move Python < 2.6 fallback into _markerlib

--HG--
branch : distribute
extra : rebase_source : 901265cef35594b3ab20ae4ba3b270a5da6f9ea7

1 files changed, 16 insertions, 2 deletions

diff --git a/_markerlib/__init__.py b/_markerlib/__init__.py
index a7b26037..d13e4d5a 100644
--- a/_markerlib/__init__.py
+++ b/_markerlib/__init__.py
@@ -1,2 +1,16 @@
-"""Used by pkg_resources to interpret PEP 345 environment markers."""
-from _markerlib.markers import default_environment, compile, interpret, as_function
+try:
+    import ast
+    from markerlib.markers import default_environment, compile, interpret
+except ImportError:
+    if 'ast' in globals():
+        raise
+    def default_environment():
+        return {}
+    def compile(marker):
+        def marker_fn(environment=None, override=None):
+            # 'empty markers are True' heuristic won't install extra deps.
+            return not marker.strip()
+        marker_fn.__doc__ = marker
+        return marker_fn
+    def interpret(marker, environment=None, override=None):
+        return compile(marker)()