Add test capturing expectation for #250.

setuptools/tests/test_develop.py

@@ -114,3 +114,76 @@ def test_console_scripts(self, tmpdir):
         cmd.install_dir = tmpdir
         cmd.run()
         # assert '0.0' not in foocmd_text
+
+
+import textwrap
+import subprocess
+
+
+class TestNamespaces:
+    @staticmethod
+    def build_namespace_package(tmpdir, name):
+        src_dir = tmpdir / name
+        src_dir.mkdir()
+        setup_py = src_dir / 'setup.py'
+        namespace, sep, rest = name.partition('.')
+        script = textwrap.dedent("""
+            import setuptools
+            setuptools.setup(
+                name={name!r},
+                version="1.0",
+                namespace_packages=[{namespace!r}],
+                packages=[{namespace!r}],
+            )
+            """).format(**locals())
+        setup_py.write_text(script, encoding='utf-8')
+        ns_pkg_dir = src_dir / namespace
+        ns_pkg_dir.mkdir()
+        pkg_init = ns_pkg_dir / '__init__.py'
+        tmpl = '__import__("pkg_resources").declare_namespace({namespace!r})'
+        decl = tmpl.format(**locals())
+        pkg_init.write_text(decl, encoding='utf-8')
+        pkg_mod = ns_pkg_dir / (rest + '.py')
+        some_functionality = 'name = {rest!r}'.format(**locals())
+        pkg_mod.write_text(some_functionality, encoding='utf-8')
+        return src_dir
+
+    @staticmethod
+    def make_site_dir(target):
+        """
+        Add a sitecustomize.py module in target to cause
+        target to be added to site dirs such that .pth files
+        are processed there.
+        """
+        sc = target / 'sitecustomize.py'
+        target_str = str(target)
+        tmpl = '__import__("site").addsitedir({target_str!r})'
+        sc.write_text(tmpl.format(**locals()), encoding='utf-8')
+
+    def test_namespace_package_importable(self, tmpdir):
+        """
+        Installing two packages sharing the same namespace, one installed
+        naturally using pip or `--single-version-externally-managed`
+        and the other installed using `develop` should leave the namespace
+        in tact and both packages reachable by import.
+        """
+        pkg_A = self.build_namespace_package(tmpdir, 'myns.pkgA')
+        pkg_B = self.build_namespace_package(tmpdir, 'myns.pkgB')
+        target = tmpdir / 'packages'
+        # use pip to install to the target directory
+        install_cmd = [
+            'pip',
+            'install',
+            '-e', str(pkg_B),
+            str(pkg_A),
+            '-t', str(target),
+        ]
+        subprocess.check_call(install_cmd)
+        self.make_site_dir(target)
+        try_import = [
+            sys.executable,
+            '-c', 'import myns.pkgA; import myns.pkgB',
+        ]
+        env = dict(PYTHONPATH=str(target))
+        subprocess.check_call(try_import, env=env)
+