[Python-Dev] adding key argument to min and max

[Raymond Hettinger]

> I don't want to put words into your mouth, so is this a vote against a
> key= argument for min and max?

Right.  I don't think there is any need.

 
> If nsmallest/nlargest get key= arguments, this would definitely cover
> the same cases. 

Right.


> If a key= argument gets vetoed for min and max, I'd
> at least like to add a bit of documentation pointing users of min/max
> to nsmallest/nlargest if they want a key= argument...

Sounds reasonable.



Raymond


P.S.  In case you're interested, here is the patch:


Index: heapq.py
===================================================================
RCS file: /cvsroot/python/python/dist/src/Lib/heapq.py,v
retrieving revision 1.27
diff -u -r1.27 heapq.py
--- heapq.py	29 Nov 2004 05:54:47 -0000	1.27
+++ heapq.py	2 Dec 2004 01:32:44 -0000
@@ -307,6 +307,31 @@
 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)
+    it = ((key(r), i, r) for i, r in enumerate(iterable))   # decorate
+    result = _nsmallest(n, it)
+    return [r for k, i, r in 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)
+    it = ((key(r), i, r) for i, r in enumerate(iterable))   # decorate
+    result = _nlargest(n, it)
+    return [r for k, i, r in result]                        #
undecorate
+
 if __name__ == "__main__":
     # Simple sanity test
     heap = []
Index: test/test_heapq.py
===================================================================
RCS file: /cvsroot/python/python/dist/src/Lib/test/test_heapq.py,v
retrieving revision 1.16
diff -u -r1.16 test_heapq.py
--- test/test_heapq.py	29 Nov 2004 05:54:48 -0000	1.16
+++ test/test_heapq.py	2 Dec 2004 01:32:44 -0000
@@ -105,13 +105,19 @@
 
     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):
         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])
 
 
 
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