Gotcha with Hash.new when providing a default value

For a project I am working on at the moment, I needed a Hash that returned a different default value, i.e. not nil. Specifically, I needed it to return another Hash, and for that internal Hash’s default value to be an Array, like so:

hash = # ...
p hash.class                              #=> Hash
p hash[:non_existant_key].class           #=> Hash
p hash[:another][:non_existant_key].class #=> Array

Naïvely, I threw this into my code, and went along my merry way: entries = Hash.new(Hash.new([])). A few days later, I came back to this code to finish it and realised that something was awry with the values I was getting out of the Hash. It took me a while to figure it out because the values were all integers being summed together and I was never looking or setting the values directly.

However, eventually I saw a pattern — many of the values were the same. For instance, given the following assignments, I should expect [ruby]p entries[153][:monday] #=> [12.hours]:

entries[153][:monday]  << 12.hours
entries[153][:tuesday] << 3.hours
entries[87][:monday]   << 7.hours
entries[87][:tuesday]  << 2.5.hours

However, what I found was the following:

p entries[153][:monday]  #=> [12.hours, 3.hours, 7.hours, 2.5.hours]
p entries[153][:tuesday] #=> [12.hours, 3.hours, 7.hours, 2.5.hours]
p entries[87][:monday]   #=> [12.hours, 3.hours, 7.hours, 2.5.hours]
p entries[87][:tuesday]  #=> [12.hours, 3.hours, 7.hours, 2.5.hours]

# And in fact:
p entries[:any][:thing]  #=> [12.hours, 3.hours, 7.hours, 2.5.hours]

Let that soak in for a second. Hash.new({}) uses the same instance of the internal Array as the default value for each of the inner-most keys. It also doesn’t set the key you want so p entries still printed {}. In retrospect, it is blatantly obvious that it does this, but the ramifications are still huge.

The way to get the intended effect is of course to use the block syntax of [ruby]Hash.new which, while much uglier, definitely works as expected:

entries = Hash.new {|hash, key| hash[key] = Hash.new {|h, k| h[k] = [] }}
entries[153][:monday]  << 12.hours
entries[153][:tuesday] << 3.hours
entries[87][:monday]   << 7.hours
entries[87][:tuesday]  << 2.5.hours

p entries #=> {87=>{:monday=>[7.hours], :tuesday=>[2.5.hours]}, 153=>{:monday=>[12.hours], :tuesday=>[3.hours]}}

Wow. Obvious, but good to remember. I am sure most of you have had this issue before, but I think it is still worthy of mention…