Create a PEP 302 importer for managing conditional import of vendored packages from the 'extern' namespace. This technique avoids the use of 'imp' and works even when setuptools is installed as a zipped egg. Ref #229.

--HG--
branch : feature/issue-229

1 files changed, 42 insertions, 0 deletions

diff --git a/setuptools/extern/__init__.py b/setuptools/extern/__init__.py
index e69de29b..803d9e9a 100644
--- a/setuptools/extern/__init__.py
+++ b/setuptools/extern/__init__.py
@@ -0,0 +1,42 @@
+import sys
+
+_VENDORED_NAMES = 'six',
+_SEARCH_PATH = 'setuptools._vendor.', ''
+
+class VendorImporter:
+    """
+    A PEP 302 meta path importer for finding optionally-vendored
+    or otherwise naturally-installed packages from __name__.
+    """
+    def find_module(self, fullname, path=None):
+        root, base, target = fullname.partition(__name__ + '.')
+        if root:
+            return
+        if not any(map(target.startswith, _VENDORED_NAMES)):
+            return
+        return self
+
+    def load_module(self, fullname):
+        root, base, target = fullname.partition(__name__ + '.')
+        for prefix in _SEARCH_PATH:
+            try:
+                __import__(prefix + target)
+                mod = sys.modules[prefix + target]
+                sys.modules[fullname] = mod
+                return mod
+            except ImportError:
+                pass
+        else:
+            raise ImportError(
+                "The '{target}' package is required; "
+                "normally this is bundled with this package so if you get "
+                "this warning, consult the packager of your "
+                "distribution.".format(**locals())
+            )
+
+    @classmethod
+    def install(cls):
+        if not any(isinstance(imp, cls) for imp in sys.meta_path):
+            sys.meta_path.append(cls())
+
+VendorImporter.install()