rebase & cleanup, fixup some edge cases

closes #16402

Author: jreback

Diff for: doc/source/whatsnew/v0.21.0.txt

@@ -84,7 +84,8 @@ Bug Fixes
 Conversion
 ^^^^^^^^^^
 
-- Bug in assignment against datetime-like data with ``int`` may incorrectly converted to datetime-like (:issue:`14145`)
+- Inconsistent behavior in ``.where()`` with datetimelikes which would raise rather than coerce to ``object`` (:issue:`16402`)
+- Bug in assignment against datetime-like data with ``int`` may incorrectly convert to datetime-like (:issue:`14145`)
 - Bug in assignment against ``int64`` data with ``np.ndarray`` with ``float64`` dtype may keep ``int64`` dtype (:issue:`14001`)
 
 

Diff for: pandas/_libs/index.pyx

@@ -19,6 +19,7 @@ cimport tslib
 from hashtable cimport *
 from pandas._libs import tslib, algos, hashtable as _hash
 from pandas._libs.tslib import Timestamp, Timedelta
+from datetime import datetime, timedelta
 
 from datetime cimport (get_datetime64_value, _pydatetime_to_dts,
                        pandas_datetimestruct)
@@ -507,24 +508,30 @@ cdef class TimedeltaEngine(DatetimeEngine):
         return 'm8[ns]'
 
 cpdef convert_scalar(ndarray arr, object value):
+    # we don't turn intgers
+    # into datetimes/timedeltas
+
     if arr.descr.type_num == NPY_DATETIME:
         if isinstance(value, np.ndarray):
             pass
-        elif isinstance(value, Timestamp):
-            return value.value
+        elif isinstance(value, datetime):
+            return Timestamp(value).value
         elif value is None or value != value:
             return iNaT
-        else:
+        elif util.is_string_object(value):
             return Timestamp(value).value
+        raise ValueError("cannot set a Timestamp with a non-timestamp")
+
     elif arr.descr.type_num == NPY_TIMEDELTA:
         if isinstance(value, np.ndarray):
             pass
-        elif isinstance(value, Timedelta):
-            return value.value
+        elif isinstance(value, timedelta):
+            return Timedelta(value).value
         elif value is None or value != value:
             return iNaT
-        else:
+        elif util.is_string_object(value):
             return Timedelta(value).value
+        raise ValueError("cannot set a Timedelta with a non-timedelta")
 
     if issubclass(arr.dtype.type, (np.integer, np.bool_)):
         if util.is_float_object(value) and value != value:

Diff for: pandas/core/dtypes/cast.py

@@ -6,7 +6,7 @@
 import warnings
 
 from pandas._libs import tslib, lib
-from pandas._libs.tslib import iNaT
+from pandas._libs.tslib import iNaT, Timestamp
 from pandas.compat import string_types, text_type, PY3
 from .common import (_ensure_object, is_bool, is_integer, is_float,
                      is_complex, is_datetimetz, is_categorical_dtype,
@@ -1028,13 +1028,25 @@ def find_common_type(types):
     return np.find_common_type(types, [])
 
 
-def _cast_scalar_to_array(shape, value, dtype=None):
+def cast_scalar_to_array(shape, value, dtype=None):
     """
     create np.ndarray of specified shape and dtype, filled with values
+
+    Parameters
+    ----------
+    shape : tuple
+    value : scalar value
+    dtype : np.dtype, optional
+        dtype to coerce
+
+    Returns
+    -------
+    ndarray of shape, filled with value, of specified / inferred dtype
+
     """
 
     if dtype is None:
-        dtype, fill_value = _infer_dtype_from_scalar(value)
+        dtype, fill_value = infer_dtype_from_scalar(value)
     else:
         fill_value = value
 

Diff for: pandas/core/frame.py

@@ -25,14 +25,15 @@
 import numpy.ma as ma
 
 from pandas.core.dtypes.cast import (
-    maybe_upcast, infer_dtype_from_scalar,
+    maybe_upcast,
     maybe_cast_to_datetime,
     maybe_infer_to_datetimelike,
     maybe_convert_platform,
     maybe_downcast_to_dtype,
     invalidate_string_dtypes,
     coerce_to_dtypes,
     maybe_upcast_putmask,
+    cast_scalar_to_array,
     find_common_type)
 from pandas.core.dtypes.common import (
     is_categorical_dtype,
@@ -343,8 +344,8 @@ def __init__(self, data=None, index=None, columns=None, dtype=None,
                 raise_with_traceback(exc)
 
             if arr.ndim == 0 and index is not None and columns is not None:
-                values = _cast_scalar_to_array((len(index), len(columns)),
-                                               data, dtype=dtype)
+                values = cast_scalar_to_array((len(index), len(columns)),
+                                              data, dtype=dtype)
                 mgr = self._init_ndarray(values, index, columns,
                                          dtype=values.dtype, copy=False)
             else:
@@ -2734,8 +2735,8 @@ def reindexer(value):
 
         else:
             # upcast the scalar
-            value = _cast_scalar_to_array(len(self.index), value)
-            value = _possibly_cast_to_datetime(value, value.dtype)
+            value = cast_scalar_to_array(len(self.index), value)
+            value = maybe_cast_to_datetime(value, value.dtype)
 
         # return internal types directly
         if is_extension_type(value):

Diff for: pandas/core/generic.py

@@ -12,7 +12,6 @@
 from pandas._libs import tslib, lib
 from pandas.core.dtypes.common import (
     _ensure_int64,
-    needs_i8_conversion,
     is_scalar,
     is_number,
     is_integer, is_bool,
@@ -5030,48 +5029,6 @@ def _where(self, cond, other=np.nan, inplace=False, axis=None, level=None,
                 raise NotImplemented("cannot align with a higher dimensional "
                                      "NDFrame")
 
-        elif is_list_like(other):
-
-            if self.ndim == 1:
-
-                # try to set the same dtype as ourselves
-                try:
-                    new_other = np.array(other, dtype=self.dtype)
-                except ValueError:
-                    new_other = np.array(other)
-                except TypeError:
-                    new_other = other
-
-                # we can end up comparing integers and m8[ns]
-                # which is a numpy no no
-                is_i8 = needs_i8_conversion(self.dtype)
-                if is_i8:
-                    matches = False
-                else:
-                    matches = (new_other == np.array(other))
-
-                if matches is False or not matches.all():
-
-                    # coerce other to a common dtype if we can
-                    if needs_i8_conversion(self.dtype):
-                        try:
-                            other = np.array(other, dtype=self.dtype)
-                        except:
-                            other = np.array(other)
-                    else:
-                        other = np.asarray(other)
-                        other = np.asarray(other,
-                                           dtype=np.common_type(other,
-                                                                new_other))
-
-                    # we need to use the new dtype
-                    try_quick = False
-                else:
-                    other = new_other
-            else:
-
-                other = np.array(other)
-
         if isinstance(other, np.ndarray):
 
             if other.shape != self.shape: