python - Why does my original list change? -

i wrote function swapcities able swap entries 3 , 4 in list.

so f.e. [0,1,2,3,4] should become [0,1,2,4,3]. function works perfectly, strangely original list changes not want.

this 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 

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print "start" incumbentsolution = list(x x in range(0,numberofcities)) print incumbentsolution  print "after swap" newsolution = swapcities(incumbentsolution) print newsolution  print "original solution" print incumbentsolution 

i 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 change?!) 

as can see original solution changed should not do.

i have no clue why happens. when change code such changes applied copy of original list result. explain doing wrong?

incumbentsolution = list(x x in range(0,numberofcities)) print "start" print incumbentsolution  print "after swap" tmpsolution = incumbentsolution newsolution = swapcities(tmpsolution) print newsolution  print "original solution" print incumbentsolution 

swapcities mutating contents of solution. since solution points same list incumbentsolution, values inside incumbentsolution altered too.

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to preserve original values in incumbentsolution, make new copy of list:

tmpsolution = list(incumbentsolution) 

makes shallow copy of the original list. since contents of incumbentsolution immutable numbers, shallow copy suffices. if contents included, say, dicts being mutated, need make deep copy of list:

import copy tmpsolution = copy.deepcopy(incumbentsolution)