How to find out the default values of a particular function's argument in another function in Python?

Let's suppose we have a function like this:

def myFunction(arg1='a default value'):
  pass

We can use introspection to find out the names of the arguments that myFunction() takes using myFunction.func_code.co_varnames, but how to find out the default value of arg1 (which is 'a default value' in the above example)?