From eafd9ea11cab2d87e92d70ae00b28aca01ec596b Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 7 Nov 2020 00:54:16 +0100
Subject: [PATCH 01/16] Start fixing dtype setting

---
 pandas/core/frame.py  | 8 +-------
 pandas/core/series.py | 3 ++-
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 049d2c4888a69..4518649b3ef3b 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -3202,13 +3202,7 @@ def _set_value(self, index, col, value, takeable: bool = False):
                 return
 
             series = self._get_item_cache(col)
-            engine = self.index._engine
-            loc = engine.get_loc(index)
-            validate_numeric_casting(series.dtype, value)
-
-            series._values[loc] = value
-            # Note: trying to use series._set_value breaks tests in
-            #  tests.frame.indexing.test_indexing and tests.indexing.test_partial
+            series._set_value(index, value, takeable)
         except (KeyError, TypeError):
             # set using a non-recursive method & reset the cache
             if takeable:
diff --git a/pandas/core/series.py b/pandas/core/series.py
index e4a805a18bcdb..4f2cca9e503e8 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -43,7 +43,7 @@
 from pandas.core.dtypes.cast import (
     convert_dtypes,
     maybe_cast_to_extension_array,
-    validate_numeric_casting,
+    validate_numeric_casting, infer_dtype_from_scalar,
 )
 from pandas.core.dtypes.common import (
     ensure_platform_int,
@@ -1110,6 +1110,7 @@ def _set_value(self, label, value, takeable: bool = False):
             else:
                 loc = self.index.get_loc(label)
                 validate_numeric_casting(self.dtype, value)
+                dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
                 self._values[loc] = value
         except KeyError:
 

From c95dcc94805e2f20afbe05b95d3bd92ff5d18448 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 7 Nov 2020 01:28:30 +0100
Subject: [PATCH 02/16] Fix dtype setting for incompatible dtypes

---
 pandas/core/frame.py             |  1 +
 pandas/core/series.py            |  7 ++++++-
 pandas/tests/indexing/test_at.py | 18 +++++++++++++++++-
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 4518649b3ef3b..521a86b2ce5a2 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -3202,6 +3202,7 @@ def _set_value(self, index, col, value, takeable: bool = False):
                 return
 
             series = self._get_item_cache(col)
+            self.index._engine.get_loc(index)
             series._set_value(index, value, takeable)
         except (KeyError, TypeError):
             # set using a non-recursive method & reset the cache
diff --git a/pandas/core/series.py b/pandas/core/series.py
index 4f2cca9e503e8..a24d1ad000209 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -43,7 +43,8 @@
 from pandas.core.dtypes.cast import (
     convert_dtypes,
     maybe_cast_to_extension_array,
-    validate_numeric_casting, infer_dtype_from_scalar,
+    validate_numeric_casting,
+    infer_dtype_from_scalar,
 )
 from pandas.core.dtypes.common import (
     ensure_platform_int,
@@ -57,6 +58,7 @@
     is_object_dtype,
     is_scalar,
     validate_all_hashable,
+    is_dtype_equal,
 )
 from pandas.core.dtypes.generic import ABCDataFrame
 from pandas.core.dtypes.inference import is_hashable
@@ -1111,6 +1113,9 @@ def _set_value(self, label, value, takeable: bool = False):
                 loc = self.index.get_loc(label)
                 validate_numeric_casting(self.dtype, value)
                 dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
+                if not is_dtype_equal(self.dtype, dtype):
+                    self.loc[label] = value
+                    return
                 self._values[loc] = value
         except KeyError:
 
diff --git a/pandas/tests/indexing/test_at.py b/pandas/tests/indexing/test_at.py
index 2e06d8c73d7d1..18c42772b1944 100644
--- a/pandas/tests/indexing/test_at.py
+++ b/pandas/tests/indexing/test_at.py
@@ -3,7 +3,7 @@
 import numpy as np
 import pytest
 
-from pandas import DataFrame
+from pandas import DataFrame, Series
 import pandas._testing as tm
 
