Is it possible to get the default value for keyword arguments in Ruby?

Question

I am looking to get the default values for a function with keyword arguments. Example:

def some_method(foo: 1, bar: 2) end

My expected output would be something like

{ foo: 1, bar: 2 }

The parameters method defined on method only provides parameter names.

I don't think there is : stackoverflow.com/questions/34751724 — romainsalles
– romainsalles, Commented Jun 2, 2021 at 18:22

Todd A. Jacobs · Accepted Answer · 2021-06-02 20:30:27Z

1

You can get the keys, but not the values, because they aren't actually set until the method is actually invoked. You can get the method's parameters using Method#parameters. For example, using your example code from above:

method(:some_method).parameters #=> [[:key, :foo], [:key, :bar]]

However, the :foo and :bar keys aren't actually set until the method is run, so there's no way to get the "value" except possibly to introspect the method definition itself.

answered Jun 2, 2021 at 20:30

Todd A. Jacobs

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3 Comments

Mark Ramasco Over a year ago

I can use introspection to do it, but I am having difficulty coming up with a good solution.

Todd A. Jacobs Over a year ago

@MarkRamasco If you're running from a file, you might be able to do it with RBS and/or TypeProf, or simply grepping for the method definition and parsing it. However, from a REPL, you can't access the source in irb, and would probably need to hijack stdout from pry (which is generally a bad idea) to match against output from show-source or show-method. Since I explained that Ruby doesn't set the default arguments until invoked, you might get better answers if you explain your use case; this may be an X/Y problem.

Mark Ramasco Over a year ago

Without going into too much detail, I had to write an adapter around a custom legacy framework for writing tests. This was built on years of work, so it's not a scenario that we can easily change. Essentially, the framework allows someone to write a description that uses string interpolation to fill in values from the test input parameters. This is one of those scenarios where everything is wrong, but I needed to find a work around solution. I ended up with a really janky work around where I use sorcerer to parse the source.

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