`pip install .` leaves a build directory that is causing issues for some tools

[omry]

Environment

• pip version: 20.1
• Python version: 3.6+
• OS: Any

Description
Build-in-place leaves a build directory in the working directory, which can sometimes confuse tools like pytest when they discover pyc files inside.

Expected behavior

How to Reproduce

pip install .
pytest

Output

pytest
================================================================================== test session starts ==================================================================================
platform linux -- Python 3.8.2, pytest-5.4.1, py-1.8.1, pluggy-0.13.1
rootdir: /home/omry/dev/hydra, inifile: pytest.ini
plugins: snail-0.1.0
collected 899 items / 1 error / 898 selected                                                                                                                                            

======================================================================================== ERRORS =========================================================================================
_______________________________________________________________ ERROR collecting build/lib/hydra/test_utils/test_utils.py _______________________________________________________________
import file mismatch:
imported module 'hydra.test_utils.test_utils' has this __file__ attribute:
  /home/omry/dev/hydra/hydra/test_utils/test_utils.py
which is not the same as the test file we want to collect:
  /home/omry/dev/hydra/build/lib/hydra/test_utils/test_utils.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
================================================================================ short test summary info ================================================================================
ERROR build/lib/hydra/test_utils/test_utils.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
=================================================================================== 1 error in 0.64s ==

[lucasrijllart]

Hey @omry, I'm getting a different error message but also coming from the build directory issue. Using version pip 20.1 and pytest 3.6.3. I'm running:

pip install -r requirements.txt
pytest

And getting

ImportMismatchError: ('functional.conftest', '/app/tests/build/lib/interaction/functional/conftest.py', local('/app/tests/interaction/functional/conftest.py'))

It seems to me that version 20.0.2 did not create build directories when doing a pip install, and this behaviour has newly arrived in version 20.1.

[pfmoore]

It seems to me that version 20.0.2 did not create build directories when doing an build-in-place

In-place builds are new in 20.1, the reason 20.0.2 did not create a build directory is because the build was done in a temporary location.

[pfmoore]

Presumably the workaround is simply to use the --ignore=build flag to pytest?

[lucasrijllart]

Thanks for the quick reply @pfmoore, that's worked on my end!

[omry]

Presumably the workaround is simply to use the --ignore=build flag to pytest?

Yes, or to use pytest.ini:

[pytest]
norecursedirs = build

[omry]

It seems to me that version 20.0.2 did not create build directories when doing an build-in-place

In-place builds are new in 20.1, the reason 20.0.2 did not create a build directory is because the build was done in a temporary location.

I think it would make sense to build from the current directory but place the output in a temporary location (assuming whatever is consuming that output can also be adjusted).

[McSinyx]

@omry, I've just found out about this and hope you like it too: https://discuss.python.org/t/proposal-adding-a-persistent-cache-directory-to-pep-517-hooks/2303

[rksatyam]

Is there an option to pip to use the temporary location as in 20.0.2?

[pradyunsg]

I'd suggest filing an issue w/ pytest, to ask for a change to the discovery logic -- ignore the build directory by default.

[hsharrison]

It's worth noting that this affects more than pytest -- git, docker, linters, IDEs, other test runners, etc. Of course they all have options to ignore a directory, but it's a hassle. It would be better if there was an option (or default) for pip to clean up the build directory after installing.

[McSinyx]

@hsharrison, I hope the discussion on IRC yesterday provide you some extra info:

[13:25:17] <cnx> in continuation of the build dir thing, could we please have pip clean?
[13:26:43] <cnx> afaik atm pip only create build/ during build process, but it'd be easier for users to be able to have `pip install . && pip clean .` or `pip install --auto-clean .`
[13:35:48] <toad_polo> cnx: pip doesn't create that directory.
[13:35:55] <toad_polo> It wouldn't know what to clean.
[13:37:41] <toad_polo> You're probably best off using `.gitignore` to exclude `build/`, then running `git clean` to remove files not tracked by git.

Per my understanding, pip does not know what the build backend does and what they create in the project directory, so it cannot clean them. The previous behavior of copy everything to a tmp dir and build them there might not pollute the dir, however the implementation was not perfect (and there was not yet any standard to guide it), i.e. there are edge cases where it's broken (GH-7555) and multi-stage build was not possible.

[hsharrison]

Hmm, so I suppose the solution in Dockerfiles is pip install . && rm -r build

[omry]

Proposed solution:

1. By default, pip creates the build directory in a temporary dir (just the output, input can still be the source tree).
2. pip config allow the definition of a persistent build dir for cases (future cases?) when we need incremental build.