 
@@ -39,3 +39,19 @@ def test_at_with_duplicate_axes_requires_scalar_lookup(self):
             df.at[1, ["A"]] = 1
         with pytest.raises(ValueError, match=msg):
             df.at[:, "A"] = 1
+
+
+def test_at_assign_float_to_int_frame():
+    # GH: 26395
+    obj = DataFrame([0, 0, 0], index=["A", "B", "C"], columns=["D"])
+    obj.at["C", "D"] = 44.5
+    expected = DataFrame([0, 0, 44.5], index=["A", "B", "C"], columns=["D"])
+    tm.assert_frame_equal(obj, expected)
+
+
+def test_at_assign_float_to_int_series():
+    # GH: 26395
+    obj = Series([0, 0, 0], index=["A", "B", "C"])
+    obj.at["C"] = 44.5
+    expected = Series([0, 0, 44.5], index=["A", "B", "C"])
+    tm.assert_series_equal(obj, expected)

From 3788c4df7ff0c5c1a714a8ca5d0b0916687e3ffb Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 7 Nov 2020 01:29:47 +0100
Subject: [PATCH 03/16] Add whatsnew

---
 doc/source/whatsnew/v1.2.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index 690e6b8f725ad..b97a38588a3d1 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -465,6 +465,7 @@ Indexing
 - Bug in indexing on a :class:`Series` or :class:`DataFrame` with a :class:`MultiIndex` with a level named "0" (:issue:`37194`)
 - Bug in :meth:`Series.__getitem__` when using an unsigned integer array as an indexer giving incorrect results or segfaulting instead of raising ``KeyError`` (:issue:`37218`)
 - Bug in :meth:`Index.where` incorrectly casting numeric values to strings (:issue:`37591`)
+- Bug in :meth:`DataFrame.at` and :meth:`Series.at` did not adjust dtype when float was assigned to integer column (:issue:`26395`)
 
 Missing
 ^^^^^^^

From 27d9e35a3b6b4999990080584761f9704bd8569a Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 7 Nov 2020 01:33:17 +0100
Subject: [PATCH 04/16] Fix imports

---
 pandas/core/frame.py  | 1 -
 pandas/core/series.py | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 521a86b2ce5a2..7200ea4c14ea3 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -91,7 +91,6 @@
     maybe_downcast_to_dtype,
     maybe_infer_to_datetimelike,
     maybe_upcast,
-    validate_numeric_casting,
 )
 from pandas.core.dtypes.common import (
     ensure_int64,
diff --git a/pandas/core/series.py b/pandas/core/series.py
index a24d1ad000209..8699ce58cd5b5 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -51,6 +51,7 @@
     is_bool,
     is_categorical_dtype,
     is_dict_like,
+    is_dtype_equal,
     is_extension_array_dtype,
     is_integer,
     is_iterator,
@@ -58,7 +59,6 @@
     is_object_dtype,
     is_scalar,
     validate_all_hashable,
-    is_dtype_equal,
 )
 from pandas.core.dtypes.generic import ABCDataFrame
 from pandas.core.dtypes.inference import is_hashable

From 23cc926b146f1f9e6fea86a78b6fdbb6be87ebcf Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 7 Nov 2020 01:39:19 +0100
Subject: [PATCH 05/16] Change import order

---
 pandas/core/series.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/series.py b/pandas/core/series.py
index 8699ce58cd5b5..d79264f9ff649 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -42,9 +42,9 @@
 
 from pandas.core.dtypes.cast import (
     convert_dtypes,
+    infer_dtype_from_scalar,
     maybe_cast_to_extension_array,
     validate_numeric_casting,
-    infer_dtype_from_scalar,
 )
 from pandas.core.dtypes.common import (
     ensure_platform_int,

From cee9fc2b410a2a66702eca29a10e5e93ff45c30d Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sat, 7 Nov 2020 20:40:53 +0100
Subject: [PATCH 06/16] Add check for numeric dtype

---
 pandas/core/series.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/core/series.py b/pandas/core/series.py
index d79264f9ff649..40b2b9c446911 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -56,6 +56,7 @@
     is_integer,
     is_iterator,
     is_list_like,
+    is_numeric_dtype,
     is_object_dtype,
     is_scalar,
     validate_all_hashable,
@@ -1113,7 +1114,7 @@ def _set_value(self, label, value, takeable: bool = False):
                 loc = self.index.get_loc(label)
                 validate_numeric_casting(self.dtype, value)
                 dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
-                if not is_dtype_equal(self.dtype, dtype):
+                if not is_dtype_equal(self.dtype, dtype) and is_numeric_dtype(dtype):
                     self.loc[label] = value
                     return
                 self._values[loc] = value

