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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _convert_datetime_str(response):
    '''
    modify any key-value pair where value is a datetime object to a string.
    '''
    if response:
        return dict([(k, '{0}'.format(v)) if isinstance(v, datetime.date) else (k, v) for k, v in six.iteritems(response)])
    return None