.first_name = "Rhianon"
ree.last_name = "Farrow"
puts ree.full_name
puts ree.initial_and_last_name

The initial_and_last_name can be called from an instance outside its class definition and it makes use of a method that is private to format its return value.

Additionally, the public keyword was not used since it can be omitted, any methods above the private keyword will be public.

There could also be protected methods by adding the keyword to the class.

# classes/method_access_control.rb
class Person
  attr_accessor :first_name, :last_name
  
  def full_name
    "#{first_name} #{last_name}"
  end  

  def initial_and_last_name
    "#{get_initial(first_name)} #{last_name}"
  end

  protected
    def another_method
      "A protected method"
    end

  private
    def get_initial(name)
      name.chars.first + '.'
    end
end

When defining classes it is important to think about the access control of its methods as it is best practice to only allow access to methods when necessary.

Initialize Method

The initialize method can be used to instantiate objects with default or customizations. It is called automatically whenever a new instance of a class is created. For example, when Animal.new is called, initialize will be called with it.

# classes/initialize_method.rb
class Animal
  attr_accessor :noise

  def initialize
    @noise = 'Ooof!'
    puts "New animal instantiated."
  end
end

It is useful to set attributes to a default or specific state. This is done by passing arguments to the new method which get passed on to the initialize method. Typically, this is done using an options hash.

# classes/initialize_method.rb
def initialize(options={})
  @noise = options[:noise] || 'Ooof!'
end
#...

pig = Animal.new({noise: "Oink!"})
puts pig.noise

There's a challenge available for this chapter: Dice

Class Attributes and Methods

Subsequent sections will introduce class attributes and methods. These are different to instance attributes and methods since they can run without a class being instantiated.

Class Methods

Class methods are used to define behaviors related to a class generally and not a specific instance. These methods are called directly on the class, not an instance.

An example is the new method. When Animal.new is called an instance hasn't been created yet. The new method is a class method that is being called directly on the Animal class.

Perhaps a more practical example would be Bicycle.all_brands. Calling this method would list all brands a bicycle can have. However, an instance only has one. The all_brands method does not apply to a specific bicycle but returns information of what brand one might have.

To define a class method, self is added to the method name. The name of the class would also work but most Rubyists would prefer self.

# class-attributes-and-methods/class_methods.rb
class Animal
  def self.list_classes
    [
      'mammalia', 
      'actinopterygii', 
      'chondrichthyes', 
      'aves', 
      'amphibia', 
      'anthropods', 
      'reptilia'
    ]
  end
end

puts Animal.list_classes

The class method returns array of classes an animal can belong to, and can be called on the Animal class itself without using an instance.

A common pattern is to use class methods to create instances of class with custom values. This is called the factory pattern:

# class-attributes-and-methods/class_methods.rb
class Product
  attr_accessor :name, :price

  def initialize(price=0.0)
    @price = price
  end

  def self.exclusive
    Product.new(45.0)
  end

  def self.standard
    Product.new(30.0)
  end

  def self.discounted
    Product.new(15.0)
  end
end

puts "Deals of the day: "
exclusive_product = Product.exclusive
discounted_product = Product.discounted
puts "Exclusive: #{exclusive_product.price}"
puts "Discounted: #{discounted_product.price}"

The example above uses class methods to create instances of different types of products whilst using instance attributes for the name and price as these can be different for each instance.

Ruby documentation convention on class and instance methods

In Ruby documentation there is a convention for referencing both class and instance methods that aid in knowing which one is being used. Typically, when using a class method, dot notation is used (Array.new) and when using an instance method a hash sign is used instead (Array#size):

• Class method: Array.new
• Instance method: Array#size

This only applies to documentation and dot notation should be used on both cases when writing Ruby scripts.

Class Attributes

Similar to class methods, class attributes relate to a class generally and not to any specific instance. This is what makes them different from instance attributes.

These are stored in a class and become shared values among all instances of the class and any instance can access it. Class attributes are used less frequently than class methods.

