SF patch #969791: Add nlargest() and nsmallest() to heapq.

2 files changed, 44 insertions, 3 deletions

diff --git a/Lib/heapq.py b/Lib/heapq.py
index 3eb69d8274..d1aad98a24 100644
--- a/Lib/heapq.py
+++ b/Lib/heapq.py
@@ -30,7 +30,7 @@ without surprises: heap[0] is the smallest item, and heap.sort()
 maintains the heap invariant!
 """
 
-# Original code by Kevin O'Connor, augmented by Tim Peters
+# Original code by Kevin O'Connor, augmented by Tim Peters and Raymond Hettinger
 
 __about__ = """Heap queues
 
@@ -126,7 +126,10 @@ Believe me, real good tape sorts were quite spectacular to watch!
 From all times, sorting has always been a Great Art! :-)
 """
 
-__all__ = ['heappush', 'heappop', 'heapify', 'heapreplace']
+__all__ = ['heappush', 'heappop', 'heapify', 'heapreplace', 'nlargest',
+           'nsmallest']
+
+from itertools import islice, repeat
 
 def heappush(heap, item):
     """Push item onto heap, maintaining the heap invariant."""
@@ -168,6 +171,35 @@ def heapify(x):
     for i in reversed(xrange(n//2)):
         _siftup(x, i)
 
+def nlargest(iterable, n):
+    """Find the n largest elements in a dataset.
+
+    Equivalent to:  sorted(iterable, reverse=True)[:n]
+    """
+    it = iter(iterable)
+    result = list(islice(it, n))
+    if not result:
+        return result
+    heapify(result)
+    _heapreplace = heapreplace
+    sol = result[0]         # sol --> smallest of the nlargest
+    for elem in it:
+        if elem <= sol:
+            continue
+        _heapreplace(result, elem)
+        sol = result[0]
+    result.sort(reverse=True)
+    return result
+
+def nsmallest(iterable, n):
+    """Find the n smallest elements in a dataset.
+
+    Equivalent to:  sorted(iterable)[:n]
+    """
+    h = list(iterable)
+    heapify(h)
+    return map(heappop, repeat(h, min(n, len(h))))
+
 # 'heap' is a heap at all indices >= startpos, except possibly for pos.  pos
 # is the index of a leaf with a possibly out-of-order value.  Restore the
 # heap invariant.
diff --git a/Lib/test/test_heapq.py b/Lib/test/test_heapq.py
index 8f3c6f9ac9..f04ea9452f 100644
--- a/Lib/test/test_heapq.py
+++ b/Lib/test/test_heapq.py
@@ -2,7 +2,7 @@
 
 from test.test_support import verify, vereq, verbose, TestFailed
 
-from heapq import heappush, heappop, heapify, heapreplace
+from heapq import heappush, heappop, heapify, heapreplace, nlargest, nsmallest
 import random
 
 def check_invariant(heap):
@@ -84,6 +84,15 @@ def test_main():
         data.sort()
         sorted = [heappop(heap) for i in range(size)]
         vereq(data, sorted)
+
+    # 7) Check nlargest() and nsmallest()
+    data = [random.randrange(2000) for i in range(1000)]
+    copy = data[:]
+    copy.sort(reverse=True)
+    vereq(nlargest(data, 400), copy[:400])
+    copy.sort()
+    vereq(nsmallest(data, 400), copy[:400])
+
     # Make user happy
     if verbose:
         print "All OK"