Shen - 1 year ago 96
Python Question

# Deep copy a list in Python

I have some problem with a List copy:

So After I got

`E0`
from
`'get_edge'`
, I make a copy of
`E0`
by calling
`'E0_copy = list(E0)'`
. Here I guess
`E0_copy`
is a deep copy of
`E0`
, and I pass
`E0_copy`
into
`'karger(E)'`
. But in the main function.

Why does the result of
`'print E0[1:10]'`
before the for loop is not the same with that after the for loop?

Below is my code:

``````def get_graph():
f=open('kargerMinCut.txt')
G={}
for line in f:
ints = [int(x) for x in line.split()]
G[ints[0]]=ints[1:len(ints)]
return G

def get_edge(G):
E=[]
for i in range(1,201):
for v in G[i]:
if v>i:
E.append([i,v])
print id(E)
return E

def karger(E):
import random
count=200
while 1:
if count == 2:
break
edge = random.randint(0,len(E)-1)
v0=E[edge][0]
v1=E[edge][1]
E.pop(edge)
if v0 != v1:
count -= 1
i=0
while 1:
if i == len(E):
break
if E[i][0] == v1:
E[i][0] = v0
if E[i][1] == v1:
E[i][1] = v0
if E[i][0] == E[i][1]:
E.pop(i)
i-=1
i+=1

mincut=len(E)
return mincut

if __name__=="__main__":
import copy
G = get_graph()
results=[]
E0 = get_edge(G)
print E0[1:10]               ## this result is not equal to print2
for k in range(1,5):
E0_copy=list(E0)         ## I guess here E0_coypy is a deep copy of E0
results.append(karger(E0_copy))
#print "the result is %d" %min(results)
print E0[1:10]               ## this is print2
``````

`E0_copy` is not a deep copy. You don't make a deep copy using `list()` (Both `list(...)` and `testList[:]` are shallow copies).

You use `copy.deepcopy(...)` for deep copying a list.

``````deepcopy(x, memo=None, _nil=[])
Deep copy operation on arbitrary Python objects.
``````

See the following snippet -

``````>>> a = [[1, 2, 3], [4, 5, 6]]
>>> b = list(a)
>>> a
[[1, 2, 3], [4, 5, 6]]
>>> b
[[1, 2, 3], [4, 5, 6]]
>>> a[0][1] = 10
>>> a
[[1, 10, 3], [4, 5, 6]]
>>> b   # b changes too -> Not a deepcopy.
[[1, 10, 3], [4, 5, 6]]
``````

Now see the `deepcopy` operation

``````>>> b = copy.deepcopy(a)
>>> a
[[1, 10, 3], [4, 5, 6]]
>>> b
[[1, 10, 3], [4, 5, 6]]
>>> a[0][1] = 9
>>> a
[[1, 9, 3], [4, 5, 6]]
>>> b    # b doesn't change -> Deep Copy
[[1, 10, 3], [4, 5, 6]]
``````
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