From d1bc870eab49c55feccf4c22f7b17c97f05c63ff Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sun, 8 Nov 2020 01:22:01 +0100
Subject: [PATCH 07/16] Parametrize tests

---
 pandas/tests/indexing/test_at.py | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/indexing/test_at.py b/pandas/tests/indexing/test_at.py
index 72e5a3ad7c477..21c90b6340df9 100644
--- a/pandas/tests/indexing/test_at.py
+++ b/pandas/tests/indexing/test_at.py
@@ -110,17 +110,19 @@ def test_at_frame_raises_key_error2(self):
             df.loc["a", 0]
 
 
-def test_at_assign_float_to_int_frame():
+@pytest.mark.parametrize("func", ["at", "loc"])
+def test_at_assign_float_to_int_frame(func):
     # GH: 26395
     obj = DataFrame([0, 0, 0], index=["A", "B", "C"], columns=["D"])
-    obj.at["C", "D"] = 44.5
+    getattr(obj, func)["C", "D"] = 44.5
     expected = DataFrame([0, 0, 44.5], index=["A", "B", "C"], columns=["D"])
     tm.assert_frame_equal(obj, expected)
 
 
-def test_at_assign_float_to_int_series():
+@pytest.mark.parametrize("func", ["at", "loc"])
+def test_at_assign_float_to_int_series(func):
     # GH: 26395
     obj = Series([0, 0, 0], index=["A", "B", "C"])
-    obj.at["C"] = 44.5
+    getattr(obj, func)["C"] = 44.5
     expected = Series([0, 0, 44.5], index=["A", "B", "C"])
     tm.assert_series_equal(obj, expected)

From 4d91e666652c36715c253a601ec7bfaa3d560954 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 10 Nov 2020 22:05:21 +0100
Subject: [PATCH 08/16] Improve dtype casting for at and iat

---
 pandas/core/series.py                        | 12 +++++++++---
 pandas/tests/indexing/test_at.py             | 12 ++++++++++++
 pandas/tests/series/indexing/test_setitem.py |  8 ++++++++
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/pandas/core/series.py b/pandas/core/series.py
index 77d19bddb14ef..380c04d287e0a 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -1046,7 +1046,11 @@ def _set_with_engine(self, key, value):
         # fails with AttributeError for IntervalIndex
         loc = self.index._engine.get_loc(key)
         validate_numeric_casting(self.dtype, value)
-        self._values[loc] = value
+        dtype, _ = infer_dtype_from_scalar(value)
+        if is_dtype_equal(self.dtype, dtype):
+            self._values[loc] = value
+        else:
+            self.loc[key] = value
 
     def _set_with(self, key, value):
         # other: fancy integer or otherwise
@@ -1108,12 +1112,14 @@ def _set_value(self, label, value, takeable: bool = False):
         takeable : interpret the index as indexers, default False
         """
         try:
-            if takeable:
+            dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
+            if takeable and is_dtype_equal(self.dtype, dtype):
                 self._values[label] = value
+            elif takeable:
+                self.iloc[label] = value
             else:
                 loc = self.index.get_loc(label)
                 validate_numeric_casting(self.dtype, value)
-                dtype, _ = infer_dtype_from_scalar(value, pandas_dtype=True)
                 if not is_dtype_equal(self.dtype, dtype) and is_numeric_dtype(dtype):
                     self.loc[label] = value
                     return
diff --git a/pandas/tests/indexing/test_at.py b/pandas/tests/indexing/test_at.py
index 21c90b6340df9..466b65aef2ee5 100644
--- a/pandas/tests/indexing/test_at.py
+++ b/pandas/tests/indexing/test_at.py
@@ -126,3 +126,15 @@ def test_at_assign_float_to_int_series(func):
     getattr(obj, func)["C"] = 44.5
     expected = Series([0, 0, 44.5], index=["A", "B", "C"])
     tm.assert_series_equal(obj, expected)
+
+
+def test_assign_float_to_int_series_takeable():
+    # GH: 20643
+    ser = Series([0, 1, 2], index=list('abc'))
+    ser.iat[1] = 3.1
+    expected = Series([0, 3.1, 2], index=list('abc'))
+    tm.assert_series_equal(ser, expected)
+
+    ser = Series([0, 1, 2], index=list('abc'))
+    ser.at["b"] = 3.1
+    tm.assert_series_equal(ser, expected)
diff --git a/pandas/tests/series/indexing/test_setitem.py b/pandas/tests/series/indexing/test_setitem.py
index 7e25e5200d610..c56bd961381d3 100644
--- a/pandas/tests/series/indexing/test_setitem.py
+++ b/pandas/tests/series/indexing/test_setitem.py
@@ -248,3 +248,11 @@ def test_setitem_slice_into_readonly_backing_data():
         series[1:3] = 1
 
