A long time ago I found a bug in Ruby core. If you’re on any p version of 1.9.3, you’re affected by this.

def foo(bizz=:bizz, baz=:baz, buzz=:buzz)

Object.send(:define_method, :bar) do |bizz=:bizz, baz=:baz, buzz=:buzz|

method(:foo).arity # expected: -1, actual: -1
method(:bar).arity # expected: -1, actual: 0

The “arity” of a function is the number of arguments it accepts. arity in ruby respects this definition if the method only has required arguments. If there are optional arguments (ex. arg=:arg), it should return -n-1 where n is the number of required arguments.