A Python class before and after optimization, as inspired by GitHub Copilot.

indexformatter1.py

class IndexFormatter:
    def __init__(self, volumeId: int):
        self.itemPages = (
            ItemPage.objects.filter(volume__id=volumeId)
            .order_by('item__topic__name', F('page') * 1, 'item__name'))

    def format(self) -> str:
        """
        Format the index for printing.
        """
        previousLetter = ''
        indexText = ''

        # unable to use `.distinct()` here for some reason
        # using `list(dict.fromkeys(…))` trick to mimic an ordered set
        topics = list(dict.fromkeys(self.itemPages.values_list(
            'item__topic__id', 'item__topic__name')))

        for topicId, topicName in topics:
            if topicName[0].upper() != previousLetter:
                previousLetter = topicName[0].upper()
                indexText += f'\n{previousLetter}\n\n'

            indexText += (
                    f'{topicName}, '
                    + '; '.join([f'{itemName}, {page}' for page, itemName in 
                                 self.itemPages.filter(item__topic=topicId)
                                .values_list('page', 'item__name')]) 
                    + '\n')
        return indexText

indexformatter2.py

class IndexFormatter:
    def __init__(self, volumeId: int):
        self.itemPages = (
            ItemPage.objects.filter(volume__id=volumeId)
            .order_by('item__topic__name', F('page') * 1, 'item__name'))

    def format(self) -> str:
        """
        Format the index for printing.
        """
        previousLetter = ''
        indexText = ''

        for (topic, topicLetter), itemPages in groupby(
                self.itemPages, lambda ip: (
                        (n := ip.item.topic.name), n[0].upper())):
            indexText += (
                    (f'\n{(previousLetter := topicLetter)}\n\n'
                     if previousLetter != topicLetter else '')
                    + f'{topic}, '
                    + '; '.join([f'{itemPage.item.name}, {itemPage.page}'
                                 for itemPage in itemPages])
                    + '\n')

        return indexText