     assert not array.any()
+
+
+def test_setitem_float_to_int():
+    # GH 20643
+    ser = Series([0, 1, 2], index=list('abc'))
+    ser["b"] = 3.1
+    expected = Series([0, 3.1, 2], index=list('abc'))
+    tm.assert_series_equal(ser, expected)

From d6084747ad0f228d84e598c857ccc6175544338e Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 10 Nov 2020 22:05:59 +0100
Subject: [PATCH 09/16] Adjust whatsnew

---
 doc/source/whatsnew/v1.2.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index 250c9fb0f7328..0c89f12b5a587 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -469,7 +469,7 @@ Indexing
 - Bug in :meth:`Index.where` incorrectly casting numeric values to strings (:issue:`37591`)
 - Bug in :meth:`Series.loc` and :meth:`DataFrame.loc` raises when numeric label was given for object :class:`Index` although label was in :class:`Index` (:issue:`26491`)
 - Bug in :meth:`DataFrame.loc` returned requested key plus missing values when ``loc`` was applied to single level from :class:`MultiIndex` (:issue:`27104`)
-- Bug in :meth:`DataFrame.at` and :meth:`Series.at` did not adjust dtype when float was assigned to integer column (:issue:`26395`)
+- Bug in :meth:`DataFrame.at` and :meth:`Series.at` did not adjust dtype when float was assigned to integer column (:issue:`26395`, :issue:`20643`)
 
 Missing
 ^^^^^^^

From 760c6a12a38630557e2a15f9f1a3e4c9beb032a1 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 10 Nov 2020 22:06:37 +0100
Subject: [PATCH 10/16] Run black

---
 pandas/tests/indexing/test_at.py             | 6 +++---
 pandas/tests/series/indexing/test_setitem.py | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/pandas/tests/indexing/test_at.py b/pandas/tests/indexing/test_at.py
index e802ee0152b6b..df32f78055642 100644
--- a/pandas/tests/indexing/test_at.py
+++ b/pandas/tests/indexing/test_at.py
@@ -148,11 +148,11 @@ def test_at_assign_float_to_int_series(func):
 
 def test_assign_float_to_int_series_takeable():
     # GH: 20643
-    ser = Series([0, 1, 2], index=list('abc'))
+    ser = Series([0, 1, 2], index=list("abc"))
     ser.iat[1] = 3.1
-    expected = Series([0, 3.1, 2], index=list('abc'))
+    expected = Series([0, 3.1, 2], index=list("abc"))
     tm.assert_series_equal(ser, expected)
 
-    ser = Series([0, 1, 2], index=list('abc'))
+    ser = Series([0, 1, 2], index=list("abc"))
     ser.at["b"] = 3.1
     tm.assert_series_equal(ser, expected)
diff --git a/pandas/tests/series/indexing/test_setitem.py b/pandas/tests/series/indexing/test_setitem.py
index fbf61b873a686..44c992e5ae771 100644
--- a/pandas/tests/series/indexing/test_setitem.py
+++ b/pandas/tests/series/indexing/test_setitem.py
@@ -261,7 +261,7 @@ def test_setitem_slice_into_readonly_backing_data():
 
 def test_setitem_float_to_int():
     # GH 20643
-    ser = Series([0, 1, 2], index=list('abc'))
+    ser = Series([0, 1, 2], index=list("abc"))
     ser["b"] = 3.1
-    expected = Series([0, 3.1, 2], index=list('abc'))
+    expected = Series([0, 3.1, 2], index=list("abc"))
     tm.assert_series_equal(ser, expected)

From 870e927db890e9c32d84482e7e7f9531a475cada Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 10 Nov 2020 23:01:20 +0100
Subject: [PATCH 11/16] Fix tests with new dtype conversion

