django error __str__ returned non-string (type __proxy__)

Question

I have some models that use GenericForeignKey, and when I try to access them with Django Admin util, I can see the list of records, but when I click on one of them, I get this error: __str__ returned non-string (type __proxy__).

This is the code of one of the models:

class ReservationComponent(models.Model):
    reservation = models.ForeignKey(Reservation, on_delete=models.PROTECT, related_name='components', verbose_name=_(''))
    day = models.DateField(verbose_name=_('Day'))
    content_type = models.ForeignKey(ContentType, on_delete=models.PROTECT)
    object_id = models.PositiveIntegerField()
    content_object = GenericForeignKey('content_type', 'object_id')

    class Meta:
        verbose_name = _("Reservation Component")
        verbose_name_plural = _("Reservations Components")

    def __str__(self):
        return "[{}][{}]{} - [{}]{}".format(self.id, self.reservation, self.day, self.content_type, self.object_id)

This is the whole traceback:

Internal Server Error: /es/admin/ReservationsManagerApp/reservationcomponent/14/change/
Traceback (most recent call last):
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/core/handlers/exception.py", line 35, in inner
    response = get_response(request)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/core/handlers/base.py", line 158, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/core/handlers/base.py", line 156, in _get_response
    response = response.render()
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/response.py", line 106, in render
    self.content = self.rendered_content
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/response.py", line 83, in rendered_content
    content = template.render(context, self._request)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/backends/django.py", line 61, in render
    return self.template.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 175, in render
    return self._render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
    return self.nodelist.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
    bit = node.render_annotated(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 155, in render
    return compiled_parent._render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
    return self.nodelist.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
    bit = node.render_annotated(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 155, in render
    return compiled_parent._render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
    return self.nodelist.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
    bit = node.render_annotated(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 67, in render
    result = block.nodelist.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
    bit = node.render_annotated(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 67, in render
    result = block.nodelist.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
    bit = node.render_annotated(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 211, in render
    nodelist.append(node.render_annotated(context))
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/loader_tags.py", line 194, in render
    return template.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 177, in render
    return self._render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 167, in _render
    return self.nodelist.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
    bit = node.render_annotated(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 211, in render
    nodelist.append(node.render_annotated(context))
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 211, in render
    nodelist.append(node.render_annotated(context))
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/defaulttags.py", line 314, in render
    return nodelist.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 943, in render
    bit = node.render_annotated(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 910, in render_annotated
    return self.render(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 993, in render
    output = self.filter_expression.resolve(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 676, in resolve
    obj = self.var.resolve(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 802, in resolve
    value = self._resolve_lookup(context)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/template/base.py", line 864, in _resolve_lookup
    current = current()
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/contrib/admin/helpers.py", line 137, in label_tag
    contents = conditional_escape(self.field.label)
  File "/Users/hugovillalobos/Documents/Code/IntellibookProject/IntellibookVenv/lib/python3.6/site-packages/django/utils/html.py", line 78, in conditional_escape
    text = str(text)
TypeError: __str__ returned non-string (type __proxy__)
[22/Aug/2018 02:52:14] "GET /es/admin/ReservationsManagerApp/reservationcomponent/14/change/ HTTP/1.1" 500 489781

Answer 1

I tried to reproduce the problem and I did. The problem is in ugettext_lazy.

You should use ugettext instead of ugettext_lazy. Hence your import should be,

from django.utils.translation import ugettext as _

Test environment was Django==1.11

Answer 2

You should use gettext_lazy in models declaration. It's very helpful for verbose_name translation (When should I use ugettext_lazy?). So, to bypass the problem in __str__ method, you can simply use str() built-in function to force to return string instead of __proxy__. And if you need to use string format (example: 'My object is: {}'.format(object)), you must use format_lazy and also force string with str() function.

For example:

from django.db import models
from django.utils.text import format_lazy
from django.utils.translation import gettext_lazy as _

class MyThing(models.Model):
    name = models.CharField(help_text=_(
        'help text for MyThing model', 'This is the help text'))

    class Meta:
        verbose_name = _("My thing")

    def __str__(self):
        if self.name:
            # Force str to avoid __proxy__
            return str(format_lazy('{txt} {name}', txt=_('My name is'), name=self.name))
        # Force str to avoid __proxy__
        return str(_("No name"))