Proof of Concept: Bootstrap backend from specified directory

pep517/_in_process.py

@@ -33,10 +33,18 @@ def _build_backend():
     """Find and load the build backend"""
     ep = os.environ['PEP517_BUILD_BACKEND']
     mod_path, _, obj_path = ep.partition(':')
+    backend_locn = os.environ.get('PEP517_BACKEND_LOCN', None)
+    if backend_locn is not None:
+        sys.path.insert(0, backend_locn)
+
     try:
         obj = import_module(mod_path)
     except ImportError:
         raise BackendUnavailable(traceback.format_exc())
+    finally:
+        if backend_locn is not None:
+            sys.path.pop(0)
+
     if obj_path:
         for path_part in obj_path.split('.'):
             obj = getattr(obj, path_part)

pep517/wrappers.py

@@ -47,9 +47,10 @@ class Pep517HookCaller(object):
     source_dir : The path to the source directory, containing pyproject.toml.
     backend : The build backend spec, as per PEP 517, from pyproject.toml.
     """
-    def __init__(self, source_dir, build_backend):
+    def __init__(self, source_dir, build_backend, bootstrap_backend_locn=None):
         self.source_dir = abspath(source_dir)
         self.build_backend = build_backend
+        self.bootstrap_backend_locn = bootstrap_backend_locn
         self._subprocess_runner = default_subprocess_runner
 
     # TODO: Is this over-engineered? Maybe frontends only need to
@@ -137,16 +138,18 @@ def build_sdist(self, sdist_directory, config_settings=None):
         })
 
     def _call_hook(self, hook_name, kwargs):
+        envvars = {'PEP517_BUILD_BACKEND': self.build_backend}
+        if self.bootstrap_backend_locn is not None:
+            envvars['PEP517_BACKEND_LOCN'] = self.bootstrap_backend_locn
+
         # On Python 2, pytoml returns Unicode values (which is correct) but the
         # environment passed to check_call needs to contain string values. We
         # convert here by encoding using ASCII (the backend can only contain
         # letters, digits and _, . and : characters, and will be used as a
         # Python identifier, so non-ASCII content is wrong on Python 2 in
         # any case).
         if sys.version_info[0] == 2:
-            build_backend = self.build_backend.encode('ASCII')
-        else:
-            build_backend = self.build_backend
+            envvars = {k: v.encode('ASCII') for (k, v) in envvars.items()}
 
         with tempdir() as td:
             compat.write_json({'kwargs': kwargs}, pjoin(td, 'input.json'),
@@ -156,7 +159,7 @@ def _call_hook(self, hook_name, kwargs):
             self._subprocess_runner(
                 [sys.executable, _in_proc_script, hook_name, td],
                 cwd=self.source_dir,
-                extra_environ={'PEP517_BUILD_BACKEND': build_backend}
+                extra_environ=envvars,
             )
 
             data = compat.read_json(pjoin(td, 'output.json'))

tests/samples/bootstrap_pkg/mybackend.py

@@ -0,0 +1,20 @@
+# A "bootstrap backend" for testing purposes
+def get_requires_for_build_wheel(config_settings=None):
+    return ['wheel', 'sentinel']
+
+
+def get_requires_for_build_sdist(config_settings=None):
+    return ['sdist_sentinel']
+
+
+def prepare_metadata_for_build_wheel(metadata_directory, config_settings=None):
+    pass
+
+
+def build_wheel(wheel_directory, config_settings=None,
+                metadata_directory=None):
+    pass
+
+
+def build_sdist(sdist_directory, config_settings):
+    pass

tests/samples/bootstrap_pkg/pyproject.toml

@@ -0,0 +1,4 @@
+[build-system]
+requires=[]
+build-backend="mybackend"
+bootstrap-backend-location="."

tests/test_call_hooks.py

@@ -18,7 +18,11 @@ def get_hooks(pkg):
     source_dir = pjoin(SAMPLES_DIR, pkg)
     with open(pjoin(source_dir, 'pyproject.toml')) as f:
         data = pytoml.load(f)
-    return Pep517HookCaller(source_dir, data['build-system']['build-backend'])
+    buildsys = data['build-system']
+    return Pep517HookCaller(
+        source_dir, buildsys['build-backend'],
+        bootstrap_backend_locn=buildsys.get('bootstrap-backend-location')
+    )
 
 
 def test_missing_backend_gives_exception():
@@ -103,3 +107,13 @@ def test_build_sdist_unsupported():
         with modified_env({'PYTHONPATH': BUILDSYS_PKGS}):
             with pytest.raises(UnsupportedOperation):
                 hooks.build_sdist(sdistdir, {'test_unsupported': True})
+
+
+def test_bootstrap_backend():
+    hooks = get_hooks('bootstrap_pkg')
+    with modified_env({'PYTHONPATH': ''}):
+        wheel_reqs = hooks.get_requires_for_build_wheel({})
+        assert wheel_reqs == ['wheel', 'sentinel']
+
+        sdist_reqs = hooks.get_requires_for_build_sdist({})
+        assert sdist_reqs == ['sdist_sentinel']