> There are examples of the opposite too, though. Ruby's Hash does not insert the entry: It does if you initialise the hash with a block to do so: m = Has

> There are examples of the opposite too, though. Ruby's Hash does not insert the entry: It does if you initialise the hash with a block to do so: m = Hash.new {|h, k| h[k] = 0} m["nonexistent"] m.length # => 1 That is, essentially, what defaultdict does. Just through subclassing (because `dict` does not provide such a fallback, but it has a magic method you can override to customise the mapping's behaviour in that case).