Ruby hash equivalent to Python dict setdefault

Question

In Python it is possible to read a dictionary/hash key while at the same time setting the key to a default value if one does not already exist.

For example:

Answer 1

You can simply pass a block to the Hash constructor:

hash = Hash.new do |hash, key|
  hash[key] = :default
end

The block will be invoked when an attempt to access a non-existent key is made. It will be passed the hash object and the key. You can do anything you want with them; set the key to a default value, derive a new value from the key, etc.

If you already have a Hash object, you can use the default_proc= method:

hash = { key: 'value' }

# ...

hash.default_proc = proc do |hash, key|
  hash[key] = :default
end