Relative imports don't work in pytest-playwright

[frere-jacques]

I have written a shiny app as a package and use relative imports to load the code from other modules.

This works fine and I can run the app via uvicorn myap.app:app.

However, if I want to run tests with pytest, I get the relative import error. Which is strange, because my app is an actual package.

I assume the reason is that the fixture forces me to give the path to the app.py. I gues the module is then tested as a standalone module and not the project loaded as package.

Another thing I observed is that for shiny express the relative import works for testing, but not for running the app.

[wch]

The behavior for relative imports can be a little confusing. It stems from how uvicorn handles relative imports (or doesn't, depending on how you launch the app).

First, when you use shiny run, there are four different ways you could launch an app::

1. shiny run app.py: launching it as a file from the app's directory
2. shiny run myapp/app.py: launching it as a file from the app's parent directory
3. shiny run app:app: launching it as a module from the app's directory
4. shiny run myapp.app:app: launching it as a module from the app's parent directory

For the first two, shiny run internally does something like the following: it goes to the app directory, and then runs uvicorn run app:app. For the latter two, it basically just calls uvicorn run app:app or uvicorn run myapp.app:app.

With that background, for Shiny Core apps:

• If you launch it from the app directory with uvicorn app:app, then the relative import won't work.
• If you launch it from the parent directory with uvicorn myap.app:app, then the relative import will work.

Shiny Express are a bit different, because they are actually wrapped into a Shiny Core app. For Shiny Express, we recently added the ability to use relative imports. However, for Shiny Express apps, you can only launch apps as files, not as modules. So you can do (1) shiny run app.py and (2)shiny run myapp/app.py, but not (3) shiny run app:app or (4) shiny run myapp.app:app. The reason for this is because Shiny Express needs to evaluate the code in a special way to capture the result of each line, similar to how Jupyter notebooks evaluate the code. We can't just pass app:app to uvicorn and have it work; we actually need to wrap up the code in a special evaluator inside of a Shiny Core app, and then pass that Shiny Core app to uvicorn.

If you want to launch a Shiny Express app from Python (instead of from the command line), it is possible, but I won't go into detail right here.

The PR that added this functionality to Shiny Express is #1464. That PR has some other information that may be useful.

This spreadsheet shows in which conditions you can do which kind of import (apologies if it is somewhat confusing to read): https://docs.google.com/spreadsheets/d/1jIVoEDr234uo_ALOPiilSIoU-bj18L-R6YP_39ORXwM/edit?gid=0#gid=0

It looks like kind of a mess, and again, that's in part because of how uvicorn handles relative imports.

We generally expect people to:

• Use global imports like import utils, instead of relative/submodule imports, like from . import utils.
• Launch their apps as files, with shiny run app.py or shiny run myapp/app.py.

If you do those things, the imports should work.

[frere-jacques]

Okay, I see then, why it works with run and not with the app fixture.

Is there any other way to create the fixture from running the module instead of linking to the py file?

If not then just using the absolute path might work to have a testable package.