Reduce import time overhead

* Fix subclass traversals to not run classes multiple times

* switch compiler visitor to use an attrgetter, to avoid
  an eval() at startup time

* don't pre-generate traversal functions, there's lots of these
  which are expensive to generate at once and most applications
  won't use them all; have it generate them on first use instead

* Some ideas about removing asyncio imports, they don't seem to
  be too signficant, apply some more simplicity to the overall
  "greenlet fallback" situation

Fixes: #5681
Change-Id: Ib564ddaddb374787ce3e11ff48026e99ed570933

1 files changed, 31 insertions, 16 deletions

diff --git a/lib/sqlalchemy/util/concurrency.py b/lib/sqlalchemy/util/concurrency.py
index e0883aa68..f78c0971c 100644
--- a/lib/sqlalchemy/util/concurrency.py
+++ b/lib/sqlalchemy/util/concurrency.py
@@ -1,25 +1,40 @@
 from . import compat
 
+have_greenlet = False
 
 if compat.py3k:
-    import asyncio
-    from ._concurrency_py3k import await_only
-    from ._concurrency_py3k import await_fallback
-    from ._concurrency_py3k import greenlet
-    from ._concurrency_py3k import greenlet_spawn
-    from ._concurrency_py3k import AsyncAdaptedLock
-else:
-    asyncio = None
-    greenlet = None
-
-    def await_only(thing):
+    try:
+        import greenlet  # noqa F401
+    except ImportError:
+        pass
+    else:
+        have_greenlet = True
+        from ._concurrency_py3k import await_only
+        from ._concurrency_py3k import await_fallback
+        from ._concurrency_py3k import greenlet_spawn
+        from ._concurrency_py3k import AsyncAdaptedLock
+        from ._concurrency_py3k import asyncio  # noqa F401
+
+if not have_greenlet:
+
+    asyncio = None  # noqa F811
+
+    def _not_implemented():
+        if not compat.py3k:
+            raise ValueError("Cannot use this function in py2.")
+        else:
+            raise ValueError(
+                "the greenlet library is required to use this function."
+            )
+
+    def await_only(thing):  # noqa F811
         return thing
 
-    def await_fallback(thing):
+    def await_fallback(thing):  # noqa F81
         return thing
 
-    def greenlet_spawn(fn, *args, **kw):
-        raise ValueError("Cannot use this function in py2.")
+    def greenlet_spawn(fn, *args, **kw):  # noqa F81
+        _not_implemented()
 
-    def AsyncAdaptedLock(*args, **kw):
-        raise ValueError("Cannot use this function in py2.")
+    def AsyncAdaptedLock(*args, **kw):  # noqa F81
+        _not_implemented()