BUG: Fix for passing multiple ints as levels in DataFrame.stack() (#7660)

doc/source/reshaping.rst

@@ -160,10 +160,34 @@ the level numbers:
 
    stacked.unstack('second')
 
+.. _reshaping.stack_multiple:
+
 You may also stack or unstack more than one level at a time by passing a list
 of levels, in which case the end result is as if each level in the list were
 processed individually.
 
+.. ipython:: python
+
+    columns = MultiIndex.from_tuples([
+            ('A', 'cat', 'long'), ('B', 'cat', 'long'),
+            ('A', 'dog', 'short'), ('B', 'dog', 'short')
+        ],
+        names=['exp', 'animal', 'hair_length']
+    )
+    df = DataFrame(randn(4, 4), columns=columns)
+    df
+
+    df.stack(level=['animal', 'hair_length'])
+
+The list of levels can contain either level names or level numbers (but
+not a mixture of the two).
+
+.. ipython:: python
+
+    # df.stack(level=['animal', 'hair_length'])
+    # from above is equivalent to:
+    df.stack(level=[1, 2])
+
 These functions are intelligent about handling missing data and do not expect
 each subgroup within the hierarchical index to have the same set of labels.
 They also can handle the index being unsorted (but you can make it sorted by

doc/source/v0.15.0.txt

@@ -30,6 +30,11 @@ users upgrade to this version.
 API changes
 ~~~~~~~~~~~
 
+- Passing multiple levels to `DataFrame.stack()` will now work when multiple level
+  numbers are passed (:issue:`7660`), and will raise a ``ValueError`` when the
+  levels aren't all level names or all level numbers. See
+  :ref:`Reshaping by stacking and unstacking <reshaping.stack_multiple>`.
+
 .. _whatsnew_0150.cat:
 
 Categoricals in Series/DataFrame

pandas/core/frame.py

@@ -3311,13 +3311,10 @@ def stack(self, level=-1, dropna=True):
         -------
         stacked : DataFrame or Series
         """
-        from pandas.core.reshape import stack
+        from pandas.core.reshape import stack, stack_multiple
 
         if isinstance(level, (tuple, list)):
-            result = self
-            for lev in level:
-                result = stack(result, lev, dropna=dropna)
-            return result
+            return stack_multiple(self, level, dropna=dropna)
         else:
             return stack(self, level, dropna=dropna)
 

pandas/core/index.py

@@ -2490,6 +2490,12 @@ def _get_level_number(self, level):
                 raise KeyError('Level %s not found' % str(level))
             elif level < 0:
                 level += self.nlevels
+                if level < 0:
+                    orig_level = level - self.nlevels
+                    raise IndexError(
+                        'Too many levels: Index has only %d levels, '
+                        '%d is not a valid level number' % (self.nlevels, orig_level)
+                    )
             # Note: levels are zero-based
             elif level >= self.nlevels:
                 raise IndexError('Too many levels: Index has only %d levels, '

pandas/core/reshape.py

@@ -513,9 +513,7 @@ def stack(frame, level=-1, dropna=True):
                    "names are not unique.".format(level))
             raise ValueError(msg)
 
-    if isinstance(level, int) and level < 0:
-        level += frame.columns.nlevels
-
+    # Will also convert negative level numbers and check if out of bounds.
     level = frame.columns._get_level_number(level)
 
     if isinstance(frame.columns, MultiIndex):
@@ -547,6 +545,45 @@ def stack(frame, level=-1, dropna=True):
     return Series(new_values, index=new_index)
 
 
+def stack_multiple(frame, level, dropna=True):
+    # If all passed levels match up to column names, no
+    # ambiguity about what to do
+    if all(lev in frame.columns.names for lev in level):
+        result = frame
+        for lev in level:
+            result = stack(result, lev, dropna=dropna)
+
+    # Otherwise, level numbers may change as each successive level is stacked
