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

Doc/lib/libheapq.tex

@@ -83,6 +83,30 @@ True
 >>>
 \end{verbatim}
 
+The module also offers two general purpose functions based on heaps.
+
+\begin{funcdesc}{nlargest}{iterable, n}
+Return a list with the \var{n} largest elements from the dataset defined
+by \var{iterable}. Equivalent to:  \code{sorted(iterable, reverse=True)[:n]}
+\versionadded{2.4}              
+\end{funcdesc}
+
+\begin{funcdesc}{nsmallest}{iterable, n}
+Return a list with the \var{n} smallest elements from the dataset defined
+by \var{iterable}. Equivalent to:  \code{sorted(iterable)[:n]}
+\versionadded{2.4}              
+\end{funcdesc}
+
+Though the above functions appear symmetrical, they each have different
+speed and space requirements.  In particular, \function{nsmallest()}
+operates on a full copy of the dataset.  In contrast, \function{nlargest()}
+only requires storage space for \var{n} elements.
+
+Both functions perform best for smaller values of \var{n}.  For larger
+values, it is more efficient to use the \function{sorted()} function.  Also,
+when \code{n==1}, it is more efficient to use the builtin \function{min()}
+and \function{max()} functions.
+
 
 \subsection{Theory}
 

Doc/whatsnew/whatsnew24.tex

@@ -449,7 +449,10 @@ improved performance:  \module{Queue}, \module{mutex}, \module{shlex}
 
 \item The \module{heapq} module has been converted to C.  The resulting
    tenfold improvement in speed makes the module suitable for handling
-   high volumes of data.
+   high volumes of data.  In addition, the module has two new functions
+   \function{nlargest()} and \function{nsmallest()} that use heaps to
+   find the largest or smallest n values in a dataset without the
+   expense of a full sort.
 
 \item The \module{imaplib} module now supports IMAP's THREAD command.
 (Contributed by Yves Dionne.)

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.

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"

Misc/NEWS

@@ -416,7 +416,9 @@ Library
   os.path.exists(), switched to using os.lstat() directly if possible.
 
 - bisect.py and heapq.py now have underlying C implementations
-  for better performance
+  for better performance.
+
+- heapq.py has two new functions, nsmallest() and nlargest().                                                                  
 
 - traceback.format_exc has been added (similar to print_exc but it returns
   a string).