Error

[inshu chauhan]

for this code m getting this error :

CODE :
def ComputeClasses(data):
    radius = .5
    points = []
    for cy in xrange(0, data.height):
        for cx in xrange(0, data.width):
            if data[cy,cx] != (0.0,0.0,0.0):
                centre = data[cy, cx]
                points.append(centre)


            change = True

            while change:

                for ring_number in xrange(1, 1000):
                    change = False
                    new_indices = GenerateRing(cx, cy, ring_number)


                    for idx in new_indices:
                        point = data[idx[0], idx[1]]

                        if point == (0.0, 0.0, 0.0 ):
                          continue
                        else:
                            dist = distance(centre, point)
                            if  dist < radius :
                                print point
                                points.append(point)
                                change = True
                                print change


            break


ERROR :
Traceback (most recent call last):
  File "Z:\modules\classification2.py", line 74, in <module>
    ComputeClasses(data)
  File "Z:\modules\classification2.py", line 56, in ComputeClasses
    dist = distance(centre, point)
UnboundLocalError: local variable 'centre' referenced before assignment

And i am unable to understand .. WHY ?
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