python - Why does my original list change?

Question

I a wrote a function SwapCities that is able to swap entries 3 and 4 in a list.

So f.e. [0,1,2,3,4] should become [0,1,2,4,3]. This function works perfectly, but strangely my original list also changes which I do not want.

This is my code:

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

I get the following result:

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?!)

As you can see my original solution changed which it should not do.

I have no clue why this happens. Even when I change the code such that the changes to are applied to a copy of the original list I get this result. Could someone explain what I am doing wrong?

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 is mutating the contents of solution. Since solution points to the same list as IncumbentSolution, the values inside IncumbentSolution are altered too.

---

To preserve the original values in IncumbentSolution, make a new copy of the list:

tmpsolution = list(IncumbentSolution)

makes a shallow copy of the the original list. Since the contents of IncumbentSolution are immutable numbers, a shallow copy suffices. If the contents included, say, dicts which were also being mutated, then you would need to make a deep copy of the list:

import copy
tmpsolution = copy.deepcopy(IncumbentSolution)