How to assert exception using django.test.TestCase?

Question

I am using Django 3 (latest) and tried to assert exception using unittest assertRaises as in the documentation.

My code is raising django.template.TemplateSyntaxError when it gets an IndexError:

from django import template

register = template.Library()
@register.tag(name="autopaginate")
def do_autopaginate(parser, token):
    split = token.split_contents()
    as_index = None
    context_var = None
    for i, bit in enumerate(split):
        if bit == "as":
            as_index = i
            break
    if as_index is not None:
        try:
            context_var = split[as_index + 1]
        except IndexError:
            raise template.TemplateSyntaxError(
                (
                    f"Context variable assignment "
                    f"must take the form of {{% {split[0]} object.example_set.all ... as "
                    f"context_var_name %}}"
                )
            )
        del split[as_index : as_index + 2]
    if len(split) == 2:
        return AutoPaginateNode(split[1])
    if len(split) == 3:
        return AutoPaginateNode(split[1], paginate_by=split[2], context_var=context_var)
    if len(split) == 4:
        try:
            orphans = int(split[3])
        except ValueError:
            raise template.TemplateSyntaxError(f"Got {split[3]}, but expected integer.")
        return AutoPaginateNode(
            split[1], paginate_by=split[2], orphans=orphans, context_var=context_var
        )
    raise template.TemplateSyntaxError(
        f"{split[0]} tag takes one required argument and one optional argument"
    )

My test code following documentation is:

from django import template
from django.http import HttpRequest
from django.test import TestCase

class TestHttpRequest(HttpRequest):
    __test__ = False
    page = 1

class TestTemplatePaginateTags(TestCase):
    def test_render_range_by_var_as_index_error(self):
        t = template.Template(
            "{% load pagination_tags %}{% autopaginate var by as %}{{ foo }}"
        )
        c = template.Context({"var": range(21), "request": TestHttpRequest()})
        with self.assertRaises(template.TemplateSyntaxError):
            t.render(c)

Full code is in this PR.

Unfortunately, instead of passing the test it is failing saying that it is throwing the exception:

FAILED tests/test_pagination_tags.py::TestTemplatePaginateTags::test_render_range_by_var_as_index_error

django.template.exceptions.TemplateSyntaxError: Context variable assignment must take the form of {% autopaginate object.example_set.all ... as context_var_name %}

This is exactly what I expect to make this test pass. I don't know what is wrong here.

willeM_ Van Onsem · Accepted Answer · 2020-07-04 22:28:02Z

3

The error will be raised when you construct the Template object, not when you render it. You thus expect the error to raise with t.render(c), but the error is in the t = template.Template(…), so you need to wrap another statement in the context manager:

class TestTemplatePaginateTags(TestCase):
    def test_render_range_by_var_as_index_error(self):
        with self.assertRaises(template.TemplateSyntaxError):
            template.Template(
                "{% load pagination_tags %}{% autopaginate var by as %}{{ foo }}"
            )

answered Jul 4, 2020 at 22:28

willeM_ Van Onsem

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