Hash defaults and mutation

The idea

A hash associates keys with values and preserves insertion order. Reading an absent key normally returns nil, which may be indistinguishable from a deliberately stored nil unless you use key? or fetch. Fetch is useful when a missing entry is a contract violation or needs an explicit fallback. Symbol and string keys are distinct, so normalize external keys only according to a documented policy.

Hash.new([]) supplies the same default array on every missing lookup; it does not store a separate array for each key. Use a default block that assigns a fresh array to the requested key when building groups. Assignment and shallow duplication can also preserve references to nested objects. Mutation should be intentional and documented; a bang in a method name is a convention, not a universal guarantee.

groups = Hash.new { |hash, key| hash[key] = [] }
groups[:open] << 1
groups[:closed] << 2
p groups[:open]
p groups[:closed]

[1]
[2]

The default block runs for each missing key and stores a new array, so adding a closed item does not change the open group.