lazy_dict.py

from copy import copy


class LazyDict(dict):
    """
    A dict class with default value for unfound keys

    >>> ldict = LazyDict(0)
    >>> ldict
    {}
    >>> ldict['x'] += 1
    >>> ldict
    {'x': 1}
    >>> ldict['x'] += 1
    >>> ldict
    {'x': 2}
    >>> ldict['y'] += 10
    >>> ldict == {'x': 2, 'y': 10}
    True
    >>> ldict['z'] == 0
    True
    >>> ldict == {'x': 2, 'y': 10, 'z': 0}
    True
    >>> ldict = LazyDict([])
    >>> ldict
    {}
    >>> ldict['a'].append(20)
    >>> ldict['a'].append(20)
    >>> ldict
    {'a': [20, 20]}
    >>> ldict['b'].append(6)
    >>> ldict == {'a': [20, 20], 'b': [6]}
    True
    """
    def __init__(self, default_value):
        super(LazyDict, self).__init__()
        self._default_value = default_value

    def __getitem__(self, item):
        if not item in self:
            self.__setitem__(item, copy(self._default_value))
        return super(LazyDict, self).__getitem__(item)