IndentationError expected an indented block

Question

I keep getting this error irrespective of all my effort in my code.

The code is as follows:

import numpy as np
import math
import random
PI = math.pi
class Grassfire:
START =  0
DEST  = -1
UNVIS = -2
OBST  = -3
PATH  = -4
COLOR_STARTN = np.array([0,0.75,0])
COLOR_DEST = np.array([0.75,0,0])
COLOR_VIS = np.array([0, 0.5, 1])
COLOR_UNVIS = np.array([1,1,1])
def random_ grid( self, rows=16, cols=16, obstacleProb =0.3):
obstacleGrid = np.random_sample(( rows, cols))

Error:

File "<string>", line 6
    START =  0
    ^
IndentationError: expected an indented block