Add unstack() support for non-multiindexed dataframes(#7054)

Closes #6694 

When `unstack()` receives a dataframe with "single" index, returns a series to match pandas behavior.

Authors:
  - Michael Wang <[email protected]>

Approvers:
  - null

URL: #7054

python/cudf/cudf/core/reshape.py

@@ -902,6 +902,11 @@ def unstack(df, level, fill_value=None):
     Pivots the specified levels of the index labels of df to the innermost
     levels of the columns labels of the result.
 
+    * If the index of ``df`` has multiple levels, returns a ``Dataframe`` with
+      specified level of the index pivoted to the column levels.
+    * If the index of ``df`` has single level, returns a ``Series`` with all
+      column levels pivoted to the index levels.
+
     Parameters
     ----------
     df : DataFrame
@@ -913,7 +918,7 @@ def unstack(df, level, fill_value=None):
 
     Returns
     -------
-    DataFrame with specified index levels pivoted to column levels
+    Series or DataFrame
 
     Examples
     --------
@@ -964,18 +969,43 @@ def unstack(df, level, fill_value=None):
     a
     1     5  <NA>     6  <NA>     7
     2  <NA>     8  <NA>     9  <NA>
+
+    Unstacking single level index dataframe:
+
+    >>> df = cudf.DataFrame({('c', 1): [1, 2, 3], ('c', 2):[9, 8, 7]})
+    >>> df.unstack()
+    c  1  0    1
+          1    2
+          2    3
+       2  0    9
+          1    8
+          2    7
+    dtype: int64
     """
+    if not isinstance(df, cudf.DataFrame):
+        raise ValueError("`df` should be a cudf Dataframe object.")
+
+    if df.empty:
+        raise ValueError("Cannot unstack an empty dataframe.")
+
     if fill_value is not None:
         raise NotImplementedError("fill_value is not supported.")
     if pd.api.types.is_list_like(level):
         if not level:
             return df
     df = df.copy(deep=False)
     if not isinstance(df.index, cudf.MultiIndex):
-        raise NotImplementedError(
-            "Calling unstack() on a DataFrame without a MultiIndex "
-            "is not supported"
-        )
+        dtype = df._columns[0].dtype
+        for col in df._columns:
+            if not col.dtype == dtype:
+                raise ValueError(
+                    "Calling unstack() on single index dataframe"
+                    " with different column datatype is not supported."
+                )
+        res = df.T.stack(dropna=False)
+        # Result's index is a multiindex
+        res.index.names = tuple(df.columns.names) + df.index.names
+        return res
     else:
         columns = df.index._poplevels(level)
         index = df.index

python/cudf/cudf/tests/test_reshape.py

@@ -1,3 +1,5 @@
+import re
+
 import numpy as np
 import pandas as pd
 import pytest
@@ -402,7 +404,7 @@ def test_pivot_multi_values():
         ),
     ],
 )
-def test_unstack(level):
+def test_unstack_multiindex(level):
     pdf = pd.DataFrame(
         {
             "foo": ["one", "one", "one", "two", "two", "two"],
@@ -417,6 +419,53 @@ def test_unstack(level):
     )
 
 
+@pytest.mark.parametrize(
+    "data",
+    [{"A": [1.0, 2.0, 3.0, 4.0, 5.0], "B": [11.0, 12.0, 13.0, 14.0, 15.0]}],
+)
+@pytest.mark.parametrize(
+    "index",
+    [
+        pd.Index(range(0, 5), name=None),
+        pd.Index(range(0, 5), name="row_index"),
+    ],
+)
+@pytest.mark.parametrize(
+    "col_idx",
+    [
+        pd.Index(["a", "b"], name=None),
+        pd.Index(["a", "b"], name="col_index"),
+        pd.MultiIndex.from_tuples([("c", 1), ("c", 2)], names=[None, None]),
+        pd.MultiIndex.from_tuples(
+            [("c", 1), ("c", 2)], names=["col_index1", "col_index2"]
+        ),
+    ],
+)
+def test_unstack_index(data, index, col_idx):
+    pdf = pd.DataFrame(data)
+    gdf = cudf.from_pandas(pdf)
+
+    pdf.index = index
+    pdf.columns = col_idx
+
+    gdf.index = cudf.from_pandas(index)
+    gdf.columns = cudf.from_pandas(col_idx)
+
+    assert_eq(pdf.unstack(), gdf.unstack())
+
+
+def test_unstack_index_invalid():
+    gdf = cudf.DataFrame({"a": [1, 2, 3], "b": ["a", "b", "c"]})
+    with pytest.raises(
+        ValueError,
+        match=re.escape(
+            "Calling unstack() on single index dataframe with "
+            "different column datatype is not supported."
+        ),
+    ):
+        gdf.unstack()
+
+
 def test_pivot_duplicate_error():
     gdf = cudf.DataFrame(
         {"a": [0, 1, 2, 2], "b": [1, 2, 3, 3], "d": [1, 2, 3, 4]}