Python constructor and default value

Question

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.

Solution 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 

Solution 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.

Solution 3

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

Solution 4

I would try:

self.wordList = list(wordList)

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

View more solutions

Comments

[Josh Bleecher Snyder]

These can also be one-liners: self.wordList = wordList if wordList is not None else [], or, slightly less safe, self.wordList = wordList or [].

[Karl Knechtel]

This is considered the Pythonic way, but I prefer krousey's way because "special cases aren't special enough".

[markdsievers]

@JoshBleecherSnyder I couldn't put my finger on it, what makes the latter less safe than the former?

[Josh Bleecher Snyder]

@markdsievers many things other than None evaluate to False (e.g. False, 0, "", {}, (), objects that specify __nonzero__()). The former is specific: None is the only special object that triggers defaulting to []. The latter will replace any false-y object with [].

[A---]

Michael J. Barber's contribution is correct - but no explanation is offered. The reason for this behavior is that the default argument is bound at function definition, not runtime. See stackoverflow.com/questions/1132941/…

[sharshofski]

So, is this a bug? This behavior is quite counterintuitive.

[user2357112]

@sharshofski: It's a terrible design decision, and I don't know of any other language that handles defaults like this, but it's not officially considered a bug.