dict with "missing" values

missing_dict.py

import collections


missing = object()

class MissingDict(dict):
    """Dictionary that supports a special 'missing' value.
    Assigning a missing value to key will remove the key from dictionary.
    Initializing a key with missing value will result in key not being
    added to dictionary at all.
    Rationale is to eliminate 'ifs' which are sometimes needed when creating
    dictionaries e.g. when sometimes we want default value for keyword argument.
    Example:
    >>> MissingDict({
        'zero': 0 or missing, 
        'one': 1,
    })
    {'one': 1}
    """
    def __init__(self, arg=None, **kwargs):
        if arg is None:
            kwargs = dict((k, v) for k, v in kwargs.iteritems()
                          if v is not missing)
            dict.__init__(self, **kwargs)
        else:
            items = (((k, arg[k]) for k in arg)
                     if isinstance(arg, collections.Mapping) else arg)
            filtered_items = ((k, v) for k, v in items
                              if v is not missing)
            dict.__init__(self, filtered_items)

    def __setitem__(self, key, obj):
        if obj is missing:
            del self[key]
        else:
            dict.__setitem__(self, key, obj)