From 44c31db0a2fff4c070591238fcb3ba7193542266 Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Mon, 23 Jan 2023 23:59:49 -0500
Subject: [PATCH 01/10] first value_counts commit

---
 pandas/core/frame.py                          |  9 ++++++++-
 .../tests/frame/methods/test_value_counts.py  | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 1d86c81745a6a..142fcb54d993f 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -6932,7 +6932,7 @@ def value_counts(
 
         Parameters
         ----------
-        subset : list-like, optional
+        subset : mapping, function, label, list of labels, optional
             Columns to use when counting unique combinations.
         normalize : bool, default False
             Return proportions rather than frequencies.
@@ -7024,6 +7024,13 @@ def value_counts(
         John        Smith          1
                     NaN            1
         dtype: int64
+
+        >>> df.value_counts("first_name")
+        first_name
+        John    2
+        Anne    1
+        Beth    1
+        dtype: int64
         """
         if subset is None:
             subset = self.columns.tolist()
diff --git a/pandas/tests/frame/methods/test_value_counts.py b/pandas/tests/frame/methods/test_value_counts.py
index 9859ffb83da66..d8b7e21ebc210 100644
--- a/pandas/tests/frame/methods/test_value_counts.py
+++ b/pandas/tests/frame/methods/test_value_counts.py
@@ -144,3 +144,22 @@ def test_data_frame_value_counts_dropna_false(nulls_fixture):
     )
 
     tm.assert_series_equal(result, expected)
+
+
+def test_data_frame_value_counts_subset(nulls_fixture):
+    # GH 50829
+    df = pd.DataFrame(
+        {
+            "first_name": ["John", "Anne", "John", "Beth"],
+            "middle_name": ["Smith", nulls_fixture, nulls_fixture, "Louise"],
+        },
+    )
+    result = df.value_counts("first_name")
+    expected = pd.Series(
+        data=[2, 1, 1],
+        index=pd.MultiIndex.from_arrays(
+            [["John", "Anne", "Beth"]], names=["first_name"]
+        ),
+    )
+
+    tm.assert_series_equal(result, expected)

From c20e994ea5e3e2c3e3426e65d4624282cbb59c79 Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Tue, 24 Jan 2023 14:52:38 -0500
Subject: [PATCH 02/10] updated test_data_frame_value_counts_subset

---
 pandas/tests/frame/methods/test_value_counts.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/pandas/tests/frame/methods/test_value_counts.py b/pandas/tests/frame/methods/test_value_counts.py
index d8b7e21ebc210..8bf54aadf340c 100644
--- a/pandas/tests/frame/methods/test_value_counts.py
+++ b/pandas/tests/frame/methods/test_value_counts.py
@@ -157,9 +157,7 @@ def test_data_frame_value_counts_subset(nulls_fixture):
     result = df.value_counts("first_name")
     expected = pd.Series(
         data=[2, 1, 1],
-        index=pd.MultiIndex.from_arrays(
-            [["John", "Anne", "Beth"]], names=["first_name"]
-        ),
+        index=pd.Index(["John", "Anne", "Beth"], name="first_name"),
     )
 
     tm.assert_series_equal(result, expected)

From da419881f698bde16bcace407909aa6f4632a44a Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Sat, 28 Jan 2023 12:29:57 -0500
Subject: [PATCH 03/10] updated subset docstring

---
 pandas/core/frame.py | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 142fcb54d993f..43e3f36170ea6 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -16,7 +16,6 @@
 import functools
 from io import StringIO
 import itertools
-import sys
 from textwrap import dedent
 from typing import (
     TYPE_CHECKING,
@@ -49,7 +48,7 @@
 from pandas._libs.hashtable import duplicated
 from pandas._libs.lib import (
     NoDefault,
-    is_range_indexer,
+    array_equal_fast,
     no_default,
 )
 from pandas._typing import (
@@ -92,17 +91,12 @@
     WriteBuffer,
     npt,
 )
-from pandas.compat import PYPY
 from pandas.compat._optional import import_optional_dependency
 from pandas.compat.numpy import (
     function as nv,
     np_percentile_argname,
 )
-from pandas.errors import (
-    ChainedAssignmentError,
-    InvalidIndexError,
-    _chained_assignment_msg,
-)
+from pandas.errors import InvalidIndexError
 from pandas.util._decorators import (
     Appender,
     Substitution,
@@ -3868,10 +3862,6 @@ def isetitem(self, loc, value) -> None:
         self._iset_item_mgr(loc, arraylike, inplace=False)
 
     def __setitem__(self, key, value):
-        if not PYPY and using_copy_on_write():
-            if sys.getrefcount(self) <= 3:
-                raise ChainedAssignmentError(_chained_assignment_msg)
-
         key = com.apply_if_callable(key, self)
 
