Define a function with a fractional power in python

Question

I want to defining the following function:

def f(x,y):
    return  pow(x, 1/3) + y

x = np.linspace(-1, 1, 10)
y = np.linspace(-1, 1, 10)

X, Y = np.meshgrid(x, y)
Z = f(X, Y)

where clearly x is raised to a fractional power. I want to keep x and y symbolic since I want to plot the corresponding surface later.

However, numpy seems to be unable to handle fractional powers:

<ipython-input-96-bb0ed6d3607a>:2: RuntimeWarning: invalid value encountered in float_power
  return  np.float_power(x, 1/2, dtype=float)

Is there a quick shortaround this problem? I have searched for similar questions on SO but they all involve np.arrays or such with prescribed types of variables.