Add an async function cache with a LIFO policy

Relates to #59

piker/_async_utils.py

@@ -0,0 +1,33 @@
+"""
+Async utils no one seems to have built into a core lib (yet).
+"""
+from collections import OrderedDict
+
+
+def alifo_cache(maxsize=128):
+    """Async ``cache`` with a LIFO policy.
+
+    Implemented my own since no one else seems to have
+    a standard. I'll wait for the smarter people to come
+    up with one, but until then...
+    """
+    cache = OrderedDict()
+
+    def decorator(fn):
+
+        async def wrapper(*args):
+            key = args
+            try:
+                return cache[key]
+            except KeyError:
+                if len(cache) >= maxsize:
+                    # discard last added new entry
+                    cache.popitem()
+
+                # do it
+                cache[key] = await fn(*args)
+                return cache[key]
+
+        return wrapper
+
+    return decorator