'Explicitly checks for the string \'True\', \'False\', \'true\',
\'false\', \'1\' and \'0\' and returns boolean True or False.
Returns None if value is not passed at all and raises an
exception for any other value.'

def to_python(self, value):

    if (value in (True, 'True', 'true', '1')):
        return True
    elif (value in (False, 'False', 'false', '0')):
        return False
    elif (not value):
        return None
    else:
        raise ValidationError('Invalid    Boolean    Value.')
