关于python 3.x：为什么这个函数不进入while循环？

Why isn't this function entering the while loop?

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def millerRabin(toTest = 3, accuracy = 5):
# Return true if toTest is prime
################################
# Find how many times toTest can be halved
print(toTest)
under = toTest - 1
loopTracker = 0
while under % 2 == 0:
print('Before halving')
# Keep halving, and keep track until hit we can no longer divide in half
under = under // 2
print('After Halving: ', under)
loopTracker += 1
print("looped")
print(loopTracker)

print(millerRabin(toTest = 144000))

Miller-Rabin的第一部分是跟踪待测数量减半的次数。但是，我无法弄清楚为什么程序没有进入while循环并输出一些打印语句。