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.

changelog.d/1652.change.rst

-Added the ``build_meta_legacy`` backend, a "compatibility mode" PEP 517 backend that can be used as the default when ``build-backend`` is left unspecified in ``pyproject.toml``.
+Added the ``build_meta:legacy`` backend, a "compatibility mode" PEP 517 backend that can be used as the default when ``build-backend`` is left unspecified in ``pyproject.toml``.

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()

setuptools/build_meta_legacy.py

-"""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.
-"""
-
-import os
-import sys
-
-from setuptools.build_meta import _BuildMetaBackend
-from setuptools.build_meta import SetupRequirementsError
-
-
-__all__ = ['get_requires_for_build_sdist',
-           'get_requires_for_build_wheel',
-           'prepare_metadata_for_build_wheel',
-           'build_wheel',
-           'build_sdist',
-           'SetupRequirementsError']
-
-
-class _BuildMetaLegacyBackend(_BuildMetaBackend):
-    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
-
-
-_BACKEND = _BuildMetaLegacyBackend()
-
-get_requires_for_build_wheel = _BACKEND.get_requires_for_build_wheel
-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

setuptools/tests/test_build_meta.py

@@ -23,7 +23,6 @@ class BuildBackendBase:
         self.env = env
         self.backend_name = backend_name
 
-
 class BuildBackend(BuildBackendBase):
     """PEP 517 Build Backend"""
 
@@ -43,12 +42,24 @@ class BuildBackend(BuildBackendBase):
 
 
 class BuildBackendCaller(BuildBackendBase):
+    def __init__(self, *args, **kwargs):
+        super(BuildBackendCaller, self).__init__(*args, **kwargs)
+
+        (self.backend_name, _,
+         self.backend_obj) = self.backend_name.partition(':')
+
     def __call__(self, name, *args, **kw):
         """Handles aribrary function invocations on the build backend."""
         os.chdir(self.cwd)
         os.environ.update(self.env)
         mod = importlib.import_module(self.backend_name)
-        return getattr(mod, name)(*args, **kw)
+
+        if self.backend_obj:
+            backend = getattr(mod, self.backend_obj)
+        else:
+            backend = mod
+
+        return getattr(backend, name)(*args, **kw)
 
 
 defns = [
@@ -259,7 +270,7 @@ class TestBuildMetaBackend:
 
 
 class TestBuildMetaLegacyBackend(TestBuildMetaBackend):
-    backend_name = 'setuptools.build_meta_legacy'
+    backend_name = 'setuptools.build_meta:legacy'
 
     # build_meta_legacy-specific tests
     def test_build_sdist_relative_path_import(self, tmpdir_cwd):