How do I create an "optional" argument in Python?

Question

I am writing a simple Python class named "Bag":

class Bag:
def __init__(self, items=None):
    if self.items == None:
        self.items = defaultdict()
    else:
        temp_dict = defaultdict()
        for key in self.items:
            temp_dict[key] += 1
        self.items = temp_dict 

The variable items takes in a list of objects, such as:

['d','a','b','d','c','b','d']

From there, "def __init__(self, items=None)" will either:

1. Initialize items as an empty defaultdict, if nothing was passed into items, or
2. Initialize the passed in argument, if a list of objects were passed into items.

For example, this should work:

b = Bag()

The absence of an argument should be fine, as items is set, by default, to _None.

However, this always raises an exception(from a script that checks for errors):

*Error: b = Bag() raised exception AttributeError: 'Bag' object has no attribute 'items'

I want to initialize Bag() without putting a passing an argument into items.

Hope everything is clear, tell me if it isn't.

Any ideas or anything wrong with the code?

Answer 1

from collections import defaultdict

class Bag(object):

def __init__(self, items=None):
    self.items = items or list()
    if not self.items:
        self.items = defaultdict()
    else:
        temp_dict = defaultdict()
        for key in self.items:
            temp_dict[key] += 1
        self.items = temp_dict

I made a few corrections:

1) create self.items and initialize with items 2) compare with None using is None , or simply not self.items, since empty lists evaluate to False

Answer 2

'Bag' object has no attribute 'items' means: somewhere in the Bag's method you're accessing self.items that has not been defined.

And indeed you do. In the constructor, you write: if self.items == None: before you make the first assignment to self.items, thus creating it.

I think it's a typo, and you meant if items is None:, referring to the parameter.

This code can be simplified, though. My take:

import collections  # Batteries included.

...
def __init__(self, items=None):
    if not items:  # Covers both None and empty list.
        self.items = {}
    else:
        self.items = collections.Counter(items)

This can be simplified even more:

     ...
     self.items = collections.Counter(items or [])

If you strictly want a to use a defaultdict, you can:

     self.items = collections.defauldict(int, collections.Counter(items or []))

(Note that defaultdict() without an argument makes rather little sense.)