What is the id function used for

Question

I read the Python 2 docs and noticed the id   function      Return the    identity    of an object  This is an integer  or long integer  which is guaranteed to be unique and constant for this object during its lifetime  Two objects with non-overlapping lifetimes may have the same id   value       CPython implementation detail  This is the address of the object in memory    So  I experimented by using id   with a list    gt  gt  gt  list    1 2 3   gt  gt  gt  id list 0   31186196  gt  gt  gt  id list 1   31907092    increased by 896  gt  gt  gt  id list 2   31907080    decreased by 12   What is the integer returned from the function  Is it synonymous to memory addresses in C  If so  why doesn t the integer correspond to the size of the data type   When is id   used in practice

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Be carefull  concerning the answer just below    That s only true because 123 is between -5 and 256     In  111   q   257                                                           In  112   id q                                                              Out 112   140020248465168  In  113   w   257                                                           In  114   id w                                                             Out 114   140020274622544  In  115   id 257                                                           Out 115   140020274622768

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As of in python 3 id is assigned to a value not a variable  This means that if you create two functions as below  all the three id s are the same    gt  gt  gt  def xyz            q 123         print id q        gt  gt  gt  def iop            w 123         print id w    gt  gt  gt  xyz   1650376736  gt  gt  gt  iop   1650376736  gt  gt  gt  id 123  1650376736

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I m a little bit late and i will talk about Python3  To understand what id   is and how it  and Python  works  consider next example    gt  gt  gt  x 1000  gt  gt  gt  y 1000  gt  gt  gt  id x   id y  False  gt  gt  gt  id x  4312240944  gt  gt  gt  id y  4312240912  gt  gt  gt  id 1000  4312241104  gt  gt  gt  x 1000  gt  gt  gt  id x  4312241104  gt  gt  gt  y 1000  gt  gt  gt  id y  4312241200   You need to think about everything on the right side as objects  Every time you make assignment - you create new object and that means new id  In the middle you can see a  wild  object which is created only for function - id 1000   So  it s lifetime is only for that line of code  If you check the next line - you see that when we create new variable x  it has the same id as that wild object  Pretty much it works like memory address

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If you re using python 3 4 1 then you get a different answer to your question   list    1 2 3  id list 0   id list 1   id list 2     returns   1705950792    1705950808    increased by 16    1705950824    increased by 16   The integers -5 to 256 have a constant id  and on finding it multiple times its id does not change  unlike all other numbers before or after it that have different id s every time you find it  The numbers from -5 to 256 have id s in increasing order and differ by 16   The number returned by id   function is a unique id given to each item stored in memory and it is analogy wise the same as the memory location in C

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The is operator uses it to check whether two objects are identical  as opposed to equal   The actual value that is returned from id   is pretty much never used for anything because it doesn t really have a meaning  and it s platform-dependent

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I have an idea to use value of id   in logging  It s cheap to get and it s quite short  In my case I use tornado and id   would like to have an anchor to group messages scattered and mixed over file by web socket

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It is the address of the object in memory  exactly as the doc says  However  it has metadata attached to it  properties of the object and location in the memory is needed to store the metadata  So  when you create your variable called list  you also create metadata for the list and its elements   So  unless you an absolute guru in the language you can t determine the id of the next element of your list based on the previous element  because you don t know what the language allocates along with the elements

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That s the identity of the location of the object in memory     This example might help you understand the concept a little more   foo   1 bar   foo baz   bar fii   1  print id foo  print id bar  print id baz  print id fii    gt  1532352  gt  1532352  gt  1532352  gt  1532352   These all point to the same location in memory  which is why their values are the same  In the example  1 is only stored once  and anything else pointing to 1 will reference that memory location

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Rob s answer  most voted above  is correct  I would like to add that in some situations using IDs is useful as it allows for comparison of objects and finding which objects refer to your objects   The later usually helps you for example to debug strange bugs where mutable objects are passed as parameter to say classes and are assigned to local vars in a class  Mutating those objects will mutate vars in a class  This manifests itself in strange behavior where multiple things change at the same time   Recently I had this problem with a Python Tkinter app where editing text in one text entry field changed the text in another as I typed     Here is an example on how you might use function id   to trace where those references are  By all means this is not a solution covering all possible cases  but you get the idea  Again IDs are used in the background and user does not see them   class democlass      classvar   24      def   init   self  var           self instancevar1   var         self instancevar2   42      def whoreferencesmylocalvars self  fromwhere           return    l       g                       for   g   in fromwhere                         if not callable   g    and id eval   g        id getattr self   l                                           for   l   in dir self                      if not callable getattr self    l     and   l   -1                                def whoreferencesthisclassinstance self  fromwhere           return    g                       for   g   in fromwhere                         if not callable   g    and id eval   g        id self                     a    1 2 3 4  b   a c   b democlassinstance   democlass a  d   democlassinstance e   d f   democlassinstance classvar g   democlassinstance instancevar2  print   My class instance is of   type democlassinstance    type    print   My instance vars are referenced by    democlassinstance whoreferencesmylocalvars globals      print   My class instance is referenced by    democlassinstance whoreferencesthisclassinstance globals        OUTPUT   My class instance is of  lt class    main   democlass  gt  type  My instance vars are referenced by    instancevar2     g     classvar     f     instancevar1     a    c    b    My class instance is referenced by    e    d    democlassinstance     Underscores in variable names are used to prevent name colisions  Functions use  fromwhere  argument so that you can let them know where to start searching for references  This argument is filled by a function that lists all names in a given namespace  Globals   is one such function

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Your post asks several questions      What is the number returned from the function     It is  an integer  or long integer  which is guaranteed to be unique and constant for this object during its lifetime    Python Standard Library - Built-in Functions  A unique number  Nothing more  and nothing less  Think of it as a social-security number or employee id number for Python objects      Is it the same with memory addresses in C     Conceptually  yes  in that they are both guaranteed to be unique in their universe during their lifetime  And in one particular implementation of Python  it actually is the memory address of the corresponding C object      If yes  why doesn t the number increase instantly by the size of the data type  I assume that it would be int     Because a list is not an array  and a list element is a reference  not an object      When do we really use id    function    Hardly ever  id    or its equivalent  is used in the is operator

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I am starting out with python and I use id when I use the interactive shell to see whether my variables are assigned to the same thing or if they just look the same   Every value is an id  which is a unique number related to where it is stored in the memory of the computer

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id   does return the address of the object being referenced  in CPython   but your confusion comes from the fact that python lists are very different from C arrays  In a python list  every element is a reference  So what you are doing is much more similar to this C code   int  arr 3   arr 0    malloc sizeof int     arr 0    1  arr 1    malloc sizeof int     arr 1    2  arr 2    malloc sizeof int     arr 2    3  printf   p  p  p   arr 0   arr 1   arr 2      In other words  you are printing the address from the reference and not an address relative to where your list is stored   In my case  I have found the id   function handy for creating opaque handles to return to C code when calling python from C  Doing that  you can easily use a dictionary to look up the object from its handle and it s guaranteed to be unique

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The answer is pretty much never  IDs are mainly used internally to Python   The average Python programmer will probably never need to use id   in their code

[python] What is the id( ) function used for?

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