The first aspect to understand is how to define a class variable. Class variables will be defined with @@ (double at sign) prefixing its name, see variable scope indicators for more whole table of variable declarations.

# class-attributes-and-methods/class_attributes.rb
class Animal
  @@species = ['cat', 'cow', 'dog', 'duck', 'horse', 'pig']

  def self.species
    @@species
  end
end

Animal.species # 

A class attribute is defined and a class method is used to return its value. Class attributes can also be used to track how many instances of a class were created:

# class-attributes-and-methods/class_attributes.rb
class Animal
  @@current_animals = []

  def initialize
    @@current_animals << self
  end

  def self.current_animals
    @@current_animals
  end

  def self.count
    @@current_animals.count
  end
end

cat = Animal.new
dog = Animal.new
chicken = Animal.new
sheep = Animal.new

Animal.count # 4
Animal.current_animals # [#<Animal:0x00007fd0292a83d8>,#<Animal:0x00007fd0292a8388>, #<Animal:0x00007fd0292a8360>, #<Animal:0x00007fd0292a8338>]

Class read/write methods

Class read/write methods are similar to instance read/write methods. However, there are not equivalent to attr_* methods except on the Ruby on Rails framework as the cattr_* but not on Ruby yet. This could be because they're not used that often.

Read/write methods are defined almost the same way as instance read/write methods with the difference of class ones making use of the self keyword:

# class-attributes-and-methods/class_read_write_methods.rb
class Animal
  @@species = ['cat', 'cow', 'dog', 'duck', 'horse', 'pig']

  def self.species
    @@species
  end

  def self.species=(array)
    return unless array.is_a(Array)
    @@species = array
  end
end

In the example above there is a method to access Animal.species and another to set it, using the same syntactic sugar mentioned when defining these methods for instance variables:

# class-attributes-and-methods/class_read_write_methods.rb
Animal.species # ['cat', 'cow', 'dog', 'duck', 'horse', 'pig']
Animal.species = ['elephant', 'crocodile']

The species class attribute can read and reset with other values when read/write methods are in place.

There's a challenge available for this chapter: Radio

Inheritance

The concept of inheritance is a fundamental part of object-oriented programming (OOP). Classes can inherit methods and attributes from others, further extending their functionality.

Class Inheritance

It is the bestowal of behaviors from another class. A class can be extended by another to make use of its methods and attributes.

In previous sections an Animal class was defined and different animals would be instantiated from it. This approach is completely valid, however, inheritance could also be applied to create classes that inherit Animal behavior. For example a Cow, Lion or Snake class could all inherit from Animal.

This means that Animal would become a superclass or parent class whilst any others that inherit from it would become subclasses or children classes.

# inheritance/class_inheritance.rb
class Animal
  attr_accessor :noise
end

class Pig < Animal
  def initialize
    @noise = "Oink!"
  end
end

class Cow < Animal
  def initialize
    @noise = "Moo!"
  end
end

In the example above the Pig and Cow classes inherit @noise from Animal. A parent class must be defined before a subclass can inherit from it, so any superclass files need to be required before any subclasses are used.

Inheritance can be useful to organize code and make it easier to stick to DRY (Don't Repeat Yourself) principles.

Override and Extend

In short, subclasses override and extend behaviors of its parent class.

When creating a subclass with inheritance, the same behaviors and attributes of its parent (superclass) are passed down. Overriding and extending a subclass's behaviors is useful in order to create more complex functionality. Otherwise, if just inheritance was used a new class would be created from its parent but everything aside from a few values would differ between the two, it would act exactly like the parent.

# inheritance/override_extend.rb
class Sofa
  @@can_open = false
  attr_accessor :width, :length

  def area
    width * length
  end
end

class SofaBed < Sofa
  @@can_open = true
  attr_accessor :length_opened
  attr_reader  :is_open

  def area
    @is_open ? width * @length_opened : width * length
  end
  
  def open
    @is_open = true
  end

  def close
    @is_open = false
  end
end

In the example above, the SofaBed class is extended to have two additional attributes. The SofaBed#area method is overridden to make use of these extra attributes.

