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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 from_db_value(self, value, expression, connection, context):
 """ Convert database value to Python value. Called when data is loaded from the database. """

        # log.debug("from_db_value: {}, {}", value, type(value))
        if value is None:
            return value
        if value == '':
            return None
        return iso_string_to_python_datetime(value)