How to pass arguments by reference in Python function?

In Python, function arguments are always passed by reference. This can be verified by checking id() of factual, formal arguments and returned object

def foo(x):
  print ("id of received argument",id(x))
  x.append("20")
  return x
a = ["10"]
print ("id of argument before calling function",id(a))
b = foo(a)
print ("id of returned object",id(b))
print (b)
print (a)

The id() of a, x inside foo() and b are found to be same.

id of argument before calling function 1475589299912
id of received argument 1475589299912
id of returned object 1475589299912