It also has two additional methods to open or close the sofa. These do not exist in the Sofa class and can only be used when a SofaBed is instantiated.

Access the superclass

This section will describe a technique to access the superclass (parent class) from a subclass (child class). After overriding methods, the parent methods can still be accessed.

There are two main reasons why this might be useful. One is to perform additional code before and/or after a parent method is executed, adding some extra steps in the beginning or cleanup in the end.

class Chef
  def make_dinner
    puts "Cook food."
  end
end

class AmateurChef < Chef
  def make_dinner
    puts "Read recipe"
    super
    puts "Clean up mess."
  end
end

In the example above, the Chef class is able to make_food in one step. AmateurChef however, has to read the recipe and clean up the mess afterwards. The AmateurChef#make_dinner is overridden to add these extra steps but Chef#make_dinner is called in between them using the super keyword.

When super is called, it will run Chef#make_dinner, and if this method changes, the AmateurChef class will inherit this behavior as well.

The second reason to use this technique is as a fallback to the parent method if a condition is not met.

class Image
  attr_accessor :resizable

  def geometry
    "800x600"
  end
end

class ProfileImage < Image
  def initialize
    @resizable = true
  end

  def geometry
    @resizable ? "100x100" : super
  end
end

In this second example Image returns a geometry. ProfileImage is defined as a subclass of Image and overrides the ProfileImage#geometry method to let Image#geometry take over when the image is not @resizable, using super.

Further notes on superclass access

Additionally, super can be assigned to a variable, for example x = super. If the parent class method takes arguments, these can also be passed to super as it is a method and works the same way.

There's a challenge available for this chapter: Secure Radio

Dates and Times

Date and time based classes are useful in Ruby and when writting program in general, this section is an introduction to these classes.

Time

Times are stored as the number of seconds since January 1st, 1970. Otherwise known as Unix time or seconds since the Epoch.

All times include fractions of a second which is important for comparisons.

The fastest way to get the current time in Ruby is using the Time.now class method. It will return the current time with the default format. Alternatively, a Unix timestamp can also be retrieved.

# dates-and-times/time.rb
Time.now # 2019-12-23 12:51:12 +0000
Time.now.to_i # 1577105472
Time.at(1577100912) # 2019-12-23 11:35:12 +0000

However, it is more common that more readable date formats are used instead. Using Time.new a custom format can be retrieved instead.

Time.new(year, month, day, hour, minute, second, utc_offset)

# dates-and-times/time.rb
Time.new(2020, 11, 05, 00, 00, 00, "+02:00") # 2020-11-05 00:00:00 +0200
Time.new(2020, 03, 01) # 2020-03-01 00:00:00 +0000

Addition and subtraction can be applied to find different times:

# dates-and-times/time.rb
a_day = (60 * 60 * 24)
yesterday = Time.now - a_day # 2019-12-23 11:05:08 +0000
tomorrow = Time.now + a_day # 2019-12-25 11:05:08 +0000
next_week = Time.now + a_day * 7 # 2019-12-31 11:05:08 +0000

Additionally, there are multiple methods that can be called to retrieve a single part of time:

# dates-and-times/time.rb
current = Time.now
current.year # 2019
current.month # 12
current.day # 24
current.hour # 12
current.min # 18
current.sec # 56
current.nsec # 516215000

There are methods that can get more specific parts of time such as the day of the year (Time#yday), the day of the week (Time#wday) and boolean methods that allow to understand if the date is on a certain day of the week (friday?):

# dates-and-times/time.rb
current.yday # 358
current.wday #  2
current.monday? # false
current.friday? # true

Ruby also supports strftime (string for time or string-formatted time). This method is common to a lot of programming languages and a format can be provided with a string, defining how a date should be returned:

# dates.and-times/time.rb
current.strftime("Printed on %m/%d/%Y") # Printed on 12/24/2019
current.strftime("at %I:%M%p") # at 12:56PM

Additional methods can be used to work with time zones:

# dates-and-times/time.rb
current.zone # WET
current.utc? # false

Consult ri or the Ruby documentation for more notes on the Time class.

