/dev/random

Ruby defaultdict

For those coming from Python, it may be surprising that the new(obj) -> new_hash method doesn't behave like Python's defaultdict.

Ruby

irb(main):001:0> foo = Hash.new([])
=> {}
irb(main):002:0> foo[:bar].push(1)
=> [1]
irb(main):003:0> foo[:baz].push(2)
=> [1, 2]
irb(main):004:0> foo
=> {}

Python

In [1]: from collections import defaultdict
In [2]: foo = defaultdict(list)
In [3]: foo['bar'].append(1)
In [4]: foo['baz'].append(2)
In [5]: foo
Out[5]: defaultdict(list, {'bar': [1], 'baz': [2]})

Instead what you want is the block form new {|hash, key| block } -> new_hash:

irb(main):001:0> foo = Hash.new {|h, k| h[k] = []}
=> {}
irb(main):002:0> foo[:bar].push(1)
=> [1]
irb(main):003:0> foo[:baz].push(2)
=> [2]
irb(main):004:0> foo
=> {:bar=>[1], :baz=>[2]}