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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 calculate_focus(self, reading):
 """ Determines what the focal point of the downloaded image should be. Returns: focal_point: (x, y) The location of the source in the middle observation, in the coordinate system of the current source reading. """

        middle_index = len(self.source.get_readings()) // 2
        middle_reading = self.source.get_reading(middle_index)
        return self.convert_source_location(middle_reading, reading)