Re-use unique_everseen from itertools recipes.

setuptools/command/build_py.py

@@ -6,10 +6,9 @@ import fnmatch
 import textwrap
 import io
 import distutils.errors
-import collections
 import itertools
 
-from setuptools.extern.six.moves import map, filter
+from setuptools.extern.six.moves import map, filter, filterfalse
 
 try:
     from setuptools.lib2to3_ex import Mixin2to3
@@ -204,14 +203,13 @@ class build_py(orig.build_py, Mixin2to3):
         # flatten the groups of matches into an iterable of matches
         matches = itertools.chain.from_iterable(match_groups)
         bad = set(matches)
-        seen = collections.defaultdict(itertools.count)
-        return [
+        keepers = (
             fn
             for fn in files
             if fn not in bad
-            # ditch dupes
-            and not next(seen[fn])
-        ]
+        )
+        # ditch dupes
+        return list(_unique_everseen(keepers))
 
     @staticmethod
     def _get_platform_patterns(spec, package, src_dir):
@@ -232,6 +230,25 @@ class build_py(orig.build_py, Mixin2to3):
         )
 
 
+# from Python docs
+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
+
+
 def assert_relative(path):
     if not os.path.isabs(path):
         return path