How do I add named parameters in a subclass or change their default in Ruby 2.2?

Question

This question is about Ruby 2.2.

Let's say I have a method which takes positional and named arguments.

class Parent
  def foo(positional, named1: "parent", named2: "parent")
    puts positional.inspect
    puts named1.inspect
    puts named2.inspect
  end
end

A subclass wants to both override some defaults, and add its own named arguments. How would I best do this? Ideally, it would not have to know details of the signature of the parent, in case the parent wants to add some optional positional arguments. My first attempt was this.

class Child < Parent
  def foo(*args, named1: "child", named3: "child" )
    super
  end
end

But that blows up because the unknown named3: is passed to the parent.

Child.new.foo({ this: 23 })

/Users/schwern/tmp/test.rb:10:in `foo': unknown keyword: this (ArgumentError)
        from /Users/schwern/tmp/test.rb:15:in `<main>'

I tried explicitly passing arguments to super, but that didn't work either. It seems the first positional argument is getting treated as a named one.

class Child < Parent
  def foo(*args, named1: "child", named3: "child" )
    super(*args, named1: "child")
  end
end

Child.new.foo({ this: 23 })

/Users/schwern/tmp/test.rb:10:in `foo': unknown keyword: this (ArgumentError)
        from /Users/schwern/tmp/test.rb:15:in `<main>'

I can make Child know about the first positional parameter, that works...

class Child < Parent
  def foo(arg, named1: "child", named3: "child" )
    super(arg, named1: "child")
  end
end

Child.new.foo({ this: 23 })
Parent.new.foo({ this: 23 })

{:this=>23}
"child"
"parent"
{:this=>23}
"parent"
"parent"

...until I pass in a named parameter.

Child.new.foo({ this: 23 }, named2: "caller")
Parent.new.foo({ this: 23 }, named2: "caller")

/Users/schwern/tmp/test.rb:10:in `foo': unknown keyword: named2 (ArgumentError)
        from /Users/schwern/tmp/test.rb:15:in `<main>'

How do I make this work and retain the benefit of named parameter checks? I'm open to turning the positional parameter into a named one.

It's hard to tell what you are asking... the code `Child.new.foo({ this: 23 })` doesn't raise an error because `named3` is passed to the parent, it raises an error because `this` is passed to the parent. Are you expecting the hash `{this: 23}` to be passed to the `positional` argument in the parent's method? — Adrian, May 01 '15 at 19:22
@Adrian Yes, I'm expecting `{this: 23}` to be passed as the positional parameter. It's being treated as a named parameter instead. I'm clearly misunderstanding something. — Schwern, May 01 '15 at 19:32

score 7 · Accepted Answer · answered May 01 '15 at 20:34

The problem here is that since the parent doesn't know anything about the child's arguments, it has no way of knowing whether the first argument you pass to it was meant to be a positional argument, or whether it was intended to provide keyword arguments to the parent method. This is because of a historical feature where Ruby allowed hashes to be passed as keyword-argument style parameters. For example:

def some_method(options={})
  puts options.inspect
end

some_method(arg1: "Some argument", arg2: "Some other argument")

Prints:

{:arg1=>"Some argument", :arg2=>"Some other argument"}

If Ruby disallowed that syntax (which would break backwards compatibility with existing programs), you could write your child method like this using the double splat operator:

class Child < Parent
  def foo(*args, named1: "child", named2: "child", **keyword_args)
    puts "Passing to parent: #{[*args, named1: named1, **keyword_args].inspect}"
    super(*args, named1: named1, **keyword_args)
  end
end

In fact, this works fine when you pass keyword arguments in addition to the positional one:

Child.new.foo({ this: 23 }, named2: "caller")

Prints:

Passing to parent: [{:this=>23}, {:named1=>"child"}]
{:this=>23}
"child"
"parent"

However, since Ruby can't tell the difference between positional arguments and keyword arguments when you only pass a single hash, Child.new.foo({ this: 23 }) results in this: 23 getting interpreted as a keyword argument by the child, and the parent method ends up interpreting both keyword arguments forwarded to it as a single positional argument (a hash) instead:

Child.new.foo({this: 23})

Prints:

Passing to parent: [{:named1=>"child", :this=>23}]
{:named1=>"child", :this=>23}
"parent"
"parent"

There are a few ways you can fix this but none of them are exactly ideal.

