Fix min/max functions with generators, and 'None' default

src/future/builtins/new_min_max.py

@@ -1,9 +1,13 @@
+import itertools
+
 from future import utils
 if utils.PY2:
     from __builtin__ import max as _builtin_max, min as _builtin_min
 else:
     from builtins import max as _builtin_max, min as _builtin_min
 
+_SENTINEL = object()
+
 
 def newmin(*args, **kwargs):
     return new_min_max(_builtin_min, *args, **kwargs)
@@ -29,21 +33,24 @@ def new_min_max(_builtin_func, *args, **kwargs):
     if len(args) == 0:
         raise TypeError
 
-    if len(args) != 1 and kwargs.get('default') is not None:
+    if len(args) != 1 and kwargs.get('default', _SENTINEL) is not _SENTINEL:
         raise TypeError
 
     if len(args) == 1:
+        iterator = iter(args[0])
         try:
-            next(iter(args[0]))
+            first = next(iterator)
         except StopIteration:
-            if kwargs.get('default') is not None:
+            if kwargs.get('default', _SENTINEL) is not _SENTINEL:
                 return kwargs.get('default')
             else:
-                raise ValueError('iterable is an empty sequence')
+                raise ValueError('{}() arg is an empty sequence'.format(_builtin_func.__name__))
+        else:
+            iterator = itertools.chain([first], iterator)
         if kwargs.get('key') is not None:
-            return _builtin_func(args[0], key=kwargs.get('key'))
+            return _builtin_func(iterator, key=kwargs.get('key'))
         else:
-            return _builtin_func(args[0])
+            return _builtin_func(iterator)
 
     if len(args) > 1:
         if kwargs.get('key') is not None:

tests/test_future/test_builtins.py

@@ -1105,6 +1105,7 @@ def test_max(self):
         with self.assertRaises(TypeError):
             max(1, 2, default=0)
         self.assertEqual(max([], default=0), 0)
+        self.assertIs(max([], default=None), None)
 
     def test_min(self):
         self.assertEqual(min('123123'), '1')
@@ -1123,6 +1124,7 @@ class BadSeq:
             def __getitem__(self, index):
                 raise ValueError
         self.assertRaises(ValueError, min, BadSeq())
+        self.assertEqual(max(x for x in [5, 4, 3]), 5)
 
         for stmt in (
             "min(key=int)",                 # no args
@@ -1149,11 +1151,15 @@ def __getitem__(self, index):
                          sorted(data, key=f)[0])
         self.assertEqual(min([], default=5), 5)
         self.assertEqual(min([], default=0), 0)
+        self.assertIs(min([], default=None), None)
         with self.assertRaises(TypeError):
             max(None, default=5)
         with self.assertRaises(TypeError):
             max(1, 2, default=0)
 
+        # Test iterables that can only be looped once #510
+        self.assertEqual(min(x for x in [5]), 5)
+
     def test_next(self):
         it = iter(range(2))
         self.assertEqual(next(it), 0)