[Python] Give side effect to only first call of Pytest patched function

I have a function that renders HTML templates. In case exception is thrown during rendering of a given template, a default template will be rendered (simplified version of the function below):

def main(path_to_template):
try:
return render_template(path_to_template)
except Exception:
return render_template('default.html')


I would like to test the exception block. In my test function I wanted to patch render_template, so that it has an Exception side effect. This must be limited to the first call only however, since I want the second call to return whatever render_template is doing.

Is it possible to create such a test?

I tried the following solutions, but none work:

def test_main():
mocker.patch("path.to.render_template", side_effect=Exception("mock exception"))
main("some_path")


The above is the first approach and it will raise Exception in both calls, hence not test what I want to test.

def test_main():
mocker.patch("path.to.render_template", side_effect=[Exception("mock exception"), None])
main("some_path")


The above also does not work because the return of render_template is not supposed to be None.

def custom_side_effect(*args):
if args[0] == "some_path":
raise Exception("mock exception")
else:
return render_template(args[0])

def test_main():
mocker.patch("path.to.render_template", side_effect=custom_side_effect)
main("some_path")


The last one results in a circular exception throwing and I have to kill the test run.

The problem seems to be something standard with an easy solution. Has anyone experienced a similar issue and can suggest the way to solve it?

EDIT 1: The original test is a functional test using Selenium, where I want to test a scenario of a server error that should result in displaying HTTP 500 Error page. Below the original function under test:

@login_required(login_url="/login/")
def pages(request):
context = {}
try:
# remove trailing slash to determine a template
path = request.path.rstrip("/")
load_template = path.split("/")[-1]
if load_template == "admin":
return HttpResponseRedirect(reverse("admin:index"))
context["segment"] = load_template
html_template = loader.get_template("home/" + load_template)
except template.TemplateDoesNotExist as e:
html_template = loader.get_template("home/page-404.html")
except Exception as e:
html_template = loader.get_template("home/page-500.html")
return HttpResponse(html_template.render(context, request))


EDIT 2: As a temporary solution I decided to do small refactoring.

def load_home_page(path_to_template):
return loader.get_template(path_to_template)


def load_404_page():
return loader.get_template("home/page-404.html")


def load_500_page():
return loader.get_template("home/page-500.html")


Then I only patch load_home_page function.

I don't think this is the best solution, so it would be great to see something cleaner.

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