Is this correct behaviour for a Ruby hash with a default value?

Question

hash = Hash.new(Hash.new([]))
hash[1][2] << 3

hash[1][2] # => [3]
hash # => {}
hash.keys # => []
hash.values # => []

What's going on? Ruby's hiding data (1.9.3p125)

Answer 1

What's going on? Ruby's hiding data (1.9.3p125)

Ruby hides neither data nor its docs.

Default value you pass into the Hash constructor is returned whenever the key is not found in the hash. But this default value is never actually stored into the hash on its own.

To get what you want you should use Hash constructor with block and store default value into the hash yourself (on both levels of your nested hash):

hash = Hash.new { |hash, key| hash[key] = Hash.new { |h, k| h[k] = [] } } 

hash[1][2] << 3

p hash[1][2]  #=> [3]
p hash        #=> {1=>{2=>[3]}}
p hash.keys   #=> [1]
p hash.values #=> [{2=>[3]}]

Answer 2

It's simple. If you pass an object to a Hash constructor, it'll become a default value for all missing keys in that hash. What's interesting is that this value is mutable. Observe:

hash = Hash.new(Hash.new([]))
# mutate default value for nested hash
hash[1][2] << 3

# default value in action
hash[1][2] # => [3]
# and again
hash[1][3] # => [3]
# and again
hash[1][4] # => [3]

# set a plain hash (without default value)
hash[1] = {}

# what? Where did my [3] go?
hash[1][2] # => nil

# ah, here it is!
hash[2][3] # => [3]

Answer 3

I get a try with this in irb. Seams Ruby does not tag element as "visible" except affecting value over default explicitly via = for instance.

hash = Hash.new(Hash.new([]))
hash[1] = Hash.new([])
hash[1][2] = [3]
hash
#=> {1=>{2=>[3]}}

May be some setters are missing this "undefaulting" behavior ...