added back mask method that does condition inversion

added condition testing to where that raised ValueError on an invalid condition (e.g. not an ndarray like object)
added tests for same

pandas/core/frame.py

@@ -4882,6 +4882,9 @@ def where(self, cond, other=NA, inplace=False):
         -------
         wh: DataFrame
         """
+        if not hasattr(cond,'shape'):
+            raise ValueError('where requires an ndarray like object for its condition')
+
         if isinstance(cond, np.ndarray):
             if cond.shape != self.shape:
                 raise ValueError('Array onditional must be same shape as self')
@@ -4901,6 +4904,21 @@ def where(self, cond, other=NA, inplace=False):
         rs = np.where(cond, self, other)
         return self._constructor(rs, self.index, self.columns)
 
+    def mask(self, cond):
+        """
+        Returns copy of self whose values are replaced with nan if the
+        inverted condition is True
+
+        Parameters
+        ----------
+        cond: boolean DataFrame or array
+
+        Returns
+        -------
+        wh: DataFrame
+        """
+        return self.where(~cond, NA)
+
 _EMPTY_SERIES = Series([])
 
 

pandas/tests/test_frame.py

@@ -5220,6 +5220,18 @@ def test_where(self):
         err2 = cond.ix[:2, :].values
         self.assertRaises(ValueError, df.where, err2, other1)
 
+        # invalid conditions
+        self.assertRaises(ValueError, df.mask, True)
+        self.assertRaises(ValueError, df.mask, 0)
+
+    def test_mask(self):
+        df = DataFrame(np.random.randn(5, 3))
+        cond = df > 0
+
+        rs = df.where(cond, np.nan)
+        assert_frame_equal(rs, df.mask(df <= 0))
+        assert_frame_equal(rs, df.mask(~cond))
+
 
     #----------------------------------------------------------------------
     # Transposing