Pip editable install reports success, doesn't actually build

[dan-fritchman]

Editably-install the hello-cpp example from https://github.com/scikit-build/scikit-build-sample-projects/tree/master/projects/hello-cpp. It reports success:

$ pip install -e .
Obtaining scikit-build-sample-projects/projects/hello-cpp
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... done
  Preparing editable metadata (pyproject.toml) ... done
Building wheels for collected packages: hello-cpp
  Building editable for hello-cpp (pyproject.toml) ... done
  Created wheel for hello-cpp: filename=hello_cpp-1.2.3-0.editable-py3-none-any.whl size=2410 sha256=4ae7f2fbb6338c6e31047184e22ef18043740e5432ba73c5d951732e6a5e4cab
  Stored in directory: /tmp/pip-ephem-wheel-cache-z__c9stq/wheels/59/b7/f0/2bf418c678571a24f6452a534896739888bed824135802ff1a
Successfully built hello-cpp
Installing collected packages: hello-cpp
  Attempting uninstall: hello-cpp
    Found existing installation: hello-cpp 1.2.3
    Uninstalling hello-cpp-1.2.3:
      Successfully uninstalled hello-cpp-1.2.3
Successfully installed hello-cpp-1.2.3

But it doesn't actually work. Doesn't look like it attempts to build the C++ at all.

$ pytest
=== test session starts ===
platform linux -- Python 3.9.16, pytest-7.3.0, pluggy-1.0.0 -- miniconda3/envs/py39/bin/python
cachedir: .pytest_cache
rootdir: scikit-build-sample-projects
configfile: pytest.ini
plugins: xdist-3.2.1, cov-4.0.0, dash-2.9.3, timeout-2.1.0, cpp-2.3.0
collected 0 items / 1 error

=== ERRORS ===
___ ERROR collecting projects/hello-cpp/tests/test_hello_cpp.py ___
ImportError while importing test module 'scikit-build-sample-projects/projects/hello-cpp/tests/test_hello_cpp.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../../miniconda3/envs/py39/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_hello_cpp.py:1: in <module>
    import hello
hello/__init__.py:1: in <module>
    from ._hello import (
E   ModuleNotFoundError: No module named 'hello._hello'

Some system & setup info.
Most notably this is on RedHat Linux, which admittedly can be weird.

$ uname -a
Linux eda-1 3.10.0-1160.83.1.el7.x86_64 #1 SMP Mon Dec 19 10:44:06 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
$ pip --version
pip 23.0.1 from miniconda3/envs/py39/lib/python3.9/site-packages/pip (python 3.9)
$ python --version
Python 3.9.16

On the same machine and setup, installing the same code non-editably does work:

$ pip install -v .
Using pip 23.0.1 from miniconda3/envs/py39/lib/python3.9/site-packages/pip (python 3.9)

# ...

  --
  -------
  ------------
  -----------------
  ----------------------
  ---------------------------
  --------------------------------
  -- Trying 'Ninja' generator - success
  --------------------------------------------------------------------------------

# ...

  Not searching for unused variables given on the command line.
  -- The C compiler identification is GNU 4.8.5
  -- The CXX compiler identification is GNU 4.8.5
  -- Detecting C compiler ABI info
  -- Detecting C compiler ABI info - done
  -- Check for working C compiler: /bin/cc - skipped
  -- Detecting C compile features
  -- Detecting C compile features - done
  -- Detecting CXX compiler ABI info
  -- Detecting CXX compiler ABI info - done
  -- Check for working CXX compiler: /bin/c++ - skipped
  -- Detecting CXX compile features
  -- Detecting CXX compile features - done
  -- Found PythonInterp: miniconda3/envs/py39/bin/python (found version "3.9.16")
  -- Found PythonLibs: miniconda3/envs/py39/lib/libpython3.9.so (found version "3.9.16")
  -- Performing Test Weak Link MODULE -> SHARED (gnu_ld_ignore) - Success
  _modinit_prefix:PyInit_
  -- Configuring done (0.9s)
  -- Generating done (0.0s)
  -- Build files have been written to: scikit-build-sample-projects/projects/hello-cpp/_skbuild/linux-x86_64-3.9/cmake-build
  [1/3] Building CXX object CMakeFiles/_hello.dir/hello/_hello.cxx.o
  [2/3] Linking CXX shared module _hello.cpython-39-x86_64-linux-gnu.so
  [2/3] Install the project...

# ...

Successfully installed hello-cpp-1.2.3

$ pytest
=================================== test session starts ===================================
platform linux -- Python 3.9.16, pytest-7.3.0, pluggy-1.0.0 -- miniconda3/envs/py39/bin/python
cachedir: .pytest_cache
rootdir: scikit-build-sample-projects
configfile: pytest.ini
plugins: xdist-3.2.1, cov-4.0.0, dash-2.9.3, timeout-2.1.0, cpp-2.3.0
collected 2 items

tests/test_hello_cpp.py::test_hello PASSED                                          [ 50%]
tests/test_hello_cpp.py::test_elevation PASSED                                      [100%]

==================================== 2 passed in 0.08s ====================================

Originally from ALIGN-analoglayout/ALIGN-public#1278

[henryiii]

We don’t support modern editable installs here, sadley. Initial support is present in scikit-build-core (native builds only ATM). You should also be able to set the envvar for classic editable builds in setuptools.

I do think we could possibly make this an error though until we can support it. (It is tricky to support because we have to produce a list of file to module mappings for setuptools).

[dan-fritchman]

Thanks for jumping on this, appreciate the quick feedback.
And understand if this is something not worth supporting.

Can you help me understand the "modern" vs "classic" build-modes you're referring to?
I don't immediately see either concept, either in skbuild's or setuptool's documentation.

Thanks again!

[henryiii]

Setuptools 64: https://setuptools.pypa.io/en/latest/userguide/development_mode.html#development-mode-a-k-a-editable-installs - you set --config-settings editable_mode=compat to get the old mode (sort of, not sure how close it is, as it's still based on the new code). The envvar SETUPTOOLS_ENABLE_FEATURES="legacy-editable" no longer works with pip 23.1, out a few days ago.