Finally, when comparing time, the following statement will always return false:

# dates-and-times/time.rb
Time.now == Time.now # false
Time.now.nsec == Time.now.nsec # false
Time.now.day == Time.now.day # true

This is because Ruby compares time down to the nanosecond so each of these instances will have a different time when they were instantiated.

Date

The Date class is similar to Time and can be useful when working with dates without the need of the time. The Date class includes methods that are not available in Time.

The Date class is part of Ruby's Standard Library. When Ruby is installed, the Date class is added as well but it is not loaded. This means Date won't be available on irb or Ruby scripts until specified.

All Core methods are loaded automatically, whilst Standard Library methods need to be referenced before use. Consult the Ruby Documentation for a complete list.

To use Date or any Standard Library class, a require statement needs to be added to irb or the Ruby script.

# dates-and-times/date.rb
require 'date'
Date.today
Date.today.month
Date.today.to_time

Additional methods are only present in the Date class:

# dates-and-times/date.rb
date = Date.new(1970, 01, 13)

date.leap?
date.cweek
date.cwday
date.cwyear

date.next_day
date.next_month
date.next_year

DateTime

The DateTime class is also part of the Ruby Stardard Library and it needs to be included in a script before being used. It is a subclass of Date so it does inherit some of its behavior.

To use DateTime the date library must be required, than similar methods to Date and Time might be used:

# dates-and-times/datetime.rb
require 'date'
DateTime.now
DateTime.new(2020, 11, 05, 00, 00, 00, "+02:00")

However, why wouldn't Time be used instead? DateTime know how to distinguish between the Gregorian and Julian calendars, so it is useful when working with historical dates. When working with modern dates, Time would be most appropriate in most cases.

If required a Time object can be converted to DateTime and vice versa:

# dates-and-times/datetime.rb
DateTime.now.to_time
Time.now.to_datetime

The Time class tends to be the most useful class when working with dates and times. However, Date and DateTime add further functionality for more complex cases.

There's a challenge available for this chapter: Secure Radio

Modules

Modules are wrappers around Ruby code like Classes. However, unlike classes, modules cannot be instantiated. Modules are typically used for namespacing and in mixins.

Namespacing

Namespacing is used to prevent conflicts when code has similarly named classes and methods.

For example, in a classroom two people might have the same name, like Sophia.

To differenciate between the two, the surname initially might be added when refering or calling them. In code, namespacing works the same way.

A pratical example could be the definition of a Date class for MakeSparks a fictitious online dating site. Date exists within Ruby's Standard Library, so the new class should be namespaced to avoid conflicts, by wrapping it in a module:

# modules/namespacing.rb
module MakeSparks
  class Date

    def initialize(double_date=false)
      @double_date = double_date
    end

    def booking
      "A date is booked for #{is_double?} at Tulsa's Botanical Garden's Restaurant by 7:30PM."
    end

    private

      def is_double?
        @double_date ? 4 : 2
      end
  end 
end

The MakeSparks module has its own Date class which can be used and instantiated by prefixing it with its namespace:

# modules/namespacing.rb
date_for_two = MakeSparks::Date.new # A date is booked for 2 at Tulsa's Botanical Garden's Restaurant by 7:30PM.

Using namespaces not only prevents conflicts with Ruby classes but also other code part of a project's codebase. As it grows there might be reasons to reuse a class name and differentiate them using namespacing.

Namespacing is frequently used to namespace classes in open-source plugins.

Mixins

Ruby only allows subclasses to inherit from a single superclass, meaning a subclass can only have one parent.

Mixing modules allow for shared functionality to be packaged up and then be mixed in one or multiple classes without having to inherit from it.

Considering an arbitrary Person class, it could be used as a parent class to other subclasses such as Customer or Supplier:

# modules/mixins.rb
class Person
  attr_accessor :first_name, :last_name
  attr_accessor :city, :state, :zip
  attr_accessor :age, :gender

  def full_name
    "#{first_name} #{last_name}"
  end

  def city_state_zip
    "#{city}, #{state} #{zip}"
  end
end 

class Customer < Person
end

class Supplier < Person
end

Using class inheritance in the example above, both Customer and Supplier will inherit all Person functionality. However, Customer and Supplier only need some functionality shared between them, Person#age and Person#gender are not needed in its subclasses.

