Function to find midpoint of two floats where one of the floats might be an empty string

Question

There must surely be a nicer way of writing this.

The wrinkle of complexity is that prev_seq or next_seq might be an empty string if the user moves the question to the first or last position, but it could also validly be '0.0' so I can't just rely on 'falsiness'.

I hope it's clear what I'm trying to achieve from the below (with the comments).

How can I improve this?

def update_sequence(self):

    prev_seq = self.request.get('prev_seq')
    next_seq = self.request.get('next_seq')
    question_id = int(self.request.get('id'))
    question = db.get(db.Key.from_path('Questions', question_id))

    try:
        # to slot a question between two numbers we compute the midpoint
        # for, example, to insert a question between 2.1 and 3.4, the
        # question would get the sequence of 2.75
        new_position = (float(prev_seq) + float(next_seq)) / 2
    except ValueError:
        # if we get here, either prev_seq or next_seq was not submitted
        # in the request which means the user pushed the question to the
        # first or last position.
        try:
            float(prev_seq)
        except ValueError:
            # in this case the question has been moved to the first
            # position so we should make:
            # the new position = the next position - 1
            new_position = float(next_seq) - 1
        try:
            float(next_seq)
        except ValueError:
            # in this case the question has been moved to the last position
            # so we should make:
            # the new position = the previous position + 1
            new_position = float(prev_seq) + 1

    question.sequence_float = new_position
    question.put()

Answer 1

def get_new_position(prev_seq, next_seq):

    if next_seq == "":
        new_position = float(prev_seq) + 1
    elif prev_seq == "":
        new_position = float(next_seq) - 1
    else:
        new_position = (float(prev_seq) + float(next_seq)) / 2

    return new_position