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 Ruby 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’re 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 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.
Example:
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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