In this case code can be packaged up in mixins and be included in other classes as needed. For example a Nameable and a ContactInfo module:

# modules/mixins.rb
module Nameable
  attr_accessor :first_name, :last_name

  def full_name
    "#{first_name} #{last_name}"
  end
end

module Contactinfo
  attr_accessor :city, :state, :zip

  def city_state_zip
    "#{city}, #{state} #{zip}"
  end
end

In this case, all functionality aside from Person#age and Person#gender has been split into modules and can now be shared across multiple classes without them inheriting unwanted behavior:

# modules/mixins.rb
class Person
  include Nameable
  include Contactinfo
  attr_accessor :age, :gender
end

class Customer
  include Nameable
  include Contactinfo
end 

class Supplier
  include Nameable
  include Contactinfo
end

Class Inheritance vs. Mixins

It might be difficult to identify when to use class inheritance and mixins as it might seem that both accomplish the same goal.

In general, class inheritance is used when a class needs to modify or extend the behavior of another class. Something similar to its parent class but with slight differences.

Mixins on the other hand are used when several classes need to utilize a single set of behaviors.

There might also be cases where it would make little difference between using one or the other and it is up to developer to make the decision on which one would be best.

Load, Require and Include

When splitting scripts in multiple files and coding with the DRY (Don't Repeat Yourself) principle in mind, it becomes important to understand different ways of adding them and use them in a script.

There are three ways of adding code from another file or module into scripts, load, require and include.

load, loads a source file everytime it is called and it returns a boolean on whether the file was loaded successfully.

It is not used very often since require is more frequently.

require works exactly the same as load but it loads a source file only once and keeps track of it for the duration of the script.

It only returns true if a file has been loaded successfully and has not been loaded before.

require must be either provided a full path to a file, or a Standard Library path which is part of $LOAD_PATH.

$LOAD_PATH is an array of paths Ruby makes use when looking to load libraries or classes part of its Standard Library. require looks in those paths for them or in a full path when provided.

include is only used when including modules in classes and cannot be used with files. An important difference when compared with the two other keywords above.

Other programming languages might change the meaning and functionality of these keywords.

require 'date'

require 'Users/grivia/project/classes/customer'

require_relative 'modules/addressable'

class Customer
  include Addressable
end

In the example above, the Date class is required from Ruby's Stardard Library and no further path is required.

The Customer class is imported into the file using require a second time. In this case it is a custom file and therefore the whole path needs to be specified.

Next the Addressable module is added to the script using require_relative. This is the same as require with the difference that the file path provided can be shortnened relatively to the file requiring it.

Lastly, Customer is overwritten and the Addressable module is added by using include.

There's a challenge available for this chapter: To-do List

Exceptions

Exceptions are classes for handling exceptional events. Unexpected behaviour or events, most often (but not always) are errors. Error exceptions can be thrown as, for instance, SyntaxErrors or NoMethodErrors.

Errors belong to Ruby's exception class and they could be referred as in "an exception as been raised". Ruby has many built in exceptions as subclasses of the Exception class.

Most common exceptions thrown in Ruby are usually a StandardError.

Handle Exceptions

A way to throw an exception in Ruby is to divide a number by zero, a ZeroDivisionError is thrown:

1 / 0 # ZeroDivisionError

When running this code in irb, a list of all the places where it broke is printed to the console. This is useful when programming and debugging code, however, this output is not useful to users and should be ommited.

To do this, additional code can be put in place to handle the exception, allowing the script to fail gracefully.

# exceptions/handle-exceptions.rb
begin
  1 / 0
rescue
  puts "Exception: Cannot divide number by zero."
end

The code which could raise an exception is put under the begin keyword, and code that handles the exception is put under the rescue keyword.

The code is really put between a begin and end block and helps letting Ruby know that rescue applies to all code in that block.