---
 pandas/tests/indexing/test_coercion.py | 42 ++++----------------------
 1 file changed, 6 insertions(+), 36 deletions(-)

diff --git a/pandas/tests/indexing/test_coercion.py b/pandas/tests/indexing/test_coercion.py
index fd6f6fbc6a4ba..0c6295fe013e2 100644
--- a/pandas/tests/indexing/test_coercion.py
+++ b/pandas/tests/indexing/test_coercion.py
@@ -114,31 +114,17 @@ def test_setitem_series_int64(self, val, exp_dtype, request):
         obj = pd.Series([1, 2, 3, 4])
         assert obj.dtype == np.int64
 
-        if exp_dtype is np.float64:
-            exp = pd.Series([1, 1, 3, 4])
-            self._assert_setitem_series_conversion(obj, 1.1, exp, np.int64)
-            mark = pytest.mark.xfail(reason="GH12747 The result must be float")
-            request.node.add_marker(mark)
-
         exp = pd.Series([1, val, 3, 4])
         self._assert_setitem_series_conversion(obj, val, exp, exp_dtype)
 
     @pytest.mark.parametrize(
-        "val,exp_dtype", [(np.int32(1), np.int8), (np.int16(2 ** 9), np.int16)]
+        "val,exp_dtype", [(np.int32(1), np.int32), (np.int16(2 ** 9), np.int16)]
     )
     def test_setitem_series_int8(self, val, exp_dtype, request):
         obj = pd.Series([1, 2, 3, 4], dtype=np.int8)
         assert obj.dtype == np.int8
 
-        if exp_dtype is np.int16:
-            exp = pd.Series([1, 0, 3, 4], dtype=np.int8)
-            self._assert_setitem_series_conversion(obj, val, exp, np.int8)
-            mark = pytest.mark.xfail(
-                reason="BUG: it must be pd.Series([1, 1, 3, 4], dtype=np.int16"
-            )
-            request.node.add_marker(mark)
-
-        exp = pd.Series([1, val, 3, 4], dtype=np.int8)
+        exp = pd.Series([1, val, 3, 4], dtype=exp_dtype)
         self._assert_setitem_series_conversion(obj, val, exp, exp_dtype)
 
     @pytest.mark.parametrize(
@@ -171,10 +157,10 @@ def test_setitem_series_complex128(self, val, exp_dtype):
     @pytest.mark.parametrize(
         "val,exp_dtype",
         [
-            (1, np.int64),
-            (3, np.int64),
-            (1.1, np.float64),
-            (1 + 1j, np.complex128),
+            (1, "object"),
+            (3, "object"),
+            (1.1, "object"),
+            (1 + 1j, "object"),
             (True, np.bool_),
         ],
     )
@@ -182,22 +168,6 @@ def test_setitem_series_bool(self, val, exp_dtype, request):
         obj = pd.Series([True, False, True, False])
         assert obj.dtype == np.bool_
 
-        mark = None
-        if exp_dtype is np.int64:
-            exp = pd.Series([True, True, True, False])
-            self._assert_setitem_series_conversion(obj, val, exp, np.bool_)
-            mark = pytest.mark.xfail(reason="TODO_GH12747 The result must be int")
-        elif exp_dtype is np.float64:
-            exp = pd.Series([True, True, True, False])
-            self._assert_setitem_series_conversion(obj, val, exp, np.bool_)
-            mark = pytest.mark.xfail(reason="TODO_GH12747 The result must be float")
-        elif exp_dtype is np.complex128:
-            exp = pd.Series([True, True, True, False])
-            self._assert_setitem_series_conversion(obj, val, exp, np.bool_)
-            mark = pytest.mark.xfail(reason="TODO_GH12747 The result must be complex")
-        if mark is not None:
-            request.node.add_marker(mark)
-
         exp = pd.Series([True, val, True, False])
         self._assert_setitem_series_conversion(obj, val, exp, exp_dtype)
 

From fa8b3313b675a560cc74cfd64f2395ffd5f66d7e Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 10 Nov 2020 23:08:30 +0100
Subject: [PATCH 12/16] Fix remaining test

