"""Store interface

Allows arbitrary classes to offer a simple get/set
store interface by deriving from the Store class in
this module
"""

class Store(object):
    """Interface for storing and retrieving simple values"""
    def __init__(self):
        self._data = {}
        self._unused = {}

    def set(self, key, value):
        """Set 'key' to 'value', overwriting 'key' if it exists already"""
        self._data[key] = value
        self._unused[key] = value

    def unused_keys(self):
        return self._unused.keys()

    def get(self, key, default=None):
        """Return the current value of 'key', or 'default' if 'key' is not set"""
        self._unused.pop(key, None)
        return self._data.get(key, default)

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