If anything goes wrong anywhere in that block, Ruby will jump to the rescue line and run the code there instead. It could run another method, or it could return another error message instead.

rescue can also be used in method definitions, def and end blocks act as the delimiters of which code rescue applied to:

# exceptions/handle-exceptions.rb
def divide(x,y)
  x / y
rescue
  puts "Exceptions raised, cannot divide #{x} with #{y}."
end

Furthermore, exceptions do not need to be handled inside the method:

# exceptions/handle-exceptions.rb
def divide(x,y)
  x / y
end

begin
  divide(1,0)
rescue
  puts "Exceptions raised when trying division."
end

This is because exceptions get returned like any other value in Ruby, and even if it was thrown in a method deep in the code of a script it will keep getting returned until it's handled or it is the end of the script.

Handle Specific Exceptions

In the previous section, only the rescue keyword was used to handle any exceptions thrown in a code block. However, rescue defaults to handling StandardError and its subclasses and sometimes it might be useful to specify exacly what kind of exception is to be handled.

This can be achieved by adding the exception name after rescue:

# exceptions/handle-specific-exceptions.rb
def divide(x,y)
  x / y
rescue ZeroDivisionError
  puts "ZeroDivisionError handled"
rescue TypeError
  puts "TypeError handled"
rescue
  puts "Some other StandardError handled"
end

Multiple exceptions may be specified, separated by a comma:

# exceptions/handle-specific-exceptions.rb
def divide(x,y)
  x / y
end

begin
  divide(4,0)
  divide(4, "2")
  divide(4)
rescue ZeroDivisionError
  puts "Cannot divide by zero"
rescue TypeError, ArgumentError
  puts "Requires two integer arguments"
end

In the example above the exceptions were moved outside of the method, since ArgumentError gets thrown even before the code inside divide has a chance to run. Depending on the exception being handled, the place where it needs to run may differ. A refactored example could look like:

def divide(x,y)
  x / y
rescue ZeroDivisionError
  puts "ZeroDivisionError handled"
rescue TypeError
  puts "TypeError handled"
end

begin
  puts divide(4,2)
  divide(4,0)
  divide(4,"2")
  divide(4)
rescue ArgumentError
  puts "Requires two integer arguments."
end

However, one gotcha is that the Exception class will also be accepted by rescue. This should not be used since it will take over other exceptions Ruby uses internally:

# Don't do this!
begin
  1 / 0
rescue Exception
  puts "Puts every exception is handled"
  puts "Even those Ruby uses internally to work"
end

Exception Methods

The Exception class contains a multitude of methods that can be used alongside rescue to aid in exception handling.

The first step is to hold an instance of Exception in a local variable (usually named e) using the hash rocket sign =>. Assigning the exception to a variable allows it to be used inside the rescue block, and methods can be called on them:

# exceptions/exceptions_methods.rb
begin
  1 / 0
rescue ZeroDivisionError => e
  puts "#{e.class} handled"
rescue => e
  puts "#{e.class} handled"
end

In the example above, the first rescue block will always print ZeroDivision handled, as the block only handles a single exception.

The subsequent block handles all StandardError exceptions and could print for instance a TypeError or an ArgumentError when using e.class.

There is a choice of methods that can be run on an Exception instance, consult Ruby's documentation for further notes:

Methods	Description
Exception#class	Useful to understand which exception was raised and when handling each exception a different way within a rescue
Exception#message	Short error message that points out what went wrong.
Exception#backtrace	Returns the path the code has taken through various files and method calls. can be useful to trackdown issues that occur when coding a script.
Exception#full_message	Combines Exception#class, Exception#message and Exception#backtrace into a single message.

Methods listed above can all be used inside a rescue block:

#exceptions/exception_methods.rb
def divide(x,y)
  x / y

  rescue ZeroDivisionError => e
    puts "ZeroDivisionError handled when dividing #{x} with #{y}"
    puts "Backtrace: #{e.backtrace}"
  rescue TypeError => e
    puts "TypeError handled: #{e.message}"
  rescue => e
    puts "#{e.class} handled"
end

Raise Exceptions

Ruby will raise exceptions when code goes wrong. Additional exceptions can also be raised intentionally in custom Ruby scripts.

