bpo-43926: Cleaner metadata with PEP 566 JSON support. (GH-25565)

* bpo-43926: Cleaner metadata with PEP 566 JSON support.

* Add blurb

* Add versionchanged and versionadded declarations for changes to metadata.

* Use descriptor for PEP 566

1 files changed, 19 insertions, 0 deletions

diff --git a/Lib/importlib/metadata/_itertools.py b/Lib/importlib/metadata/_itertools.py
new file mode 100644
index 00000000000..dd45f2f0966
--- /dev/null
+++ b/Lib/importlib/metadata/_itertools.py
@@ -0,0 +1,19 @@
+from itertools import filterfalse
+
+
+def unique_everseen(iterable, key=None):
+    "List unique elements, preserving order. Remember all elements ever seen."
+    # unique_everseen('AAAABBBCCDAABBB') --> A B C D
+    # unique_everseen('ABBCcAD', str.lower) --> A B C D
+    seen = set()
+    seen_add = seen.add
+    if key is None:
+        for element in filterfalse(seen.__contains__, iterable):
+            seen_add(element)
+            yield element
+    else:
+        for element in iterable:
+            k = key(element)
+            if k not in seen:
+                seen_add(k)
+                yield element