Identifying pure functions in python

I have a decorator @pure that registers a function as pure, for example:

@pure
def rectangle_area(a,b):
    return a*b


@pure
def triangle_area(a,b,c):
    return ((a+(b+c))(c-(a-b))(c+(a-b))(a+(b-c)))**0.5/4

Next, I want to identify a newly defined pure function

def house_area(a,b,c):
    return rectangle_area(a,b) + triangle_area(a,b,c)

Obviously house_area is pure, since it only calls pure functions.

How can I discover all pure functions automatically (perhaps by using ast)