Paginator.count() may incorrectly silence AttributeError and TypeError

[Rob Jauquet]

With this code:

from django.contrib.postgres.search import SearchQuery, Value, Func
from django.core.paginator import Paginator
from django.db.models import F, TextField, DateTimeField, Models
from django.utils import timezone

class Entry(Model):
    text = TextField()
    published_on = DateTimeField(default=timezone.now())

class Headline(Func):
    function = 'ts_headline'
    output_field = TextField()

    def __init__(self, text, query, options=None):
        extra = {}
        expressions = [text, query]
        if options:
            opt_str = ''
            for key, value in options.items():
                opt_str += f'{key}={value},'
            expressions.append(Value(opt_str[:-1]))
        super().__init__(*expressions, **extra)

entries = Entry.objects.annotate(
    text_highlighted=Headline(
        F('text'), 
        SearchQuery('house'), 
        options={'HighlightAll': False},
    )
).order_by('-published_on')

paginator = Paginator(entries, 20)

# evaluates the queryset
paginator.page(1)

Passing options by dict to the Func subclass causes the count step for pagination to fail silently in paginator.py line 85 (1) which is in turn caused by a silenced TypeError during the hash in expressions.py line 372 (2).

Then, because of the fallback to len on paginator.py line 90 (3) the entire queryset is evaluated instead of evaluating just the specified page (and in this specific case, calling ts_headline on every row instead of just the first page).

The correct way of passing options using **options instead of options=None fixes this case, but the silent error and fallback to calling len on a queryset causes an unexpected expensive query when using pagination.

(1) ​https://github.com/django/django/blob/281c0223b376d6fa1a11e0726d824ed35cfe7524/django/core/paginator.py#L85
(2) ​https://github.com/django/django/blob/281c0223b376d6fa1a11e0726d824ed35cfe7524/django/db/models/expressions.py#L372
(3) ​https://github.com/django/django/blob/281c0223b376d6fa1a11e0726d824ed35cfe7524/django/core/paginator.py#L90

[Tim Graham]

Do you have a proposal of how to address this? By the way, if you press the "y" key when viewing a GitHub page, you can get a link that includes a commit hash. Without that, the links can quickly go stale as line numbers change.

[Rob Jauquet]

Thanks for the tip. I've updated the links.

For my use case, it would make sense to not call len on a queryset in paginator.py and instead surface the exception. The len fallback could be used only if the thing passed in is not a queryset.

[Tim Graham]

It looks like the best solution would be to remove the try/except in Paginator.count(). AttributeError can be removed with hasattr(self.object_list, 'count') and TypeError with some checking that count is a method without any arguments. Perhaps a new function in django/utils/inspect.py would help with that.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In f1bf069:

Refs #29244 -- Fixed django.utils.inspect.method_has_no_args() for bound methods.

[Tim Graham <timograham@…>]

In 3767c7f:

Fixed #29244 -- Prevented Paginator.count() from silencing TypeError and AttributeError.