Keyword Arguments and Mass Assignment

Objectives

1. Define keyword arguments and learn why they are useful.
2. Define mass assignment and learn why it is useful.
3. Practice using keyword arguments and mass assignment.

Introduction

At this point, we're very familiar with the fact that methods can be defined to take in arguments. We also know that methods can be defined to take in multiple arguments. Let's take a look at an example:

def print_name_and_greeting(greeting, name)
  puts "#{greeting}, #{name}"
end

print_name_and_greeting("'sup", "Hillary Clinton")
  "'sup, Hillary Clinton"
  => nil

As it currently stands, whoever uses our method needs to remember exactly what order to pass in the arguments. They need to know that the first argument is greeting and the second argument is a name. What happens if they forget? What happens if another developer who is working on our code base doesn't see the file where the method is defined, but only sees the method being invoked? Maybe it's not clear to them which argument is which––after all, when you invoke methods, the arguments aren't labeled or anything. Let's take a look at what type of disaster would befall us:

print_name_and_greeting("Kanye", "hello")
  "Kanye, hello"
  => nil

That would be a weird way to greet Kanye. Let's take a look at another example, this time using arguments of two different data types:

def happy_birthday(name, current_age)
  puts "Happy Birthday, #{name}"
  current_age += 1
  puts "You are now #{current_age} years old"
end

happy_birthday("Beyonce", 31)
  Happy Birthday, Beyonce
  You are now 32 years old
  => nil

But what happens if we accidentally pass the arguments in in the wrong order?

happy_birthday(31, "Beyonce")
  Happy Birthday, 31
  TypeError: no implicit conversion of Fixnum into String

Oh no! We broke our program! Clearly, we have a need to regulate the passing in of multiple arguments. It would be especially helpful if we could name the arguments that we pass in, when we invoke the method. Guess what? We can! (Okay, you probably saw that one coming.)

Keyword Arguments

Keyword arguments are a special way of passing arguments into a method. They behave like hashes, pairing a key that functions as the argument name, with its value. Let's walk through it together and refactor our happy_birthday method:

def happy_birthday(name: "Beyonce", current_age: 31)
  puts "Happy Birthday, #{name}"
  current_age += 1
  puts "You are now #{current_age} years old"
end

Here, we've defined our method to take in keyword arguments. Our keyword arguments consist of two key/value pairs, :name and :current_age. We've given our keyword arguments default values of "Beyonce" and 31, but we didn't have to:

def happy_birthday(name:, current_age:)
  puts "Happy Birthday, #{name}"
  current_age += 1
  puts "You are now #{current_age} years old"
end

Notice that we can reference name and current_age inside our method body, as if they were barewords, even though they are the keys in our argument hash. That's how keyword arguments work, they allow you to name the arguments that you pass in as keys in a hash. Then, the method body can use the values of those keys, referenced by their name. Let's call our method:

happy_birthday(current_age: 31, name: "Carmelo Anthony")
 "Happy Birthday, Carmelo Anthony"
 "You are now 32 years old"

Notice that even though we changed the order of our key/value pairs, our method didn't break! Not only is this method more robust (i.e. more resistant to breakage) than the previous one, it is also more explicit. Anyone looking at its invocation can tell exactly what kind of data you are passing in.

Your turn:

???

Quiz: Keyword Arguments

?: Which rat_counter method below correctly implements the following?

The method should use keyword arguments to take in a hash as an argument: the keys of the hash are rat_count and train_line. If called with an argument of rat_count: 2, train_line: "B train", the method should return: There are 2 rats on the B train.

(x)

def rat_counter(rat_count:, train_line:)
  "There are #{rat_count} rats on the #{train_line}."
end

( )

def rat_counter(rat_count, train_line)
  "There are #{rat_count} rats on the #{train_line}."
end

( )

def rat_counter(rat_count:, train:)
  "There are #{rat_count} rats on the #{train_line}."
end

???

Mass Assignment

Another benefit of using keyword arguments is the ability to "mass assign" attributes to an object. Let's revisit our Person class from an earlier lesson. We'd like to initialize individual people with a name and an age:

class Person
  attr_accessor :name, :age

  def initialize(name, age)
    @name = name
    @age = age
  end
end

As it stands, our initialize method is vulnerable to the same issues we discussed above. Let's refactor it to take in a person's attributes as keyword arguments:

class Person
  attr_accessor :name, :age

  def initialize(name:, age:)
    @name = name
    @age = age
  end
end

Now, we have the added benefit of being able to use something called mass assignment to instantiate new people objects. If a method is defined to accept keyword arguments, we can create the hash that the method is expecting to accept as an argument, set that hash equal to a variable, and simply pass that variable in to the method as an argument. Let's take a look:

person_attributes = {name: "Sophie", age: 26}
sophie = Person.new(person_attributes)
=> #<Person:0x007f9bd5814ae8 @name="Sophie", @age=26>

This might not seem particularly useful now, but when we start building web applications, we'll understand more about how necessary this trick really is. For now, just take our word for it.

Your turn:

???

Quiz: Mass Assignment

?:Define a class, School, that initializes with a name and a location. The class should also have attr_accessors for name and location. The initialize method should use keyword arguments for those attributes.

Which snippet below is the correct implementation of School according to the above spec?

( )

class School
  attr_accessor :name, :location

  def initialize(*args)
    @name = args[0]
    @location = args[1]
  end
end

(x)

class School
  attr_accessor :name, :location

  def initialize(name:, location:)
    @name = name
    @location = location
  end
end

( )

class School
  attr_accessor :name, :location

  def initialize(name, location)
    @name = name
    @location = location
  end
end

???

View Mass Assignment on Learn.co and start learning to code for free.

View Mass Assignment on Learn.co and start learning to code for free.