Strange behavior involving Python nested lists -

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i have following code:

distances = [[100001] * 2] * 2     edge in edges:         print(distances)         distances[edge[0]][edge[1]] = edge[2]         print(distances)         print("\n") 

edges following list of tuples:

edges = [(0, 1, 10), (1, 0, -9)] 

i'm expecting output:

[[100001, 100001], [100001, 100001]] [[100001, 10], [100001, 100001]]  [[100001, 10], [100001, 100001]] [[100001, 10], [-9, 100001]] 

but i'm getting output:

[[100001, 100001], [100001, 100001]] [[100001, 10], [100001, 10]]  [[100001, 10], [100001, 10]] [[-9, 10], [-9, 10]] 

any ideas what's wrong?

try code:

>>> distances[0] distances[1] 

you true. means same objects when modify distances[0] modify distances[1] well. because distances[0] distances[1] cannot different.

how avoid it? change code to:

distances = [[100001] * 2] + [[100001] * 2]  

now distances[0] , distances[1] 2 different objects can modify 1 without modifying in same time.

now may wonder why 2*list different list+list. can explained:

lests assume have 2 lists:

a = [100001, 100001] b = [100001, 100001] c = 

it obvious when modify not modify b. modifying modify value in c because c.

now if define distances in way:

distances = a*2 # [a, a]  #so distances[0] cannot different distances[1]  

if define distances in way:

distances = a+b # [a, b]