BUG: fix unsafe dtype conversion in Series

pandas/core/common.py

@@ -760,12 +760,27 @@ def _maybe_upcast_putmask(result, mask, other, dtype=None, change=None):
 
         def changeit():
 
-            # our type is wrong here, need to upcast
+            # try to directly set by expanding our array to full
+            # length of the boolean
+            om = other[mask]
+            om_at = om.astype(result.dtype)
+            if (om == om_at).all():
+                new_other = result.values.copy()
+                new_other[mask] = om_at
+                result[:] = new_other
+                return result, False
+
+            # we are forced to change the dtype of the result as the input isn't compatible
             r, fill_value = _maybe_upcast(result, fill_value=other, dtype=dtype, copy=True)
             np.putmask(r, mask, other)
 
             # we need to actually change the dtype here
             if change is not None:
+
+                # if we are trying to do something unsafe
+                # like put a bigger dtype in a smaller one, use the smaller one
+                if change.dtype.itemsize < r.dtype.itemsize:
+                    raise Exception("cannot change dtype of input to smaller size")
                 change.dtype = r.dtype
                 change[:] = r
 

pandas/core/series.py

@@ -739,7 +739,13 @@ def where(self, cond, other=nan, inplace=False):
         if isinstance(other, Series):
             other = other.reindex(ser.index)
         elif isinstance(other, (tuple,list)):
-            other = np.array(other)
+
+            # try to set the same dtype as ourselves
+            new_other = np.array(other,dtype=self.dtype)
+            if not (new_other == np.array(other)).all():
+                other = np.array(other)
+            else:
+                other = new_other
 
         if len(other) != len(ser):
             icond = ~cond

pandas/tests/test_series.py

@@ -1127,17 +1127,44 @@ def test_where(self):
         self.assertRaises(ValueError, s.__setitem__, tuple([[[True, False]]]), [0,2,3])
         self.assertRaises(ValueError, s.__setitem__, tuple([[[True, False]]]), [])
 
+        # unsafe dtype changes
+        for dtype in [ np.int8, np.int16, np.int32, np.int64, np.float16, np.float32, np.float64 ]:
+            s = Series(np.arange(10), dtype=dtype)
+            mask = s < 5
+            s[mask] = range(2,7)
+            expected = Series(range(2,7) + range(5,10), dtype=dtype)
+            assert_series_equal(s, expected)
+            self.assertEquals(s.dtype, expected.dtype)
+
+        # these are allowed operations, but are upcasted
+        for dtype in [ np.int64, np.float64 ]:
+            s = Series(np.arange(10), dtype=dtype)
+            mask = s < 5
+            values = [2.5,3.5,4.5,5.5,6.5]
+            s[mask] = values 
+            expected = Series(values + range(5,10), dtype='float64')
+            assert_series_equal(s, expected)
+            self.assertEquals(s.dtype, expected.dtype)
+
+        # can't do these as we are forced to change the itemsize of the input to something we cannot
+        for dtype in [ np.int8, np.int16, np.int32, np.float16, np.float32 ]:
+            s = Series(np.arange(10), dtype=dtype)
+            mask = s < 5
+            values = [2.5,3.5,4.5,5.5,6.5]
+            self.assertRaises(Exception, s.__setitem__, tuple(mask), values)
+
         # GH3235
         s = Series(np.arange(10))
         mask = s < 5
-        s[mask] = range(5)
-        expected = Series(np.arange(10),dtype='float64')
-        assert_series_equal(s,expected)
+        s[mask] = range(2,7)
+        expected = Series(range(2,7) + range(5,10))
+        assert_series_equal(s, expected)
+        self.assertEquals(s.dtype, expected.dtype)
 
         s = Series(np.arange(10))
         mask = s > 5
         s[mask] = [0]*4
-        expected = Series([0,1,2,3,4,5] + [0]*4,dtype='float64')
+        expected = Series([0,1,2,3,4,5] + [0]*4)
         assert_series_equal(s,expected)
 
         s = Series(np.arange(10))
@@ -3165,7 +3192,7 @@ def test_cast_on_putmask(self):
         # need to upcast
         s = Series([1,2],index=[1,2],dtype='int64')
         s[[True, False]] = Series([0],index=[1],dtype='int64')
-        expected = Series([0,2],index=[1,2],dtype='float64')
+        expected = Series([0,2],index=[1,2],dtype='int64')
 
         assert_series_equal(s, expected)