ENH: DataFrame where and mask #2109

pandas/core/frame.py

@@ -4832,6 +4832,49 @@ def combineMult(self, other):
         """
         return self.mul(other, fill_value=1.)
 
+    def where(self, cond, other):
+        """
+        Return a DataFrame with the same shape as self and whose corresponding
+        entries are from self where cond is True and otherwise are from other.
+
+
+        Parameters
+        ----------
+        cond: boolean DataFrame or array
+        other: scalar or DataFrame
+
+        Returns
+        -------
+        wh: DataFrame
+        """
+        if isinstance(cond, np.ndarray):
+            if cond.shape != self.shape:
+                raise ValueError('Array onditional must be same shape as self')
+            cond = self._constructor(cond, index=self.index, columns=self.columns)
+        if cond.shape != self.shape:
+            cond = cond.reindex(self.index, columns=self.columns)
+            cond = cond.fillna(False)
+
+        if isinstance(other, DataFrame):
+            _, other = self.align(other, join='left', fill_value=np.nan)
+
+        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
+        corresponding entry in cond is False
+
+        Parameters
+        ----------
+        cond: boolean DataFrame or array
+
+        Returns
+        -------
+        wh: DataFrame
+        """
+        return self.where(cond, np.nan)
 
 _EMPTY_SERIES = Series([])
 

pandas/tests/test_frame.py

@@ -5063,6 +5063,34 @@ def test_align_int_fill_bug(self):
         expected = df2 - df2.mean()
         assert_frame_equal(result, expected)
 
+    def test_where(self):
+        df = DataFrame(np.random.randn(5, 3))
+        cond = df > 0
+
+        other1 = df + 1
+        rs = df.where(cond, other1)
+        for k, v in rs.iteritems():
+            assert_series_equal(v, np.where(cond[k], df[k], other1[k]))
+
+        other2 = (df + 1).values
+        rs = df.where(cond, other2)
+        for k, v in rs.iteritems():
+            assert_series_equal(v, np.where(cond[k], df[k], other2[:, k]))
+
+        other5 = np.nan
+        rs = df.where(cond, other5)
+        for k, v in rs.iteritems():
+            assert_series_equal(v, np.where(cond[k], df[k], other5))
+
+        assert_frame_equal(rs, df.mask(cond))
+
+        err1 = (df + 1).values[0:2, :]
+        self.assertRaises(ValueError, df.where, cond, err1)
+
+        err2 = cond.ix[:2, :].values
+        self.assertRaises(ValueError, df.where, err2, other1)
+
+
     #----------------------------------------------------------------------
     # Transposing