How many copies of lists creates when 2 list concatenate with + operator?

Question

I have read a lot info here and here but I still don't understand is python really make 6 copies of lists when, for example it concatenate [1,2,3,4] + [5,6] these 2 lists

A "copy" of the list would be the entire list. If the list contains 6 items, why would you make 6 copies of the entire list? — roganjosh, Aug 30 '18 at 11:12
No, how exactly Python make two lists concatenation with + operator. When we talk about itertools.chain it uses generator, but with + what is the exact mechanism? — oliinykmd, Aug 30 '18 at 11:43

score 1 · Answer 1 · answered Aug 30 '18 at 11:12

1

Python doesn't make 6 copies of the list, rather it joins or concatenates the two lists and makes them into one i.e [1,2,3,4,5,6].

List concatenation in Python: How to concatenate two lists in Python?.

answered Aug 30 '18 at 11:12

Karan Dhir

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I have read it already, but also I found this post – oliinykmd Aug 30 '18 at 11:40

score 0 · Answer 2 · answered Aug 30 '18 at 11:20

No, only the references are copied. The objects themselves aren't copied at all. See this:

class Ref:
   ''' dummy class to show references '''
   def __init__(self, ref):
       self.ref = ref
   def __repr__(self):
      return f'Ref({self.ref!r})'

x = [Ref(i) for i in [1,2,3,4]]
y = [Ref(i) for i in [5,6]]

print(x)   # [Ref(1), Ref(2), Ref(3), Ref(4)]
print(y)   # [Ref(5), Ref(6)]

z = x + y
print(z)   # [Ref(1), Ref(2), Ref(3), Ref(4), Ref(5), Ref(6)]

# edit single reference in place
x[0].ref = 100
y[1].ref = 200

# concatenated list has changed
print(z)  # Ref(100), Ref(2), Ref(3), Ref(4), Ref(5), Ref(200)]

score 0 · Answer 3 · answered Aug 30 '18 at 11:23

Let say there n number of list l1, l2, l3, ... and If you add them it has only one copy on some address.

eg:

In [8]: l1 = [1,2,3]
In [9]: l2 = [4,5,6]

In [10]: id(l1 + l2)
Out[10]: 140311052048072

In [11]: id(l1)
Out[11]: 140311052046704

In [12]: id(l2)
Out[12]: 140311052047136

id() returns identity of an object. https://docs.python.org/2/library/functions.html#id

How many copies of lists creates when 2 list concatenate with + operator?

3 Answers3