def dup_abs(f, K):
    """
    Make all coefficients positive in ``K[x]``.

    Examples
    ========

    >>> from sympy.polys import ring, ZZ
    >>> R, x = ring("x", ZZ)

    >>> R.dup_abs(x**2 - 1)
    x**2 + 1

    """
    return [K.abs(coeff) for coeff in f]