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Ruby’s Powerful Method Arguments & How To Use Them Correctly

I got an email asking for my opinion about when to use keyword arguments. I thought I’d expand my answer into a full article about method arguments so everyone can benefit!

Ruby is very flexible when it comes to method arguments…

…we have everything, from the standard required arguments to optional arguments & even keyword (named) arguments.

In this article, you are going to learn about the differences between all these argument types, and which ones to use depending on your situation.

Using the right kind of arguments will make your code easier to read & easier to work with.

When in Doubt Use Standard Arguments

Required arguments are what you get by default.

Here’ an example:

def write(file, data, mode)
end

write("cats.txt", "cats are cool!", "w")

If you don’t pass the exact number of arguments required you’ll get this familiar error message:

write("shopping_list.txt", "bacon")

ArgumentError: wrong number of arguments (2 for 3)

This means that you gave the method 2 arguments, but it needs 3!

Flexible Methods With Optional Arguments

You may want to make some of your arguments optional by providing a default value.

For example:

def write(file, data, mode = "w")
end

Now you can call write with 2 arguments, in which case mode will equal the default value ("w"), or you can pass in 3 arguments to override the default value & get different results.

This default value saves you some work when calling this method while still giving you the option to change it.

Use Keyword Arguments to Increase Clarity

One of the downsides of required & default arguments is that they are order dependent. Meaning that you have to pass in the arguments in the same order.

If you pass them in the wrong order you’ll probably get unexpected results.

For example, in the write method, you have to call it with the file first, then the data & finally the mode.

def write(file, data, mode)
end

Keyword arguments allow you to vary the order of the arguments.

Here’s an example:

def write(file:, data:, mode: "ascii")
end

This allows you to call the method in a different order:

write(data: 123, file: "test.txt")

But more importantly, when you call the method you can clearly see the meaning of the arguments.

I don’t find myself using keyword arguments very often, but they can be useful when you want to add clarity to method calls.

Let’s say that we have a Point class.

You could write it like this:

class Point
  def initialize(x: , y:)
    @x, @y = x, y
  end
end

point = Point.new(x: 10, y: 20)

This way there is no possible confusion about which of the two numbers (10 & 20) correspond to the x value or the y value.

That’s one of your main goals when writing code, to avoid confusion & misunderstanding as much as possible. If using keyword arguments helps you do that then use them.

Think about this:

“Will adding keyword arguments make this easier to understand or will it add extra work & clutter to my code?”

Another thing about keyword arguments is that they are very explicit about the arguments you are missing.

point = Point.new

# missing keywords: x, y

You can also combine keyword arguments with regular arguments. One strategy I’ve been observing on Ruby built-in methods is that new versions tend to add new, optional arguments, as keyword arguments.

Examples of this are the keyword arguments for Float#round, Kernel#clone & String#lines introduced in Ruby 2.4.

Use Variable Arguments to Capture as Many Values as Necessary

What if you want to take an unlimited amount of values?

Then you can use variable arguments:

def print_all(*args)
end

print_all(1, 2, 3, 4, 5)

This allows you to call the method with any number of arguments, including none. A variable argument is an array with all the values passed to it.

You can combine this with other types of arguments.

Example:

def print_all(title, *chapters)
end

This will take the 1st argument as title, and the rest as the chapters array. Notice that a variable argument needs to be after all your required & optional arguments, and before any keyword arguments.

Use The Correct Order

Here’s the valid order of arguments if you want to combine them & avoid a syntax error:

required -> optional -> variable -> keyword

Here’s a method that shows every possible argument type:

def testing(a, b = 1, *c, d: 1, **x)
  p a,b,c,d,x
end

testing('a', 'b', 'c', 'd', 'e', d: 2, x: 1)

The **x is the same as variable arguments, but for keyword arguments. It will be a hash instead of an array.

The Catch-All Argument

In some rare occasions you may see this:

def print_all(*)
end

This means that the method is accepting any arguments, but it’s not doing anything with them. It’s similar to using the underscore character (_) inside a block to show which arguments you aren’t using.

A practical use for this could be in combination with the super keyword:

class Food
  def nutrition(vitamins, minerals)
    puts vitamins
    puts minerals
  end
end

class Bacon < Food
  def nutrition(*)
    super
  end
end

bacon = Bacon.new

bacon.nutrition("B6", "Iron")

Now when Food’s nutrition changes its argument list, you don’t have to change arguments in Bacon too.

Summary

You’ve learned about Ruby’s powerful method arguments & how to use them! Required arguments are a good start, then you can add flexibility with optional arguments & add clarity with keyword arguments.

Hope you found this useful & interesting!

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Leave a Comment:

2 comments
Don says last month

I want to make sure I understand what you are suggesting, with keyword arguments. Are you saying that we should:

Use keyword arguments all the time except in the case when regular arguments work because no optional argument exists?

Reply
    Jesus Castello says last month

    Hey Don,

    I think there are a few factors to consider, like how different are the arguments types.

    • If all the arguments are numbers or strings, and you could easily confuse them when calling the method (like the `Point` example) then keyword arguments may be helpful.
    • If you have a combination of regular + optional arguments, the optional arguments may be best defined as keyword arguments to give you the order freedom.
    • If you have a one-argument method, a regular or optional argument should be the choice, because with one argument it should be clear enough what this needs to be.

    In any case, you want to keep your argument lists short (3 or less). If your argument lists are too long you may have some new classes hiding in there 🙂

    Reply
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