Confused about str on list in Python

Question

Coming from a Java background  I understand that   str   is something like a Python version of toString  while I do realize that Python is the older language    So  I have defined a little class along with an   str   method as follows   class Node       def   init   self  id           self id   id         self neighbours              self distance   0       def   str   self           return str self id    I then create a few instances of it   uno   Node 1      due   Node 2      tri   Node 3      qua   Node 4    Now  the expected behaviour when trying to print one of these objects is that it s associated value gets printed  This also happens   print uno   yields  1   But when I do the following   uno neighbours append   due  4    tri  5      and then  print uno neighbours   I get      lt   main   Node instance at 0x00000000023A6C48 gt   4     lt   main   Node instance at 0x00000000023A6D08 gt   5      Where I expected    2  4    3  5     What am I missing  And what otherwise cringe-worthy stuff am I doing

User · Answer

str   is only called when a string representation is required of an object   For example str uno   print   s    uno or print uno  However  there is another magic method called   repr   this is the representation of an object   When you don t explicitly convert the object to a string  then the representation is used   If you do this uno neighbors append   str due  4   str tri  5    it will do what you expect

User · Answer

Answer to the question  As pointed out in another answer and as you can read in PEP 3140  str on a list calls for each item   repr    There is not much you can do about that part    If you implement   repr    you will get something more descriptive  but if implemented correctly  not exactly what you expected   Proper implementation  The fast  but wrong solution is to alias   repr   to   str       repr   should not be set to   str   unconditionally    repr   should create a representation  that should look like a valid Python expression that could be used to recreate an object with the same value  In this case  this would rather be Node 2  than 2   A proper implementation of   repr   makes it possible to recreate the object  In this example  it should also contain the other significant members  like neighours and distance    An incomplete example   class Node       def   init   self  id  neighbours     distance 0           self id   id         self neighbours   neighbours         self distance   distance       def   str   self           return str self id        def   repr   self           return  Node id  0 id   neighbours  0 neighbours r   distance  0 distance    format self            in an elaborate implementation  members that have the default           value could be left out  but this would hide some information   uno   Node 1      due   Node 2      tri   Node 3      qua   Node 4   print uno print str uno  print repr uno   uno neighbours append   due  4    tri  5     print uno print uno neighbours print repr uno    Note  print repr uno  together with a proper implementation of   eq   and   ne   or   cmp   would allow to recreate the object and check for equality

User · Answer

The thing about classes  and setting unencumbered global variables equal to some value within the class  is that what your global variable stores is actually the reference to the memory location the value is actually stored     What you re seeing in your output is indicative of this     Where you might be able to see the value and use print without issue on the initial global variables you used because of the str method and how print works  you won t be able to do this with lists  because what is stored in the elements within that list is just a reference to the memory location of the value -- read up on aliases  if you d like to know more    Additionally  when using lists and losing track of what is an alias and what is not  you might find you re changing the value of the original list element  if you change it in an alias list -- because again  when you set a list element equal to a list or element within a list  the new list only stores the reference to the memory location  it doesn t actually create new memory space specific to that new variable    This is where deepcopy comes in handy

User · Answer

Because of the infinite superiority of Python over Java  Python has not one  but two toString operations    One is   str    the other is   repr      str   will return a human readable string    repr   will return an internal representation      repr   can be invoked on an object by calling repr obj  or by using backticks  obj    When printing lists as well as other container classes  the contained elements will be printed using   repr

User · Answer

Well  container objects    str   methods will use repr on their contents  not str  So you could use   repr   instead of   str    seeing as you re using an ID as the result

User · Answer

Python has two different ways to convert an object to a string  str   and repr     Printing an object uses str    printing a list containing an object uses str   for the list itself  but the implementation of list   str     calls repr   for the individual items   So you should also overwrite   repr       A simple    repr       str     at the end of the class body will do the trick

User · Answer

It provides human readable version  of output rather  Object   Example    class Pet object        def   init   self  name  species           self name   name         self species   species      def getName self           return self name      def getSpecies self           return self species      def Norm self           return   s is a  s     self name  self species   if   name       main         a   Pet  jax    human       print a    returns    lt   main   Pet object at 0x029E2F90 gt    while code with  str  return something different   class Pet object        def   init   self  name  species           self name   name         self species   species      def getName self           return self name      def getSpecies self           return self species      def   str   self           return   s is a  s     self name  self species   if   name       main         a   Pet  jax    human       print a    returns    jax is a human

User · Answer

print self id   str     would work for you  although not that useful for you    Your   str   method will be more useful when you say want to print out a grid or struct representation as your program develops   print self  grid   str      def   str   self               Return a string representation of the grid for debugging              grid str          for row in range self  rows           grid str    str  self  grid row            grid str      n      return grid str

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