TemplateView.get_context_data()'s kwargs returns SimpleLazyObjects that causes a crash when filtering.

[Tim L. White]

Example Code that works in 3.0, but not in 3.1:

class OfferView(TemplateView):
    template_name = "offers/offer.html"

    def get_context_data(self, **kwargs):
        offer_slug = kwargs.get("offer_slug", "")

        offer = get_object_or_404(Account, slug=offer_slug)

        return {"offer": offer, "offer_slug": offer_slug}

In order to make this work in 3.1, you have to explicitly convert the result of kwargs.get() to a string to get the SimpleLazyObject to resolve:

class OfferView(TemplateView):
    template_name = "offers/offer.html"

    def get_context_data(self, **kwargs):
        offer_slug = kwargs.get("offer_slug", "")

        offer = get_object_or_404(Account, slug=str(offer_slug))

        return {"offer": offer, "offer_slug": offer_slug}

The error generated if you don't is:

Error binding parameter 0 - probably unsupported type

from django/db/backends/sqlite3/operations.py, line 144, in _quote_params_for_last_executed_query

In both cases, the urls.py looks like:

path(
        "/offers/<slug:offer_slug>/",
        OfferView.as_view(),
        name="offer_view",
    ),

When debugging, I found that offer_slug (coming in from kwargs.get) was of type 'SimpleLazyObject' in Django 3.1, and when I explicitly converted it to a string, get_object_or_404 behaved as expected.

This is using Python 3.7.8 with SQLite.

[Mariusz Felisiak]

Thanks for the report. get_object_or_404() and QuerySet.filter() with SimpleLazyObject throw the same exception in Django 2.2 or 3.0.

TemplateView.get_context_data()'s kwargs returns SimpleLazyObjects in Django 3.1 which causes a crash. Passing URL kwargs into context is deprecated (see #19878) but should still work in Django 3.1 and 3.2.

Regression in 4ed534758cb6a11df9f49baddecca5a6cdda9311.
Reproduced at 60626162f76f26d32a38d18151700cb041201fb3.

[Adam Johnson]

Using lazy() instead of SimpleLazyObject() fixes this - PR is up.

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 20799cc0:

Fixes #31877 -- Used lazy() for TemplateView kwarg deprecation warning.

SimpleLazyObjects cause a crash when filtering.

Thanks Tim L. White for the report.
Regression in 4ed534758cb6a11df9f49baddecca5a6cdda9311.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 9ae40d81:

[3.1.x] Fixes #31877 -- Used lazy() for TemplateView kwarg deprecation warning.

SimpleLazyObjects cause a crash when filtering.

Thanks Tim L. White for the report.
Regression in 4ed534758cb6a11df9f49baddecca5a6cdda9311.

Backport of 20799cc0a6d98816b9ef0577e24691bd26b80d7d from master

[Mariusz Felisiak]

Passing deprecated keyword arguments to a queryset with lookups, e.g. Artist.objects.get(name__iexact=artist_name), still crashes on PostgreSQL:

django.db.utils.ProgrammingError: can't adapt type '__proxy__'

Thanks Mohit Solanki for the report.

[Adam Johnson]

😟 Hmm this is a tough one.

psycopg2 uses type(obj) to look for its "adapter" - in its C extension: ​https://github.com/psycopg/psycopg2/blob/e14e3385b4809ec4223894f8c7a009b1560eb41d/psycopg/microprotocols.c#L151 . So this "proxy" approach may not be feasible.

I know of a more accurate proxy wrapper that proxies more attributes than Django's lazy objects - wrapt.ObjectProxy - ​https://wrapt.readthedocs.io/en/latest/wrappers.html#object-proxy. But docs there acknowledge that it can't even make type() work, whilst it makes isinstance() work.

Any ideas of alternative approaches?

[Tom Forbes]

There isn't really a general purpose way of wrapping primitive/arbitrary types like this in Python that won't hit some corner cases. You can make an object appear to be a duck by adapting it's quacking dynamically (i.e wrapt.ObjectProxy), but if someone looks close enough they can always see that it's actually dog. And on the whole that's a good thing, IMO.

Our use of kwargs makes this harder as we lose the ability to construct a container that can trigger the deprecation warning which would be the typical easy approach. There is no way to control what lands on the other side of get_context_data() (it's always a plain kwargs dict), and there is no way to construct a wrapper value that looks _exactly_ like the value it's wrapping.

That basically leaves only "crazy" approaches, some of which are fun to consider but none of which are suitable. Here's one that uses settrace() to do what we need:

import sys

class DeprecatedWrapper(dict):
    def __getitem__(self, key):
        warnings.warn("stop right there, scoundrel!")
        return super().__getitem__(key)

def wrap_kwargs(frame, *args):
    frame.f_locals['kwargs'] = DeprecatedWrapper(frame['kwargs'])
    sys.settrace(None)

class TemplateView(...):
    def get(...):
        ...
        sys.settrace(wrap_kwargs)
        context = self.get_context_data(**context_kwargs)
        return self.render_to_response(context)

Given these issues, I'm not sure if we can go ahead with deprecating this.

[Mariusz Felisiak]

Agreed, we should revert it since we don't have a clear deprecation path.

​PR

[Adam Johnson]

I agree too.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 04e87e79:

Refs #31877 -- Reverted "Fixes #31877 -- Used lazy() for TemplateView kwarg deprecation warning."

This reverts commit 20799cc0a6d98816b9ef0577e24691bd26b80d7d.

[Mariusz Felisiak <felisiak.mariusz@…>]

In bb8f6693:

Fixed #31877 -- Reverted "Fixed #19878 -- Deprecated TemplateView passing URL kwargs into context."

This reverts commit 4ed534758cb6a11df9f49baddecca5a6cdda9311.

[Mariusz Felisiak <felisiak.mariusz@…>]

In f247c66b:

[3.1.x] Refs #31877 -- Reverted "Fixes #31877 -- Used lazy() for TemplateView kwarg deprecation warning."

This reverts commit 20799cc0a6d98816b9ef0577e24691bd26b80d7d.

Backport of 04e87e79a0bd2b1b9fdc30f884a637a3268733f0 from master

[Mariusz Felisiak <felisiak.mariusz@…>]

In e81aa7a:

[3.1.x] Fixed #31877 -- Reverted "Fixed #19878 -- Deprecated TemplateView passing URL kwargs into context."

This reverts commit 4ed534758cb6a11df9f49baddecca5a6cdda9311.

Backport of bb8f66934d93faf80cd1a2dda65aaedce21a6fc5 from master