Is a Python list guaranteed to have its elements stay in the order they are inserted in

Question

If I have the following Python code   gt  gt  gt  x       gt  gt  gt  x   x    1   gt  gt  gt  x   x    2   gt  gt  gt  x   x    3   gt  gt  gt  x  1  2  3    Will x be guaranteed to always be  1 2 3   or are other orderings of the interim elements possible

User · Accepted Answer

Yes  the order of elements in a python list is persistent

User · Answer

In short  yes  the order is preserved  In long   In general the following definitions will always apply to objects like lists   A list is a collection of elements that can contain duplicate elements and has a defined order that generally does not change unless explicitly made to do so  stacks and queues are both types of lists that provide specific  often limited  behavior for adding and removing elements  stacks being LIFO  queues being FIFO   Lists are practical representations of  well  lists of things  A string can be thought of as a list of characters  as the order is important   abc      bca   and duplicates in the content of the string are certainly permitted   aaa  can exist and     a     A set is a collection of elements that cannot contain duplicates and has a non-definite order that may or may not change over time  Sets do not represent lists of things so much as they describe the extent of a certain selection of things  The internal structure of set  how its elements are stored relative to each other  is usually not meant to convey useful information  In some implementations  sets are always internally sorted  in others the ordering is simply undefined  usually depending on a hash function    Collection is a generic term referring to any object used to store a  usually variable  number of other objects  Both lists and sets are a type of collection  Tuples and Arrays are normally not considered to be collections  Some languages consider maps  containers that describe associations between different objects  to be a type of collection as well   This naming scheme holds true for all programming languages that I know of  including Python  C    Java  C   and Lisp  in which lists not keeping their order would be particularly catastrophic   If anyone knows of any where this is not the case  please just say so and I ll edit my answer  Note that specific implementations may use other names for these objects  such as vector in C   and flex in ALGOL 68  both lists  flex is technically just a re-sizable array    If there is any confusion left in your case due to the specifics of how the   sign works here  just know that order is important for lists and unless there is very good reason to believe otherwise you can pretty much always safely assume that list operations preserve order  In this case  the   sign behaves much like it does for strings  which are really just lists of characters anyway   it takes the content of a list and places it behind the content of another   If we have  list1    0  1  2  3  4  list2    5  6  7  8  9    Then  list1   list2   Is the same as   0  1  2  3  4     5  6  7  8  9    Which evaluates to   0  1  2  3  4  5  6  7  8  9    Much like   abdcde     fghijk    Produces    abdcdefghijk

User · Answer

I suppose one thing that may be concerning you is whether or not the entries could change  so that the 2 becomes a different number  for instance  You can put your mind at ease here  because in Python  integers are immutable  meaning they cannot change after they are created    Not everything in Python is immutable  though  For example  lists are mutable---they can change after being created   So for example  if you had a list of lists   gt  gt  gt  a     1    2    3    gt  gt  gt  a 0  append 7   gt  gt  gt  a   1  7    2    3     Here  I changed the first entry of a  I added 7 to it   One could imagine shuffling things around  and getting unexpected things here if you are not careful  and indeed  this does happen to everyone when they start programming in Python in some way or another  just search this site for  modifying a list while looping through it  to see dozens of examples     It s also worth pointing out that x   x    a  and x append a  are not the same thing  The second one mutates x  and the first one creates a new list and assigns it to x  To see the difference  try setting y   x before adding anything to x and trying each one  and look at the difference the two make to y

User · Answer

aList  1 2 3     i 0    for item in aList         if i lt 2                 aList remove item         i  1     aList     2     The moral is when modifying a list in a loop driven by the list  takes two steps   aList  1 2 3  i 0 for item in aList      if i lt 2          aList i   del      i  1  aList    del    del   3  for i in range 2       del aList 0   aList  3

User · Answer

Yes lists and tuples are always ordered while dictionaries are not

User · Answer

You are confusing  sets  and  lists    A set does not guarantee order  but lists do   Sets are declared using curly brackets      In contrast  lists are declared using square brackets       mySet    a  b  c  c    Does not guarantee order  but list does   myList    a  b  c

[python] Is a Python list guaranteed to have its elements stay in the order they are inserted in?

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