Python constructor and default value [duplicate]

Question

This question already has an answer here:

• “Least Astonishment” and the Mutable Default Argument 31 answers

Somehow, in the Node class below, the wordList and adjacencyList variable is shared between all instances of Node.

>>> class Node:
...     def __init__(self, wordList = [], adjacencyList = []):
...         self.wordList = wordList
...         self.adjacencyList = adjacencyList
... 
>>> a = Node()
>>> b = Node()
>>> a.wordList.append("hahaha")
>>> b.wordList
['hahaha']
>>> b.adjacencyList.append("hoho")
>>> 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.

Answer 1

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 

Answer 2

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.
>>> class Foo:
...     def __init__(self, x=[]):
...         x.append(1)
... 
>>> Foo.__init__.__defaults__
([],)
>>> f = Foo()
>>> Foo.__init__.__defaults__
([1],)
>>> f2 = Foo()
>>> 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.

Answer 3

class Node:
    def __init__(self, wordList=None adjacencyList=None):
        self.wordList = wordList or []
        self.adjacencyList = adjacencyList or []

Answer 4

I would try:

self.wordList = list(wordList)

to force it to make a copy instead of referencing the same object.