Move build_meta_legacy to build_meta:legacy

Rather than exposing a top-level module for the legacy backend, we will
move the legacy backend into the `setuptools.build_meta` module and
specify it using the module:object syntax.

1 files changed, 35 insertions, 0 deletions

diff --git a/setuptools/build_meta.py b/setuptools/build_meta.py
index 8e31a04d..e16f319e 100644
--- a/setuptools/build_meta.py
+++ b/setuptools/build_meta.py
@@ -40,6 +40,7 @@ __all__ = ['get_requires_for_build_sdist',
            'prepare_metadata_for_build_wheel',
            'build_wheel',
            'build_sdist',
+           'legacy',
            'SetupRequirementsError']
 
 class SetupRequirementsError(BaseException):
@@ -187,6 +188,36 @@ class _BuildMetaBackend(object):
         return _file_with_extension(sdist_directory, '.tar.gz')
 
 
+class _BuildMetaLegacyBackend(_BuildMetaBackend):
+    """Compatibility backend for setuptools
+
+    This is a version of setuptools.build_meta that endeavors to maintain backwards
+    compatibility with pre-PEP 517 modes of invocation. It exists as a temporary
+    bridge between the old packaging mechanism and the new packaging mechanism,
+    and will eventually be removed.
+    """
+    def run_setup(self, setup_script='setup.py'):
+        # In order to maintain compatibility with scripts assuming that
+        # the setup.py script is in a directory on the PYTHONPATH, inject
+        # '' into sys.path. (pypa/setuptools#1642)
+        sys_path = list(sys.path)           # Save the original path
+
+        script_dir = os.path.dirname(os.path.abspath(setup_script))
+        if script_dir not in sys.path:
+            sys.path.insert(0, script_dir)
+
+        try:
+            super(_BuildMetaLegacyBackend,
+                  self).run_setup(setup_script=setup_script)
+        finally:
+            # While PEP 517 frontends should be calling each hook in a fresh
+            # subprocess according to the standard (and thus it should not be
+            # strictly necessary to restore the old sys.path), we'll restore
+            # the original path so that the path manipulation does not persist
+            # within the hook after run_setup is called.
+            sys.path[:] = sys_path
+
+# The primary backend
 _BACKEND = _BuildMetaBackend()
 
 get_requires_for_build_wheel = _BACKEND.get_requires_for_build_wheel
@@ -194,3 +225,7 @@ get_requires_for_build_sdist = _BACKEND.get_requires_for_build_sdist
 prepare_metadata_for_build_wheel = _BACKEND.prepare_metadata_for_build_wheel
 build_wheel = _BACKEND.build_wheel
 build_sdist = _BACKEND.build_sdist
+
+
+# The legacy backend
+legacy = _BuildMetaLegacyBackend()