What's the Ruby equivalent of Python's defaultdict?

Question

In Python, I can make a hash where each element has a default value when it's first referenced (also know as "autovivification"). Here's an example:

from collections import defaultdict
d = defaultdict(int)
d["new_key"] += 1
print d

Printing the dict shows the value for "new_key" is 1.

What's the equivalent in Ruby? This code throws an error:

d = {}
d[:new_key] += 1
puts d

test.rb:3:in `<main>': undefined method `+' for nil:NilClass (NoMethodError)

Answer 1

You can use the first argument of the Hash.new method for that:

d = Hash.new 0
d[:new_key] += 1
d[:new_key] #=> 1
d[:foo]     #=> 0

Be careful - you might accidentally change the default value:

h = Hash.new("Go Fish")
h[:unknown_key]         #=> "Go Fish"
h[:unknown_key].upcase! #=> "GO FISH"
h[:next_key]            #=> "GO FISH"

As "mu is too short" pointed out in his answer, you should better use a proc, as in:

h = Hash.new { |h, k| h[k] = 0 }