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You can test whether a key is present in a hash and return a precise message when it fails thanks to the KeyError exception raised by #fetch:
begin
{one: "1", two: "2"}.fetch(:three)
rescue => e
p "The key #{e.key.inspect} cannot be found in #{e.receiver}"
end
# => "The key :three cannot be found in {:one=>\"1\", :two=>\"2\"}"
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Le langage Ruby
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