How do I extract a sub-hash from a hash?

Question

ActiveSupport, at least since 2.3.8, provides four convenient methods: #slice, #except and their destructive counterparts: #slice! and #except!. They were mentioned in other answers, but to sum them in one place:

x = {a: 1, b: 2, c: 3, d: 4}
# => {:a=>1, :b=>2, :c=>3, :d=>4}

x.slice(:a, :b)
# => {:a=>1, :b=>2}

x
# => {:a=>1, :b=>2, :c=>3, :d=>4}

x.except(:a, :b)
# => {:c=>3, :d=>4}

x
# => {:a=>1, :b=>2, :c=>3, :d=>4}

Note the return values of the bang methods. They will not only tailor existing hash but also return removed (not kept) entries. The Hash#except! suits best the example given in the question:

x = {a: 1, b: 2, c: 3, d: 4}
# => {:a=>1, :b=>2, :c=>3, :d=>4}

x.except!(:c, :d)
# => {:a=>1, :b=>2}

x
# => {:a=>1, :b=>2}

ActiveSupport does not require whole Rails, is pretty lightweight. In fact, a lot of non-rails gems depend on it, so most probably you already have it in Gemfile.lock. No need to extend Hash class on your own.

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