Python constructor and default value

Question

Somehow  in the Node class below  the wordList and adjacencyList variable is shared between all instances of Node    gt  gt  gt  class Node          def   init   self  wordList       adjacencyList                    self wordList   wordList             self adjacencyList   adjacencyList       gt  gt  gt  a   Node    gt  gt  gt  b   Node    gt  gt  gt  a wordList append  hahaha    gt  gt  gt  b wordList   hahaha    gt  gt  gt  b adjacencyList append  hoho    gt  gt  gt  a adjacencyList   hoho     Is there any way I can keep using the default value  empty list in this case  for the constructor parameters but to get both a and b to have their own wordList and adjacencyList variables   I am using python 3 1 2

User · Accepted Answer

Mutable default arguments don t generally do what you want  Instead  try this   class Node       def   init   self  wordList None  adjacencyList None           if wordList is None              self wordList              else               self wordList   wordList          if adjacencyList is None              self adjacencyList              else               self adjacencyList   adjacencyList

User · Answer

Let s illustrate what s happening here   Python 3 1 2  r312 79147  Sep 27 2010  09 45 41    GCC 4 4 3  on linux2 Type  help    copyright    credits  or  license  for more information   gt  gt  gt  class Foo          def   init   self  x                  x append 1        gt  gt  gt  Foo   init     defaults          gt  gt  gt  f   Foo    gt  gt  gt  Foo   init     defaults     1     gt  gt  gt  f2   Foo    gt  gt  gt  Foo   init     defaults     1  1      You can see that the default arguments are stored in a tuple which is an attribute of the function in question  This actually has nothing to do with the class in question and goes for any function  In python 2  the attribute will be func func defaults    As other posters have pointed out  you probably want to use None as a sentinel value and give each instance it s own list

User · Answer

I would try   self wordList   list wordList    to force it to make a copy instead of referencing the same object

User · Answer

class Node      def   init   self  wordList None adjacencyList None           self wordList   wordList or            self adjacencyList   adjacencyList or

[python] Python constructor and default value

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