Add key= argument to heapq.nsmallest() and heapq.nlargest().

2 files changed, 36 insertions, 2 deletions

diff --git a/Lib/heapq.py b/Lib/heapq.py
index b4ebb91..04725cd 100644
--- a/Lib/heapq.py
+++ b/Lib/heapq.py
@@ -129,7 +129,8 @@ From all times, sorting has always been a Great Art! :-)
 __all__ = ['heappush', 'heappop', 'heapify', 'heapreplace', 'nlargest',
            'nsmallest']
 
-from itertools import islice, repeat
+from itertools import islice, repeat, count, imap, izip, tee
+from operator import itemgetter
 import bisect
 
 def heappush(heap, item):
@@ -307,6 +308,33 @@ try:
 except ImportError:
     pass
 
+# Extend the implementations of nsmallest and nlargest to use a key= argument
+_nsmallest = nsmallest
+def nsmallest(n, iterable, key=None):
+    """Find the n smallest elements in a dataset.
+
+    Equivalent to:  sorted(iterable, key=key)[:n]
+    """
+    if key is None:
+        return _nsmallest(n, iterable)
+    in1, in2 = tee(iterable)
+    it = izip(imap(key, in1), count(), in2)                 # decorate
+    result = _nsmallest(n, it)
+    return map(itemgetter(2), result)                       # undecorate
+
+_nlargest = nlargest
+def nlargest(n, iterable, key=None):
+    """Find the n largest elements in a dataset.
+
+    Equivalent to:  sorted(iterable, key=key, reverse=True)[:n]
+    """
+    if key is None:
+        return _nlargest(n, iterable)
+    in1, in2 = tee(iterable)
+    it = izip(imap(key, in1), count(), in2)                 # decorate
+    result = _nlargest(n, it)
+    return map(itemgetter(2), result)                       # undecorate
+
 if __name__ == "__main__":
     # Simple sanity test
     heap = []
diff --git a/Lib/test/test_heapq.py b/Lib/test/test_heapq.py
index 68003e7..2da4f8c 100644
--- a/Lib/test/test_heapq.py
+++ b/Lib/test/test_heapq.py
@@ -105,13 +105,19 @@ class TestHeap(unittest.TestCase):
 
     def test_nsmallest(self):
         data = [random.randrange(2000) for i in range(1000)]
+        f = lambda x:  x * 547 % 2000
         for n in (0, 1, 2, 10, 100, 400, 999, 1000, 1100):
             self.assertEqual(nsmallest(n, data), sorted(data)[:n])
+            self.assertEqual(nsmallest(n, data, key=f),
+                             sorted(data, key=f)[:n])
 
-    def test_largest(self):
+    def test_nlargest(self):
         data = [random.randrange(2000) for i in range(1000)]
+        f = lambda x:  x * 547 % 2000
         for n in (0, 1, 2, 10, 100, 400, 999, 1000, 1100):
             self.assertEqual(nlargest(n, data), sorted(data, reverse=True)[:n])
+            self.assertEqual(nlargest(n, data, key=f),
+                             sorted(data, key=f, reverse=True)[:n])
 
 
 #==============================================================================