         # see if we can slice the rows
@@ -6734,7 +6724,7 @@ def sort_values(
             else:
                 return self.copy(deep=None)
 
-        if is_range_indexer(indexer, len(indexer)):
+        if array_equal_fast(indexer, np.arange(0, len(indexer), dtype=indexer.dtype)):
             if inplace:
                 return self._update_inplace(self)
             else:
@@ -6932,8 +6922,8 @@ def value_counts(
 
         Parameters
         ----------
-        subset : mapping, function, label, list of labels, optional
-            Columns to use when counting unique combinations.
+        subset : label or list of labels, optional
+            Column(s) to use when counting unique combinations.
         normalize : bool, default False
             Return proportions rather than frequencies.
         sort : bool, default True
@@ -10887,7 +10877,11 @@ def quantile(
                 f"Invalid method: {method}. Method must be in {valid_method}."
             )
         if method == "single":
-            res = data._mgr.quantile(qs=q, axis=1, interpolation=interpolation)
+            # error: Argument "qs" to "quantile" of "BlockManager" has incompatible type
+            # "Index"; expected "Float64Index"
+            res = data._mgr.quantile(
+                qs=q, axis=1, interpolation=interpolation  # type: ignore[arg-type]
+            )
         elif method == "table":
             valid_interpolation = {"nearest", "lower", "higher"}
             if interpolation not in valid_interpolation:

From efae1bf006277f87e2bfa44ab80ddb0397fc659e Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Sun, 29 Jan 2023 22:40:14 -0500
Subject: [PATCH 04/10] Back to pre updated subset docstring commit

---
 pandas/core/frame.py | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 43e3f36170ea6..142fcb54d993f 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -16,6 +16,7 @@
 import functools
 from io import StringIO
 import itertools
+import sys
 from textwrap import dedent
 from typing import (
     TYPE_CHECKING,
@@ -48,7 +49,7 @@
 from pandas._libs.hashtable import duplicated
 from pandas._libs.lib import (
     NoDefault,
-    array_equal_fast,
+    is_range_indexer,
     no_default,
 )
 from pandas._typing import (
@@ -91,12 +92,17 @@
     WriteBuffer,
     npt,
 )
+from pandas.compat import PYPY
 from pandas.compat._optional import import_optional_dependency
 from pandas.compat.numpy import (
     function as nv,
     np_percentile_argname,
 )
-from pandas.errors import InvalidIndexError
+from pandas.errors import (
+    ChainedAssignmentError,
+    InvalidIndexError,
+    _chained_assignment_msg,
+)
 from pandas.util._decorators import (
     Appender,
     Substitution,
@@ -3862,6 +3868,10 @@ def isetitem(self, loc, value) -> None:
         self._iset_item_mgr(loc, arraylike, inplace=False)
 
     def __setitem__(self, key, value):
+        if not PYPY and using_copy_on_write():
+            if sys.getrefcount(self) <= 3:
+                raise ChainedAssignmentError(_chained_assignment_msg)
+
         key = com.apply_if_callable(key, self)
 
         # see if we can slice the rows
@@ -6724,7 +6734,7 @@ def sort_values(
             else:
                 return self.copy(deep=None)
 
-        if array_equal_fast(indexer, np.arange(0, len(indexer), dtype=indexer.dtype)):
+        if is_range_indexer(indexer, len(indexer)):
             if inplace:
                 return self._update_inplace(self)
             else:
@@ -6922,8 +6932,8 @@ def value_counts(
 
         Parameters
         ----------
-        subset : label or list of labels, optional
-            Column(s) to use when counting unique combinations.
+        subset : mapping, function, label, list of labels, optional
+            Columns to use when counting unique combinations.
         normalize : bool, default False
             Return proportions rather than frequencies.
         sort : bool, default True
@@ -10877,11 +10887,7 @@ def quantile(
                 f"Invalid method: {method}. Method must be in {valid_method}."
             )
         if method == "single":
-            # error: Argument "qs" to "quantile" of "BlockManager" has incompatible type
-            # "Index"; expected "Float64Index"
-            res = data._mgr.quantile(
-                qs=q, axis=1, interpolation=interpolation  # type: ignore[arg-type]
-            )
+            res = data._mgr.quantile(qs=q, axis=1, interpolation=interpolation)
         elif method == "table":
             valid_interpolation = {"nearest", "lower", "higher"}
             if interpolation not in valid_interpolation:

