something wrong about T.gt()

In order to obtain the usage of T.gt(), i wrote a toy code.

def f(data):
# return T.gt(data, 0)
if T.gt(data, 0):
    print "1"
    return -data
else:
    print "2"
    return data


a = T.scalar()
t = f(a)

print t.eval({a:-4})

I expect the returned value is -4 when a=-4 and -4 when a=4, but it always satisfy the condition and run the return -data. I don't know why. Can you help me? thanks for your time.