为什么原始列表会发生变化？

Question

我写了一个函数SwapCities,它能够在列表中交换条目3和4.

所以fe [0,1,2,3,4]应该变成[0,1,2,4,3].这个功能很完美,但奇怪的是我的原始列表也改变了我不想要的.

这是我的代码:

def SwapCities(solution):
    n = 3##randint(0,NumberOfCities-1)
    m = 4##randint(0,NumberOfCities-1)
    result = solution
    temp1 = solution[n]
    temp2 = solution[m]
    result[n] = temp2
    result[m] = temp1
    return result

---

print "Start"
IncumbentSolution = list(x for x in range(0,NumberOfCities))
print IncumbentSolution

print "After swap" NewSolution = SwapCities(IncumbentSolution)
print NewSolution

print "Original solution"
print IncumbentSolution

我得到以下结果:

How many cities?
8 Start [0, 1, 2, 3, 4, 5, 6, 7]
After swap [0, 1, 2, 4, 3, 5, 6, 7]
Original solution [0, 1, 2, 4, 3, 5, 6, 7]   (why did this change?!)

正如您所看到的那样,我原来的解决方案发生了变化.

我不知道为什么会这样.即使我更改代码以使更改应用于原始列表的副本,我也会得到此结果.有人能解释我做错了吗？

IncumbentSolution = list(x for x in range(0,NumberOfCities))
print "Start"
print IncumbentSolution

print "After swap"
tmpsolution = IncumbentSolution
NewSolution = SwapCities(tmpsolution)
print NewSolution

print "Original solution"
print IncumbentSolution

Answer 1

SwapCities正在改变的内容solution.由于solution指向同一列表IncumbentSolution,因此内部的值IncumbentSolution也会改变.

要保留原始值IncumbentSolution,请创建列表的新副本:

tmpsolution = list(IncumbentSolution)

制作原始列表的浅表副本.由于内容IncumbentSolution是不可变数字,因此浅拷贝就足够了.如果包含的内容,例如dicts哪些内容也在变异,那么您需要制作列表的深层副本:

import copy
tmpsolution = copy.deepcopy(IncumbentSolution)