From 245526bef90462c3cba486620f5dc44c0708852c Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Sun, 29 Jan 2023 23:09:04 -0500
Subject: [PATCH 05/10] added integer column label test

---
 pandas/core/frame.py                            | 2 +-
 pandas/tests/frame/methods/test_value_counts.py | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 142fcb54d993f..f4ae2b4e3a946 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -6932,7 +6932,7 @@ def value_counts(
 
         Parameters
         ----------
-        subset : mapping, function, label, list of labels, optional
+        subset : label or list of labels, optional
             Columns to use when counting unique combinations.
         normalize : bool, default False
             Return proportions rather than frequencies.
diff --git a/pandas/tests/frame/methods/test_value_counts.py b/pandas/tests/frame/methods/test_value_counts.py
index 8bf54aadf340c..b21f5d4530d87 100644
--- a/pandas/tests/frame/methods/test_value_counts.py
+++ b/pandas/tests/frame/methods/test_value_counts.py
@@ -160,4 +160,13 @@ def test_data_frame_value_counts_subset(nulls_fixture):
         index=pd.Index(["John", "Anne", "Beth"], name="first_name"),
     )
 
+    df = pd.DataFrame(
+        {100: [2, 100, 5, 9], 200: [2, 6, 2, 6], 300: [4, 6, 2, 1]},
+    )
+    result = df.value_counts([200])
+    expected = pd.Series(
+        data=[2, 2],
+        index=pd.Index([2, 6], name=200),
+    )
+
     tm.assert_series_equal(result, expected)

From e5f96b236589c6e4ef292c0eb2588d41592d7c36 Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Sun, 29 Jan 2023 23:37:35 -0500
Subject: [PATCH 06/10] updated value_counts subset test

---
 pandas/tests/frame/methods/test_value_counts.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/frame/methods/test_value_counts.py b/pandas/tests/frame/methods/test_value_counts.py
index b21f5d4530d87..9e32755821486 100644
--- a/pandas/tests/frame/methods/test_value_counts.py
+++ b/pandas/tests/frame/methods/test_value_counts.py
@@ -160,13 +160,15 @@ def test_data_frame_value_counts_subset(nulls_fixture):
         index=pd.Index(["John", "Anne", "Beth"], name="first_name"),
     )
 
+    tm.assert_series_equal(result, expected)
+
     df = pd.DataFrame(
         {100: [2, 100, 5, 9], 200: [2, 6, 2, 6], 300: [4, 6, 2, 1]},
     )
     result = df.value_counts([200])
     expected = pd.Series(
         data=[2, 2],
-        index=pd.Index([2, 6], name=200),
+        index=pd.MultiIndex.from_arrays([[2, 6]], names=[200]),
     )
 
     tm.assert_series_equal(result, expected)

From 80d0576146fa821ae30b5840926232a566d10f12 Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Tue, 31 Jan 2023 21:14:29 -0500
Subject: [PATCH 07/10] parameterized tests and fixed implimentation for
 integers

---
 pandas/core/frame.py                          |  2 +-
 .../tests/frame/methods/test_value_counts.py  | 23 ++++++-------------
 2 files changed, 8 insertions(+), 17 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index f4ae2b4e3a946..0f44fc1e634d0 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -7043,7 +7043,7 @@ def value_counts(
             counts /= counts.sum()
 
         # Force MultiIndex for single column
-        if len(subset) == 1:
+        if is_list_like(subset) and len(subset) == 1:
             counts.index = MultiIndex.from_arrays(
                 [counts.index], names=[counts.index.name]
             )
diff --git a/pandas/tests/frame/methods/test_value_counts.py b/pandas/tests/frame/methods/test_value_counts.py
index 9e32755821486..1b861bee9a667 100644
--- a/pandas/tests/frame/methods/test_value_counts.py
+++ b/pandas/tests/frame/methods/test_value_counts.py
@@ -1,4 +1,5 @@
 import numpy as np
+import pytest
 
 import pandas as pd
 import pandas._testing as tm
@@ -146,29 +147,19 @@ def test_data_frame_value_counts_dropna_false(nulls_fixture):
     tm.assert_series_equal(result, expected)
 
