List vs tuple when to use each

Question

In Python  when should you use lists and when tuples   Sometimes you don t have a choice  for example if you have   hello  s you are  s years old    x   then x must be a tuple   But if I am the one who designs the API and gets to choose the data types  then what are the guidelines

User · Answer

Must it be mutable   Use a list  Must it not be mutable   Use a tuple   Otherwise  it s a question of choice   For collections of heterogeneous objects  like a address broken into name  street  city  state and zip  I prefer to use a tuple   They can always be easily promoted to named tuples   Likewise  if the collection is going to be iterated over  I prefer a list   If it s just a container to hold multiple objects as one  I prefer a tuple

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There s a strong culture of tuples being for heterogeneous collections  similar to what you d use structs for in C  and lists being for homogeneous collections  similar to what you d use arrays for   But I ve never quite squared this with the mutability issue mentioned in the other answers   Mutability has teeth to it  you actually can t change a tuple   while homogeneity is not enforced  and so seems to be a much less interesting distinction

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I believe  and I am hardly well-versed in Python  that the main difference is that a tuple is immutable  it can t be changed in place after assignment  and a list is mutable  you can append  change  subtract  etc    So  I tend to make my tuples things that shouldn t change after assignment and my lists things that can

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Tuples are fixed size in nature whereas lists are dynamic  In other words  a tuple is immutable whereas a list is mutable    You can t add elements to a tuple  Tuples have no append or extend method  You can t remove elements from a tuple  Tuples have no remove or pop method  You can find elements in a tuple  since this doesn   t change the tuple   You can also use the in operator to check if an element exists in the tuple       Tuples are faster than lists  If you re defining a constant set of values and all you re ever going to do with it is iterate through it  use a tuple instead of a list  It makes your code safer if you    write-protect    data that does not need to be changed  Using a tuple instead of a list is like having an implied assert statement that this data is constant  and that special thought  and a specific function  is required to override that  Some tuples can be used as dictionary keys  specifically  tuples that contain immutable values like strings  numbers  and other tuples   Lists can never be used as dictionary keys  because lists are not immutable     Source  Dive into Python 3

User · Answer

The first thing you need to decide is whether the data structure needs to be mutable or not  As has been mentioned  lists are mutable  tuples are not  This also means that tuples can be used for dictionary keys  wheres lists cannot   In my experience  tuples are generally used where order and position is meaningful and consistant  For example  in creating a data structure for a choose your own adventure game  I chose to use tuples instead of lists because the position in the tuple was meaningful  Here is one example from that data structure   pages     foyer     text     some text               choices       open the door    rainbow                           go left into the kitchen    bottomless pit                           stay put   foyer2         The first position in the tuple is the choice displayed to the user when they play the game and the second position is the key of the page that choice goes to and this is consistent for all pages   Tuples are also more memory efficient than lists  though I m not sure when that benefit becomes apparent   Also check out the chapters on lists and tuples in Think Python

User · Answer

But if I am the one who designs the API and gets to choose the data types  then what are the guidelines    For input parameters it s best to accept the most generic interface that does what you need  It is seldom just a tuple or list - more often it s sequence  sliceable or even iterable  Python s duck typing usually gets it for free  unless you explicitly check input types  Don t do that unless absolutely unavoidable   For the data that you produce  output parameters  just return what s most convenient for you  e g  return whatever datatype you keep or whatever your helper function returns    One thing to keep in mind is to avoid returning a list  or any other mutable  that s part of your state  e g   class ThingsKeeper     def   init   self           self   things           def things self           return self   things   outside objects can now modify your state      def safer self           return self   things      it s copy-on-write  shouldn t hurt performance

User · Answer

A minor but notable advantage of a list over a tuple is that lists tend to be slightly more portable   Standard tools are less likely to support tuples   JSON  for example  does not have a tuple type   YAML does  but its syntax is ugly compared to its list syntax  which is quite nice     In those cases  you may wish to use a tuple internally then convert to list as part of an export process   Alternately  you might want to use lists everywhere for consistency

[python] List vs tuple, when to use each?

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