This can be achieved either by using Ruby's built-in exception classes or by writting custom exceptions.

The Exception class has a list of subclasses available in Ruby's documentation.

When raising an exception, the developer can pick which exception is being raised from that list of subclasses. However, the default is RuntimeError if no particular exception is specified:

# exceptions/raise_exceptions.rb
def even_numbers(array)
  unless array.is_a?(Array)
    raise ArgumentError
  end

  if array.length == 0
    raise StandardError.new("Too few elements: #{array.length}")
  end

  array.find_all {|item| item.to_i % 2 == 0}
end

The example above will raise an ArgumentError exception when the data type of its argument is not an Array. It will also raise a StandardError exception for when the array provided is empty. StandardError.new takes in a message which will print to the console when the exception is raised.

In previous examples, a Radio class has been used. In it's volume method it wouldn't return anything unless the volume was between 1 and 10. An exception could be raised here:

class Radio
  attr_accessor :volume

  def volume=(value)
    if value < 1 || value > 10
      raise "Too loud!"
    end
    @volume = value
  end
end

begin
  radio = Radio.new
  radio.volume = 20
rescue RuntimeError => e
  puts e.message
end

The raise "Too loud!" statement raises an RuntimeError exception by default and is taking a message as its argument. This exception is rescued later on inside a begin and end block.

Custom Exceptions

Custom exceptions can be defined by inheriting from on of Ruby's exception sub-classes.

class TooLoudError < StandardError
end

To raise a custom exception, the statement is written the same way as any other exception:

class Radio
  attr_accessor :volume

  def volume=(value)
    if value < 1 || value > 10
      raise TooLoudError
    end
    @volume = value
  end
end

begin
  radio = Radio.new
  radio.volume = 20
rescue TooLoudError
  puts "Too Loud!"
end

The custom exception class may then be extended with methods or some functionality may be overriden from it's parent class. In this case StandardError#message is overriden with TooLoudError#message:

class TooLoudError < StandardError
  def message
    "Too loud!"
  end
end

However, most of the time it is probably desired to set the message as soon as a custom exception is instantiated:

# exceptions/custom_exceptions.rb
class TooLoudError < StandardError
  def initialize(msg=nil)
    super(msg || "Too loud!")
  end
end

Using super will let the parent class set the message. However if no message is passed in, "Too loud!" will be set instead:

# exceptions/custom_exceptions.rb
class Radio
  attr_accessor :volume

  def volume=(value)
    if value < 1 || value > 10
      raise TooLoudError
    end
    @volume = value
  end
end

begin
  radio = Radio.new
  radio.volume = 20
rescue TooLoudError => e
  puts e.message
end

Custom exceptions can be further developed to have other behavior or relevant values. In the example below TooLoudError is extended to receive a volume argument, which can later be used when handling the exception:

# exceptions/custom_exceptions.rb
class TooLoudError < StandardError
  attr_reader :volume

  def initialize(value, msg=nil)
    super(msg || "Too loud!")
    @volume = value
  end
end

Now the TooLoudError exception will be aware of what volume was being set when raised:

# exceptions/custom_exceptions.rb
class Radio
  attr_accessor :volume

  def volume=(value)
    if value < 1 || value > 10
      raise TooLoudError.new(value)
    end
    @volume = value
  end
end

begin
  radio = Radio.new
  radio.volume = 20
rescue TooLoudError => e
  puts "Volume #{e.volume}: #{e.message}"
end

Further extending custom exception can be used to keep a log of what when wrong in a script. The log could then be saved to a file for later reference.

There's a challenge available for this chapter: Custom Exceptions

Further Resources

• Ruby Fundamentals
• Ruby: Classes and Modules - LinkedIn Learning
• Ruby: Files, Formats, Templates - LinkedIn Learning
• Ruby Monstas: Ruby for beginners
• Ruby on Rails
• Ruby API Documentation