import math


def compare_dictionaries(one, two, rel_tol=1e-4, abs_tol=1e-9):
    """Compare two dictionaries with form ``{str: float}``, and return a set of keys where differences where present.

    Tolerance values are inputs to `math.isclose <https://docs.python.org/3/library/math.html#math.isclose>`__."""
    return (
        set(one)
        .symmetric_difference(set(two))
        .union(
            {
                key
                for key in one
                if key in two
                and not math.isclose(
                    a=one[key], b=two[key], rel_tol=rel_tol, abs_tol=abs_tol
                )
            }
        )
    )