PEP 517 Backend bootstrapping

[pganssle (Paul Ganssle)]

@ncoghlan

I don’t understand why you’re concerned about folks potentially using bootstrap-backend-location to manipulate sys.path, but aren’t concerned about them doing the same thing by writing a __build_backend__.py that looks like:

For one thing, in the spec I mentioned above, this is not even guaranteed to work, but also because I am not trying to actively prevent people from being able to add '' to their sys.path, I don’t care about that. I just want it to happen in the backend, not in the frontend. Among the problems with build-backend-location='.' just adding the source root to the PATH is that it’s way too broadly scoped.

@ncoghlan

Again, doing it this way seems to make things more difficult for self-bootstrapping backend implementors, and make bootstrapping diverge further from regular source builds, for no readily apparent reaso

We should 100% be optimizing for people implementing front-ends, not self-bootstrapping backends. Pretty much the only person who ever has to implement this is setuptools. Everyone else is opting in. On the other hand, every single front-end must implement this functionality.

There is zero reason to make this any more general than it absolutely needs to be. The proposal of a fixed __build_backend__.py can be simply implemented like this:

if build_backend_name == "<bootstrap>":
    import sys
    sys.path.insert(0, '')
    build_backend = __import__('__build_backend__.py')
    sys.path[:] = sys.path[1:]    # Not required by the spec
else:
    build_backend = __import__(build_backend_name)

No fuss, no muss.

Frankly, if the implementation gets too complicated, the simplest thing is to change PEP 517 to say that backends must be built from a wheel and be done with it. --no-binary :all: would just have to be changed so that it doesn’t apply to the PEP 517 build requirements.