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

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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

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?.

Python, Let's see how to concatenate two lists using different methods in Python. In this case, a new list is created, but this method is a one liner alternative to the loop method Any no. of lists can be concatenated and returned in a new list using this operator. Output: Concatenated list using * operator : [1, 4, 5, 6, 5, 3, 5, 7, 2, 5]. However, here, you will get examples to concatenate only two lists variables in Python. There are two methods by which you combine elements of two list variables. The first is the plus(+) operator and the second is the extend in Python. Let’s start describing each method one by one. Using Plus(+) Operator to Concatenate Two List Variables in

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)]

Concatenating two lists in Python - Noteworthy, extend operator can be used to concatenate one list into another. It is not May 6, 2017 · 2 min read. Lists in Python can be concatenated in two ways. Using +  I was wondering if there was an easy way to combine two lists in dart to create a brand new list object. I couldn't find anything and something like this: var newList = list1 + list2; Isn't valid.

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

10.6. Concatenation and Repetition, Again, as with strings, the + operator concatenates lists. It is important to see that these operators create new lists from the elements of the operand lists. Similarly, repetition of a list of 2 items 4 times will give a list with 8 items. it refers to a completely new list formed by making copies of the items from fruit and numlist . sequence and list together. There is no special function for this. If the sequence is first and the list is the second, then you have to choose between converting the first one to list (and then copying it when appending using List.append) or using Seq.append followed by List.ofSeq which will copy both lists.

Making Music with Computers: Creative Programming in Python, 3.4.1 List Concatenation Python uses the “+” operator with lists for concatenation For example, >>> [1, 2] + [3, 4] [1, 2, 3, 4] >>> [2, "be or not"] + [2.0, "be?"] [2 In a way, this makes sense, as “multiplying” a list results in many copies of the list​  Assume list1 references a list. After the following statement executes, list1 and list2 will reference two identical but seperate lists in memory: list 2 = list1

How to concatenate Lists In Python? – Mkyong.com, If we concatenate both the lists, then, the result may be [1, 2, 3, 4, 5, 6] or [4, 5, Concatenation Operator(+); Loops; List Comprehensions Copy. 1.2 We can concatenate any number of lists, we want to use the concatenation operator + . 2.2 We can also create a new list with the elements from both lists. Start studying Introduction to Programming - Chapter 7. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

Python Programming: Problem Solving, Packages and Libraries, Similarly, lists and tuples can also be concatenated to each other. You cannot use the + operator to join two sequences of different types. So you cannot use + to join say a string to a list. 1 2 3 4 5 6 7 8 >>>'abc' * 3 # Creates 3 copies of the string 'abc' 'abcabcabc' >>> ['a', 'b', 'c'] * 3 # Creates 3 copies of the list ['a', 'b',  FYI cl-concatenate uses append when TYPE is list, so both answers are producing the same result. – phils May 8 '17 at 1:20 Yess, it surely does the job. Many thanks to everybody for your help.

Comments
  • Why would python make 6 copies of those 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?
  • Do you mean 6 elements inside that list?
  • 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?
  • I have read it already, but also I found this post <mathieularose.com/how-not-to-flatten-a-list-of-lists-in-python>