From 82e90e38a599218ac70b79d12eb6434a292c9c7d Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Fri, 1 Nov 2019 17:12:34 -0700
Subject: [PATCH 1/3] REF: Implement maybe_promote_scalar

---
 pandas/core/dtypes/cast.py        | 6 ++++++
 pandas/core/internals/blocks.py   | 8 ++++----
 pandas/core/internals/managers.py | 4 ++--
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 3e92906be706c..8fec99c41e8e6 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -353,6 +353,12 @@ def maybe_promote(dtype, fill_value=np.nan):
             fill_value = np.nan
             dtype = np.dtype(np.object_)
 
+    return maybe_promote_scalar(dtype, fill_value)
+
+
+def maybe_promote_scalar(dtype, fill_value=np.nan):
+    # maybe_promote, but restricted to fill_value being non-ndarray
+
     # returns tuple of (dtype, fill_value)
     if issubclass(dtype.type, np.datetime64):
         if isinstance(fill_value, datetime) and fill_value.tzinfo is not None:
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 51108d9a5a573..b494aab9dda27 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -22,7 +22,7 @@
     maybe_downcast_numeric,
     maybe_downcast_to_dtype,
     maybe_infer_dtype_type,
-    maybe_promote,
+    maybe_promote_scalar,
     maybe_upcast,
     soft_convert_objects,
 )
@@ -856,7 +856,7 @@ def setitem(self, indexer, value):
 
         # cast the values to a type that can hold nan (if necessary)
         if not self._can_hold_element(value):
-            dtype, _ = maybe_promote(arr_value.dtype)
+            dtype, _ = maybe_promote_scalar(arr_value.dtype)
             values = values.astype(dtype)
 
         if transpose:
@@ -994,7 +994,7 @@ def f(mask, val, idx):
                         n = np.array(new)
 
                     # type of the new block
-                    dtype, _ = maybe_promote(n.dtype)
+                    dtype, _ = maybe_promote_scalar(n.dtype)
 
                     # we need to explicitly astype here to make a copy
                     n = n.astype(dtype)
@@ -3169,7 +3169,7 @@ def _putmask_preserve(nv, n):
         return _putmask_preserve(v, n)
 
     # change the dtype if needed
-    dtype, _ = maybe_promote(n.dtype)
+    dtype, _ = maybe_promote_scalar(n.dtype)
 
     if is_extension_type(v.dtype) and is_object_dtype(dtype):
         v = v._internal_get_values(dtype)
diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index c47aaf7c773c4..d6b41a884a15d 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -14,7 +14,7 @@
     find_common_type,
     infer_dtype_from_scalar,
     maybe_convert_objects,
-    maybe_promote,
+    maybe_promote_scalar,
 )
 from pandas.core.dtypes.common import (
     _NS_DTYPE,
@@ -1292,7 +1292,7 @@ def _slice_take_blocks_ax0(self, slice_or_indexer, fill_tuple=None):
                 return [blk.getitem_block(slobj, new_mgr_locs=slice(0, sllen))]
             elif not allow_fill or self.ndim == 1:
                 if allow_fill and fill_tuple[0] is None:
-                    _, fill_value = maybe_promote(blk.dtype)
+                    _, fill_value = maybe_promote_scalar(blk.dtype)
                     fill_tuple = (fill_value,)
 
                 return [

From 9fae53bcc1a6cdd712f32ff81d1ad3574d12977e Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Sat, 2 Nov 2019 08:59:44 -0700
Subject: [PATCH 2/3] docstring

---
 pandas/core/dtypes/cast.py | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 8fec99c41e8e6..8d5ccaf86185c 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -357,7 +357,22 @@ def maybe_promote(dtype, fill_value=np.nan):
 
 
 def maybe_promote_scalar(dtype, fill_value=np.nan):
-    # maybe_promote, but restricted to fill_value being non-ndarray
+    """
+    Find the minimal dtype that can hold both the given dtype and fill_value.
+
+    Parameters
+    ----------
+    dtype : np.dtype or ExceptionDtype
+    fill_value : scalar, default np.nan
+
+    Returns
+    -------
+    dtype
+        Upcasted from dtype argument if necessary.
+    fill_value
+        Upcasted from fill_value argument if necessary.
+    """
+    assert is_scalar(fill_value)
 
     # returns tuple of (dtype, fill_value)
     if issubclass(dtype.type, np.datetime64):

From 3e5b72208c0251fc9c76ddc952f522a7fb74cb6c Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Sat, 2 Nov 2019 11:59:31 -0700
Subject: [PATCH 3/3] revert assertion

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

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 9bde5298a67e6..b34f25267184a 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -376,7 +376,6 @@ def maybe_promote_scalar(dtype, fill_value=np.nan):
     fill_value
         Upcasted from fill_value argument if necessary.
     """
-    assert is_scalar(fill_value)
 
     # returns tuple of (dtype, fill_value)
     if issubclass(dtype.type, np.datetime64):