Errors from python property attributes are ignored inside jinja2 templates

[kratsg]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

from fastapi import FastAPI, Request
from fastapi.templating import Jinja2Templates

templates = Jinja2Templates(directory=".")

class Prop:
    @property
    def no_crash(self) -> str:
        return 'Hello World'

    @property
    def should_crash(self) -> str:
        return self.item

class NoProp:
    def no_crash(self) -> str:
        return 'Hello World'

    def should_crash(self) -> str:
        return self.item


app = FastAPI()

@app.get("/noprop")
async def read_noprop(request: Request):
    return templates.TemplateResponse(request=request, name='noprop.html', context={'item': NoProp()})

@app.get("/prop")
async def read_prop(request: Request):
    return templates.TemplateResponse(request=request, name='prop.html', context={'item': Prop()})


import uvicorn

if __name__ == "__main__":
    uvicorn.run("__main__:app", host="0.0.0.0", port=8080, reload=True)

Description

gist with the python file and two html files here: https://gist.github.com/kratsg/3a1bb76833d1f28b0c6b8b57325cd5b8

Create two html files in the same working directory.

One called noprop.html with the contents:

{{ item.no_crash() }}

{{ item.should_crash() }}

One called prop.html with the contents:

{{ item.no_crash }}

{{ item.should_crash }}

Run the above file and navigate to both endpoints:

• http://localhost:8080/noprop (crashes)
• http://localhost:8080/prop (does not crash)

You get the following stack trace

Stack trace

INFO:     127.0.0.1:61954 - "GET /prop HTTP/1.1" 200 OK
INFO:     127.0.0.1:61954 - "GET /noprop HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/uvicorn/protocols/http/httptools_impl.py", line 411, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/uvicorn/middleware/proxy_headers.py", line 69, in __call__
    return await self.app(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/applications.py", line 123, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/routing.py", line 758, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/routing.py", line 778, in app
    await route.handle(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/routing.py", line 299, in handle
    await self.app(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/routing.py", line 79, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/routing.py", line 74, in app
    response = await func(request)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/fastapi/routing.py", line 278, in app
    raw_response = await run_endpoint_function(
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/fastapi/routing.py", line 191, in run_endpoint_function
    return await dependant.call(**values)
  File "/Users/kratsg/audrey-two/debug_template_error.py", line 27, in read_noprop
    return templates.TemplateResponse(request=request, name='noprop.html', context={'item': NoProp()})
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/templating.py", line 230, in TemplateResponse
    return _TemplateResponse(
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/starlette/templating.py", line 40, in __init__
    content = template.render(context)
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/jinja2/environment.py", line 1301, in render
    self.environment.handle_exception()
  File "/Users/kratsg/audrey-two/venv/lib/python3.10/site-packages/jinja2/environment.py", line 936, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "noprop.html", line 3, in top-level template code
    {{ item.should_crash() }}
  File "/Users/kratsg/audrey-two/debug_template_error.py", line 20, in should_crash
    return self.item
AttributeError: 'NoProp' object has no attribute 'item'

Operating System

macOS

Operating System Details

Darwin Ghost.local 22.6.0 Darwin Kernel Version 22.6.0: Fri Sep 15 13:39:52 PDT 2023; root:xnu-8796.141.3.700.8~1/RELEASE_X86_64 x86_64

FastAPI Version

0.110.0

Pydantic Version

2.6.4

Python Version

Python 3.10.13

Additional Context

I can't tell if this is a problem at the Jinja2 level, the Starlette level, or the FastAPI level. I will first start at the highest level before working downwards (I think this is maybe the safest assumption?)

[kratsg]

Indeed, this is not a FastAPI or Starlette issue. Debugging even further, it seems there's something funny happening with jinja2. This example demonstrates the somewhat strange behavior

import jinja2

class Prop:
    @property
    def no_crash(self) -> str:
        return 'Hello World'

    @property
    def should_crash(self) -> str:
        return self.item

class NoProp:
    def no_crash(self) -> str:
        return 'Hello World'

    def should_crash(self) -> str:
        return self.item


environment = jinja2.Environment()
template_noprop = environment.from_string("""{{ item.no_crash() }}

{{ item.should_crash() }}""")
template_prop = environment.from_string("""{{ item.no_crash }}

{{ item.should_crash }}""")

print('Running NoProp')
try:
    print(template_noprop.render(item=NoProp()))
except Exception:
    print('NoProp crashed')
print('-'*20)

print('Running Prop')
try:
    print(template_prop.render(item=Prop()))
except Exception:
    print('Prop crashed')

Running this gives Prop not crashing (not expected)

$ python debug_template_error_jinja2.py
Running NoProp
NoProp crashed
--------------------
Running Prop
Hello World

So I will close this and file a corresponding issue in jinja2.

[kratsg]

In fact, the default jinja2 behavior is not StrictUndefined which would at least cause the crash...

[kratsg]

The issue is correspondingly indicated here: pallets/jinja#604.