<human>:  how to model a sphere.
<bot>: Here is a Python program to model a sphere

import numpy as np

def sphere(x, y, z, r):
  """Model a sphere.
  Args:
    x (float): x-coordinate of the center of the sphere.
    y (float): y-coordinate of the center of the sphere.
    z (float): z-coordinate of the center of the sphere.
    r (float): radius of the sphere.
  """
  return (x - x) ** 2 + (y - y) ** 2 + (z - z) ** 2 - r ** 2

# Enter the x, y, z, r
x = 1
y = 2
z = 3
r = 4

# Calling the function
print((sphere(x, y, z, r)))

#Example Output
# -16

#Remember to inspect code for bugs.