immutability.py

# Python

def my_func(arg_1 = []):
    arg_1.append(1)
    print arg_1

my_func()
my_func()

# $ python test.py
# [1]
# [1, 1]

# Ruby

def my_func(arg_1=[])
  arg_1.push(1)
  p arg_1
end

my_func
my_func

# $ ruby test.rb
# [1]
# [1]