+    elif all(isinstance(lev, int) for lev in level):
+        # As each stack is done, the level numbers decrease, so we need
+        #  to account for that when level is a sequence of ints
+        result = frame
+        # _get_level_number() checks level numbers are in range and converts
+        # negative numbers to positive
+        level = [frame.columns._get_level_number(lev) for lev in level]
+
+        # Can't iterate directly through level as we might need to change
+        # values as we go
+        for index in range(len(level)):
+            lev = level[index]
+            result = stack(result, lev, dropna=dropna)
+            # Decrement all level numbers greater than current, as these
+            # have now shifted down by one
+            updated_level = []
+            for other in level:
+                if other > lev:
+                    updated_level.append(other - 1)
+                else:
+                    updated_level.append(other)
+            level = updated_level
+
+    else:
+        raise ValueError("level should contain all level names or all level numbers, "
+                         "not a mixture of the two.")
+
+    return result
+
+
 def _stack_multi_columns(frame, level=-1, dropna=True):
     this = frame.copy()
 

pandas/tests/test_frame.py

@@ -11725,6 +11725,29 @@ def test_stack_unstack(self):
         assert_frame_equal(unstacked_cols.T, self.frame)
         assert_frame_equal(unstacked_cols_df['bar'].T, self.frame)
 
+    def test_stack_ints(self):
+        df = DataFrame(
+             np.random.randn(30, 27),
+             columns=MultiIndex.from_tuples(
+                list(itertools.product(range(3), repeat=3))
+            )
+        )
+        assert_frame_equal(
+            df.stack(level=[1, 2]),
+            df.stack(level=1).stack(level=1)
+        )
+        assert_frame_equal(
+            df.stack(level=[-2, -1]),
+            df.stack(level=1).stack(level=1)
+        )
+
+        df_named = df.copy()
+        df_named.columns.set_names(range(3), inplace=True)
+        assert_frame_equal(
+            df_named.stack(level=[1, 2]),
+            df_named.stack(level=1).stack(level=1)
+        )
+
     def test_unstack_bool(self):
         df = DataFrame([False, False],
                        index=MultiIndex.from_arrays([['a', 'b'], ['c', 'l']]),

pandas/tests/test_multilevel.py

@@ -834,6 +834,12 @@ def test_count_level_corner(self):
                              columns=df.columns).fillna(0).astype(np.int64)
         assert_frame_equal(result, expected)
 
+    def test_get_level_number_out_of_bounds(self):
+        with assertRaisesRegexp(IndexError, "Too many levels"):
+            self.frame.index._get_level_number(2)
+        with assertRaisesRegexp(IndexError, "not a valid level number"):
+            self.frame.index._get_level_number(-3)
+
     def test_unstack(self):
         # just check that it works for now
         unstacked = self.ymd.unstack()
@@ -1005,6 +1011,22 @@ def test_stack_unstack_multiple(self):
         expected = self.ymd.unstack(2).unstack(1).dropna(axis=1, how='all')
         assert_frame_equal(unstacked, expected.ix[:, unstacked.columns])
 
+    def test_stack_names_and_numbers(self):
+        unstacked = self.ymd.unstack(['year', 'month'])
+
+        # Can't use mixture of names and numbers to stack
+        with assertRaisesRegexp(ValueError, "level should contain"):
+            unstacked.stack([0, 'month'])
+
+    def test_stack_multiple_out_of_bounds(self):
+        # nlevels == 3
+        unstacked = self.ymd.unstack(['year', 'month'])
+
+        with assertRaisesRegexp(IndexError, "Too many levels"):
+            unstacked.stack([2, 3])
+        with assertRaisesRegexp(IndexError, "not a valid level number"):
+            unstacked.stack([-4, -3])
+
     def test_unstack_period_series(self):
         # GH 4342
         idx1 = pd.PeriodIndex(['2013-01', '2013-01', '2013-02', '2013-02',