def dictfind(dictionary, element):
    """
    Returns a key whose value in `dictionary` is `element` 
    or, if none exists, None.
    
        >>> d = {1:2, 3:4}
        >>> dictfind(d, 4)
        3
        >>> dictfind(d, 5)
    """
    for (key, value) in dictionary.iteritems():
        if element is value: 
            return key