From 3b0a66b1e96b913b86790a8f4f487bf83cb014d1 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Sat, 19 Sep 2020 15:35:15 -0700
Subject: [PATCH] TYP: core.missing; PERF for needs_i8_conversion

---
 pandas/core/arrays/datetimelike.py | 14 ++------------
 pandas/core/dtypes/common.py       |  4 ++++
 pandas/core/missing.py             | 31 +++++++++++++++---------------
 3 files changed, 21 insertions(+), 28 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 026aad5ad6eb7..45cabe8f0b498 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -1005,19 +1005,9 @@ def fillna(self, value=None, method=None, limit=None):
                 else:
                     func = missing.backfill_1d
 
-                values = self._ndarray
-                if not is_period_dtype(self.dtype):
-                    # For PeriodArray self._ndarray is i8, which gets copied
-                    #  by `func`.  Otherwise we need to make a copy manually
-                    # to avoid modifying `self` in-place.
-                    values = values.copy()
-
+                values = self.copy()
                 new_values = func(values, limit=limit, mask=mask)
-                if is_datetime64tz_dtype(self.dtype):
-                    # we need to pass int64 values to the constructor to avoid
-                    #  re-localizing incorrectly
-                    new_values = new_values.view("i8")
-                new_values = type(self)(new_values, dtype=self.dtype)
+                new_values = self._from_backing_data(new_values)
             else:
                 # fill with value
                 new_values = self.copy()
diff --git a/pandas/core/dtypes/common.py b/pandas/core/dtypes/common.py
index 5987fdabf78bb..acbdbfd7707e3 100644
--- a/pandas/core/dtypes/common.py
+++ b/pandas/core/dtypes/common.py
@@ -1215,6 +1215,10 @@ def needs_i8_conversion(arr_or_dtype) -> bool:
     """
     if arr_or_dtype is None:
         return False
+    if isinstance(arr_or_dtype, (np.dtype, ExtensionDtype)):
+        # fastpath
+        dtype = arr_or_dtype
+        return dtype.kind in ["m", "M"] or dtype.type is Period
     return (
         is_datetime_or_timedelta_dtype(arr_or_dtype)
         or is_datetime64tz_dtype(arr_or_dtype)
diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index be66b19d10064..9b96c8f01153b 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -7,17 +7,15 @@
 import numpy as np
 
 from pandas._libs import algos, lib
+from pandas._typing import DtypeObj
 from pandas.compat._optional import import_optional_dependency
 
 from pandas.core.dtypes.cast import infer_dtype_from_array
 from pandas.core.dtypes.common import (
     ensure_float64,
-    is_datetime64_dtype,
-    is_datetime64tz_dtype,
     is_integer_dtype,
     is_numeric_v_string_like,
     is_scalar,
-    is_timedelta64_dtype,
     needs_i8_conversion,
 )
 from pandas.core.dtypes.missing import isna
@@ -72,7 +70,7 @@ def mask_missing(arr, values_to_mask):
     return mask
 
 
-def clean_fill_method(method, allow_nearest=False):
+def clean_fill_method(method, allow_nearest: bool = False):
     # asfreq is compat for resampling
     if method in [None, "asfreq"]:
         return None
@@ -543,7 +541,12 @@ def _cubicspline_interpolate(xi, yi, x, axis=0, bc_type="not-a-knot", extrapolat
 
 
 def interpolate_2d(
-    values, method="pad", axis=0, limit=None, fill_value=None, dtype=None
+    values,
+    method="pad",
+    axis=0,
+    limit=None,
+    fill_value=None,
+    dtype: Optional[DtypeObj] = None,
 ):
     """
     Perform an actual interpolation of values, values will be make 2-d if
@@ -584,18 +587,14 @@ def interpolate_2d(
     return values
 
 
-def _cast_values_for_fillna(values, dtype):
+def _cast_values_for_fillna(values, dtype: DtypeObj):
     """
     Cast values to a dtype that algos.pad and algos.backfill can handle.
     """
     # TODO: for int-dtypes we make a copy, but for everything else this
     #  alters the values in-place.  Is this intentional?
 
-    if (
-        is_datetime64_dtype(dtype)
-        or is_datetime64tz_dtype(dtype)
-        or is_timedelta64_dtype(dtype)
-    ):
+    if needs_i8_conversion(dtype):
         values = values.view(np.int64)
 
     elif is_integer_dtype(values):
@@ -605,7 +604,7 @@ def _cast_values_for_fillna(values, dtype):
     return values
 
 
-def _fillna_prep(values, mask=None, dtype=None):
+def _fillna_prep(values, mask=None, dtype: Optional[DtypeObj] = None):
     # boilerplate for pad_1d, backfill_1d, pad_2d, backfill_2d
     if dtype is None:
         dtype = values.dtype
@@ -620,19 +619,19 @@ def _fillna_prep(values, mask=None, dtype=None):
     return values, mask
 
 
-def pad_1d(values, limit=None, mask=None, dtype=None):
+def pad_1d(values, limit=None, mask=None, dtype: Optional[DtypeObj] = None):
     values, mask = _fillna_prep(values, mask, dtype)
     algos.pad_inplace(values, mask, limit=limit)
     return values
 
 
-def backfill_1d(values, limit=None, mask=None, dtype=None):
+def backfill_1d(values, limit=None, mask=None, dtype: Optional[DtypeObj] = None):
     values, mask = _fillna_prep(values, mask, dtype)
     algos.backfill_inplace(values, mask, limit=limit)
     return values
 
 
-def pad_2d(values, limit=None, mask=None, dtype=None):
+def pad_2d(values, limit=None, mask=None, dtype: Optional[DtypeObj] = None):
     values, mask = _fillna_prep(values, mask, dtype)
 
     if np.all(values.shape):
@@ -643,7 +642,7 @@ def pad_2d(values, limit=None, mask=None, dtype=None):
     return values
 
 
-def backfill_2d(values, limit=None, mask=None, dtype=None):
+def backfill_2d(values, limit=None, mask=None, dtype: Optional[DtypeObj] = None):
     values, mask = _fillna_prep(values, mask, dtype)
 
     if np.all(values.shape):