call_build_system.py

# Sample usage (with hypothetical build_wheel method that returns the name of the built wheel):
#   wheel_name = call_build_system("build/tmp", "build/tmp/venv/bin/python",
#                                  "flit:build_system_api",
#                                  "build_wheel", output_dir)

import json
import pkg_resources

from pip.utils import call_subprocess

def call_build_system(temp_dir, build_environment_python, specifier, method, *args, **kwargs):
    # We assume we have a scratch directory to work with -- like the one we
    # created to store the build environment.
    worker_path = os.path.join(temp_dir, "pip-worker.py")
    in_path = os.path.join(temp_dir, "in")
    out_path = os.path.join(temp_dir, "out")

    worker_code = pkg_resources.resource_string(__name__, "pip-worker.py")
    with open(worker_path, "w") as worker_file:
        worker_file.write(worker_code)
    with open(in_path, "w") as in_file:
        json.dump([specifier, method, args, kwargs], in_file)

    # Same machinery we use to call python setup.py whatever today:
    call_subprocess([build_environment_python, worker_path, in_path, out_path],
                    ...)

    with open(out_path) as out_file:
        return json.load(out_file)

pip-worker.py

import sys

if "" in sys.path:
    sys.path.remove("")

import json

(in_path, out_path) = sys.argv[1:]

def resolve_specifier(spec):
    # Technically "foo.bar" and "foo.bar:" are legal entry point descriptors.
    if ":" not in hook_string:
        hook_string += ":"
    module_part, dot_part = hook_string.split(":", 1)
    module_segments = module_part.split(".")
    if dot_part:
        dot_part = dot_part.split(".")
    else:
        dot_part = []

    __import__(module_part)
    segments = module_segments + dot_segments
    obj = sys.modules[segments[0]]
    for segment in segments[1:]:
        obj = getattr(hook, segment)
    return obj

with open(in_path) as in_file:
    specifier, method, args, kwargs = json.load(in_file)

build_system = resolve_specifier(specifier)
if getattr(build_system, "schema_version", 1) > 1:
    sys.exit("Uh oh")
result = getattr(build_system, method)(*args, **kwargs)

with open(out_path, "w") as out_file):
    json.dump(result, out_file)