# documentação Django 2.0.8

# Código fonte para django.core.paginator

    import collections
    import warnings
    from math import ceil

    from django.utils.functional import cached_property
    from django.utils.translation import gettext_lazy as _

    class UnorderedObjectListWarning(RuntimeWarning):
        pass

    class InvalidPage(Exception):
        pass

    class PageNotAnInteger(InvalidPage):
        pass

    class EmptyPage(InvalidPage):
        pass

    class Paginator:

        def __init__(self, object_list, per_page, orphans=0,
                     allow_empty_first_page=True):
            self.object_list = object_list
            self._check_object_list_is_ordered()
            self.per_page = int(per_page)
            self.orphans = int(orphans)
            self.allow_empty_first_page = allow_empty_first_page

        def validate_number(self, number):
            """Validate the given 1-based page number."""
            try:
                number = int(number)
            except (TypeError, ValueError):
                raise PageNotAnInteger(_('That page number is not an integer'))
            if number < 1:
                raise EmptyPage(_('That page number is less than 1'))
            if number > self.num_pages:
                if number == 1 and self.allow_empty_first_page:
                    pass
                else:
                    raise EmptyPage(_('That page contains no results'))
            return number

        def get_page(self, number):
            """
            Return a valid page, even if the page argument isn't a number or isn't
            in range.
            """
            try:
                number = self.validate_number(number)
            except PageNotAnInteger:
                number = 1
            except EmptyPage:
                number = self.num_pages
            return self.page(number)

        def page(self, number):
            """Return a Page object for the given 1-based page number."""
            number = self.validate_number(number)
            bottom = (number - 1) * self.per_page
            top = bottom + self.per_page
            if top + self.orphans >= self.count:
                top = self.count
            return self._get_page(self.object_list[bottom:top], number, self)

        def _get_page(self, *args, **kwargs):
            """
            Return an instance of a single page.

            This hook can be used by subclasses to use an alternative to the
            standard :cls:`Page` object.
            """
            return Page(*args, **kwargs)

        @cached_property
        def count(self):
            """Return the total number of objects, across all pages."""
            try:
                return self.object_list.count()
            except (AttributeError, TypeError):
                # AttributeError if object_list has no count() method.
                # TypeError if object_list.count() requires arguments
                # (i.e. is of type list).
                return len(self.object_list)

        @cached_property
        def num_pages(self):
            """Return the total number of pages."""
            if self.count == 0 and not self.allow_empty_first_page:
                return 0
            hits = max(1, self.count - self.orphans)
            return int(ceil(hits / float(self.per_page)))

        @property
        def page_range(self):
            """
            Return a 1-based range of pages for iterating through within
            a template for loop.
            """
            return range(1, self.num_pages + 1)

        def _check_object_list_is_ordered(self):
            """
            Warn if self.object_list is unordered (typically a QuerySet).
            """
            ordered = getattr(self.object_list, 'ordered', None)
            if ordered is not None and not ordered:
                obj_list_repr = (
                    '{} {}'.format(self.object_list.model, self.object_list.__class__.__name__)
                    if hasattr(self.object_list, 'model')
                    else '{!r}'.format(self.object_list)
                )
                warnings.warn(
                    'Pagination may yield inconsistent results with an unordered '
                    'object_list: {}.'.format(obj_list_repr),
                    UnorderedObjectListWarning,
                    stacklevel=3
                )

    QuerySetPaginator = Paginator   # For backwards-compatibility.

    class Page(collections.Sequence):

        def __init__(self, object_list, number, paginator):
            self.object_list = object_list
            self.number = number
            self.paginator = paginator

        def __repr__(self):
            return '<Page %s of %s>' % (self.number, self.paginator.num_pages)

        def __len__(self):
            return len(self.object_list)

        def __getitem__(self, index):
            if not isinstance(index, (int, slice)):
                raise TypeError
            # The object_list is converted to a list so that if it was a QuerySet
            # it won't be a database hit per __getitem__.
            if not isinstance(self.object_list, list):
                self.object_list = list(self.object_list)
            return self.object_list[index]

        def has_next(self):
            return self.number < self.paginator.num_pages

        def has_previous(self):
            return self.number > 1

        def has_other_pages(self):
            return self.has_previous() or self.has_next()

        def next_page_number(self):
            return self.paginator.validate_number(self.number + 1)

        def previous_page_number(self):
            return self.paginator.validate_number(self.number - 1)

        def start_index(self):
            """
            Return the 1-based index of the first object on this page,
            relative to total objects in the paginator.
            """
            # Special case, return zero if no items.
            if self.paginator.count == 0:
                return 0
            return (self.paginator.per_page * (self.number - 1)) + 1

        def end_index(self):
            """
            Return the 1-based index of the last object on this page,
            relative to total objects found (hits).
            """
            # Special case for the last page because there can be orphans.
            if self.number == self.paginator.num_pages:
                return self.paginator.count
            return self.number * self.paginator.per_page