Failure building only wheels when using build.py

[roniemartinez]

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: Mac OS X 10.15.3 (Catalina)
• Poetry version: 1.0.5

Issue

Following on existing example of build.py for poetry with Cython (see here):

1. when running poetry build, everything is successful - sdist and wheel are built successfully and working properly ✅
2. when running poetry build -f wheel only, the error below is thrown ❌ (NOTE: I hid paths)

[EnvCommandError]
Command ['/Users/<user>/Library/Caches/pypoetry/virtualenvs/<project>-VUpAFIMn-py3.7/bin/python', '/Users/<user>/Projects/<project>/setup.py', 'build', '-b', '/Users/<user>/Projects/<project>/build'] errored with the following return code 2, and output: 
/Users/<user>/Library/Caches/pypoetry/virtualenvs/<project>-VUpAFIMn-py3.7/bin/python: can't open file '/Users/<user>/Projects/<project>/setup.py': [Errno 2] No such file or directory

It seems that it cannot find setup.py which is supposedly autogenerated by poetry when building.

[roniemartinez]

Just found out it fails with just a simple build function that does nothing.

# build.py


def build(setup_kwargs):
    pass