Solution 1

As you tried to do in your third example, you could tell the child that the first argument passed will always be a positional argument, and that the rest will be keyword args:

class Child < Parent
  def foo(arg, named1: "child", named2: "child", **keyword_args)
    puts "Passing to parent: #{[arg, named1: named1, **keyword_args].inspect}"
    super(arg, named1: named1, **keyword_args)
  end
end

Child.new.foo({this: 23})
Child.new.foo({this: 23}, named1: "custom")

Prints:

Passing to parent: [{:this=>23}, {:named1=>"child"}]
{:this=>23}
"child"
"parent"
Passing to parent: [{:this=>23}, {:named1=>"custom"}]
{:this=>23}
"custom"
"parent"

Solution 2

Switch entirely to using named arguments. This avoids the problem entirely:

class Parent
  def foo(positional:, named1: "parent", named2: "parent")
    puts positional.inspect
    puts named1.inspect
    puts named2.inspect
  end
end

class Child < Parent
  def foo(named1: "child", named3: "child", **args)
    super(**args, named1: named1)
  end
end

Child.new.foo(positional: {this: 23})
Child.new.foo(positional: {this: 23}, named2: "custom")

Prints:

{:this=>23}
"child"
"parent"
{:this=>23}
"child"
"custom"

Solution 3

Write some code to figure everything out programmatically.

This solution would likely be pretty complex and would depend a lot on exactly how you want it to work, but the idea is that you would use Module#instance_method, and UnboundMethod#parameters to read the signature of the parent's foo method and pass arguments to it accordingly. Unless you really need to do it this way, I'd recommend using one of the other solutions instead.

Thanks, I ultimately went with solution 2. It's good to know solution 1. — Schwern, May 01 '15 at 21:32

score 1 · Answer 2 · answered May 01 '15 at 19:37

From what I can tell, you want to:

Use different defaults in the child's method for the same keyword arguments
Have the child's method have some separate keyword arguments which aren't passed to the parent
Not have to change the child's method definition when the signature of the parent's method definition changes

I think you problem can be solved by capturing keyword arguments which are to be passed directly to the parent's method in a separate variable in the child's method, kwargs, like this:

class Parent
  def foo(positional, parent_kw1: "parent", parent_kw2: "parent")
    puts "Positional: " + positional.inspect
    puts "parent_kw1: " + parent_kw1.inspect
    puts "parent_kw2: " + parent_kw2.inspect
  end
end

class Child < Parent
  def foo(*args, parent_kw1: "child", child_kw1: "child", **kwargs)
    # Here you can use `child_kw1`.
    # It will not get passed to the parent method.
    puts "child_kw1: " + child_kw1.inspect

    # You can also use `parent_kw1`, which will get passed
    # to the parent method along with any keyword arguments in
    # `kwargs` and any positional arguments in `args`.

    super(*args, parent_kw1: parent_kw1, **kwargs)
  end
end

Child.new.foo({this: 23}, parent_kw2: 'ABCDEF', child_kw1: 'GHIJKL')

This prints:

child_kw1: "GHIJKL"
Positional: {:this=>23}
parent_kw1: "child"
parent_kw2: "ABCDEF"

I was just going to suggest that. Problem is that it doesn't work when you don't pass any keyword arguments to the child. Try it with `Child.new.foo({this: 23})`. `:parent_kw1=>"child"` ends up as a positional argument passed to the parent (On Ruby 2.2.1 at least...). — Ajedi32, May 01 '15 at 19:42
@Ajedi32 You're right, it seems like the only way to get around that would be to write `Child.new.foo({this: 23}, **{})` which is ugly. :( I don't know if there is any other way to prevent ruby from interpreting the hash as a set of keyword arguments short of using `Parent.instance_method(:foo).parameters` in the child's method. — Adrian, May 01 '15 at 19:51

How do I add named parameters in a subclass or change their default in Ruby 2.2?

2 Answers2

Solution 1

Solution 2

Solution 3