---
 pandas/tests/indexing/test_partial.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/tests/indexing/test_partial.py b/pandas/tests/indexing/test_partial.py
index 3bf37f4cade8b..5acb1d9b6c748 100644
--- a/pandas/tests/indexing/test_partial.py
+++ b/pandas/tests/indexing/test_partial.py
@@ -47,7 +47,6 @@ def test_partial_setting(self):
         with pytest.raises(IndexError, match=msg):
             s.iloc[3] = 5.0
 
-        msg = "index 3 is out of bounds for axis 0 with size 3"
         with pytest.raises(IndexError, match=msg):
             s.iat[3] = 5.0
 

From 903be319de7dd8fe187ae45750520351a800cf55 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 10 Nov 2020 23:41:30 +0100
Subject: [PATCH 13/16] Change if condition

---
 pandas/core/series.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/series.py b/pandas/core/series.py
index 380c04d287e0a..ffb7a766a2cd1 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -1047,7 +1047,7 @@ def _set_with_engine(self, key, value):
         loc = self.index._engine.get_loc(key)
         validate_numeric_casting(self.dtype, value)
         dtype, _ = infer_dtype_from_scalar(value)
-        if is_dtype_equal(self.dtype, dtype):
+        if is_dtype_equal(self.dtype, dtype) or isna(value):
             self._values[loc] = value
         else:
             self.loc[key] = value

From 566d76921d498458b090205290b96ae16e5d898b Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Thu, 12 Nov 2020 12:43:38 +0100
Subject: [PATCH 14/16] Fix if condition

---
 pandas/core/series.py | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/pandas/core/series.py b/pandas/core/series.py
index ffb7a766a2cd1..2b28a3cc4dfa9 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -1047,10 +1047,14 @@ def _set_with_engine(self, key, value):
         loc = self.index._engine.get_loc(key)
         validate_numeric_casting(self.dtype, value)
         dtype, _ = infer_dtype_from_scalar(value)
-        if is_dtype_equal(self.dtype, dtype) or isna(value):
+        if is_dtype_equal(self.dtype, dtype):
             self._values[loc] = value
         else:
-            self.loc[key] = value
+            # This only raises when index contains tuples
+            try:
+                self.loc[key] = value
+            except KeyError:
+                self._values[loc] = value
 
     def _set_with(self, key, value):
         # other: fancy integer or otherwise

From 916faa4e5407695669f03f54774b3c3b44dd4f8f Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sun, 15 Nov 2020 18:40:24 +0100
Subject: [PATCH 15/16] Move if condition

---
 pandas/core/series.py | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/pandas/core/series.py b/pandas/core/series.py
index 2b28a3cc4dfa9..84897c6118247 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -1121,12 +1121,14 @@ def _set_value(self, label, value, takeable: bool = False):
                 self._values[label] = value
             elif takeable:
                 self.iloc[label] = value
+            elif not is_dtype_equal(self.dtype, dtype) and is_numeric_dtype(dtype):
+                loc = self.index.get_loc(label)
+                validate_numeric_casting(self.dtype, value)
+                self.loc[label] = value
+                return
             else:
                 loc = self.index.get_loc(label)
                 validate_numeric_casting(self.dtype, value)
-                if not is_dtype_equal(self.dtype, dtype) and is_numeric_dtype(dtype):
-                    self.loc[label] = value
-                    return
                 self._values[loc] = value
         except KeyError:
 

From 6e7ee82af85196043ea98150a463d4f05e9497ba Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Sun, 3 Jan 2021 19:51:47 +0100
Subject: [PATCH 16/16] Move whatsnew

---
 doc/source/whatsnew/v1.3.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 393866b92771b..beccc1e6dfe21 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -239,6 +239,7 @@ Indexing
 - Bug in :meth:`CategoricalIndex.get_indexer` failing to raise ``InvalidIndexError`` when non-unique (:issue:`38372`)
 - Bug in inserting many new columns into a :class:`DataFrame` causing incorrect subsequent indexing behavior (:issue:`38380`)
 - Bug in :meth:`DataFrame.iloc.__setitem__` and :meth:`DataFrame.loc.__setitem__` with mixed dtypes when setting with a dictionary value (:issue:`38335`)
+- Bug in :meth:`DataFrame.at` and :meth:`Series.at` did not adjust dtype when float was assigned to integer column (:issue:`26395`, :issue:`20643`)
 - Bug in :meth:`DataFrame.loc` dropping levels of :class:`MultiIndex` when :class:`DataFrame` used as input has only one row (:issue:`10521`)
 -