Raymond Hettinger reported during PyCon Canada 2016 Keynote (~20m30 sec)
That unbound local error message was inaccurate, it state that :

> local variable 'xxx' referenced before assignment

Though it can be assigned and deleted.

    for a toy example:

       def foo():
          x = 1
          del x
          print(x)

       foo()

Do the same for free variable.

    def multiplier(n):
	def multiply(x):
	    return x * n
	del n
	return multiply