#... rest of code
conv1 = Conv2D(1, (3,3), padding='same', input_shape=(1, 500, 500), kernel_constraint = CustomConstraint())
#... rest of code

class CustomConstraint(Constraint):
    def __init__(self):
        print("INIT")

    def __call__(self, w):
        print("CALL")
        return .5*(w+K.transpose(w))