 
-def test_data_frame_value_counts_subset(nulls_fixture):
+@pytest.mark.parametrize("columns", (["first_name", "middle_name"], [0, 1]))
+def test_data_frame_value_counts_subset(nulls_fixture, columns):
     # GH 50829
     df = pd.DataFrame(
         {
-            "first_name": ["John", "Anne", "John", "Beth"],
-            "middle_name": ["Smith", nulls_fixture, nulls_fixture, "Louise"],
+            columns[0]: ["John", "Anne", "John", "Beth"],
+            columns[1]: ["Smith", nulls_fixture, nulls_fixture, "Louise"],
         },
     )
-    result = df.value_counts("first_name")
+    result = df.value_counts(columns[0])
     expected = pd.Series(
         data=[2, 1, 1],
-        index=pd.Index(["John", "Anne", "Beth"], name="first_name"),
-    )
-
-    tm.assert_series_equal(result, expected)
-
-    df = pd.DataFrame(
-        {100: [2, 100, 5, 9], 200: [2, 6, 2, 6], 300: [4, 6, 2, 1]},
-    )
-    result = df.value_counts([200])
-    expected = pd.Series(
-        data=[2, 2],
-        index=pd.MultiIndex.from_arrays([[2, 6]], names=[200]),
+        index=pd.Index(["John", "Anne", "Beth"], name=columns[0]),
     )
 
     tm.assert_series_equal(result, expected)

From ff87dc8abca8d8e1cdb21aea08e8e487b097c7e5 Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Tue, 7 Feb 2023 23:02:57 -0500
Subject: [PATCH 08/10] fixed docstring

---
 pandas/core/frame.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 6ace24782fb4a..5acd6fde5dfef 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -7049,7 +7049,6 @@ def value_counts(
         John    2
         Anne    1
         Beth    1
-        dtype: int64
         Name: count, dtype: int64
         """
         if subset is None:

From a7f61c53b6367f050ca48740892ec64f2dac9354 Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Wed, 8 Feb 2023 00:34:24 -0500
Subject: [PATCH 09/10] fixed test_data_frame_value_counts_subset

---
 pandas/tests/frame/methods/test_value_counts.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pandas/tests/frame/methods/test_value_counts.py b/pandas/tests/frame/methods/test_value_counts.py
index fa38cd15bfa2f..355f05cd5156c 100644
--- a/pandas/tests/frame/methods/test_value_counts.py
+++ b/pandas/tests/frame/methods/test_value_counts.py
@@ -171,6 +171,7 @@ def test_data_frame_value_counts_subset(nulls_fixture, columns):
     expected = pd.Series(
         data=[2, 1, 1],
         index=pd.Index(["John", "Anne", "Beth"], name=columns[0]),
+        name="count",
     )
 
     tm.assert_series_equal(result, expected)

From c3adbc9624c2fc092d1abc0ebbcf4bc8a914fe56 Mon Sep 17 00:00:00 2001
From: Taylor Packard <3.t.packard@gmail.com>
Date: Sat, 18 Feb 2023 14:10:29 -0500
Subject: [PATCH 10/10] updated whatsnew and docs

---
 doc/source/whatsnew/v2.0.0.rst | 1 +
 pandas/core/frame.py           | 7 ++++---
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/doc/source/whatsnew/v2.0.0.rst b/doc/source/whatsnew/v2.0.0.rst
index ac7d30310be9e..f63563472f1f2 100644
--- a/doc/source/whatsnew/v2.0.0.rst
+++ b/doc/source/whatsnew/v2.0.0.rst
@@ -1088,6 +1088,7 @@ Removal of prior version deprecations/changes
 - Arguments after ``expr`` in :meth:`DataFrame.eval` and :meth:`DataFrame.query` are keyword-only (:issue:`47587`)
 - Removed :meth:`Index._get_attributes_dict` (:issue:`50648`)
 - Removed :meth:`Series.__array_wrap__` (:issue:`50648`)
+- Changed behavior of :meth:`.DataFrame.value_counts` to return a :class:`Series` with :class:`MultiIndex` for any list-like(one element or not) but an :class:`Index` for a single label (:issue:`50829`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_200.performance:
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 629259689e5fc..2520f54ae3d99 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -6981,9 +6981,10 @@ def value_counts(
         Notes
         -----
         The returned Series will have a MultiIndex with one level per input
-        column. By default, rows that contain any NA values are omitted from
-        the result. By default, the resulting Series will be in descending
-        order so that the first element is the most frequently-occurring row.
+        column but an Index (non-multi) for a single label. By default, rows
+        that contain any NA values are omitted from the result. By default,
+        the resulting Series will be in descending order so that the first
+        element is the most frequently-occurring row.
 
         Examples
         --------