From 830fa003eed373ae3755d315111e96c9e7d2ea2c Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Sat, 5 Dec 2020 12:47:49 -0500
Subject: [PATCH 01/21] add type hints

---
 pandas/core/missing.py | 143 ++++++++++++++++++++++++++++-------------
 1 file changed, 99 insertions(+), 44 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index e374ba435a0bd..7246af42e3098 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -2,12 +2,12 @@
 Routines for filling missing data.
 """
 from functools import partial
-from typing import Any, List, Optional, Set, Union
+from typing import Any, Callable, List, Optional, Set, Tuple, Union
 
 import numpy as np
 
 from pandas._libs import algos, lib
-from pandas._typing import ArrayLike, Axis, DtypeObj
+from pandas._typing import ArrayLike, Axis, DtypeObj, IndexLabel, Scalar
 from pandas.compat._optional import import_optional_dependency
 
 from pandas.core.dtypes.cast import infer_dtype_from_array
@@ -20,7 +20,9 @@
 from pandas.core.dtypes.missing import isna
 
 
-def mask_missing(arr: ArrayLike, values_to_mask) -> np.ndarray:
+def mask_missing(
+    arr: ArrayLike, values_to_mask: Union[List, Tuple, Scalar]
+) -> np.ndarray:
     """
     Return a masking array of same size/shape as arr
     with entries equaling any member of values_to_mask set to True
@@ -58,7 +60,7 @@ def mask_missing(arr: ArrayLike, values_to_mask) -> np.ndarray:
     return mask
 
 
-def clean_fill_method(method, allow_nearest: bool = False):
+def clean_fill_method(method: str, allow_nearest: bool = False) -> Optional[str]:
     # asfreq is compat for resampling
     if method in [None, "asfreq"]:
         return None
@@ -117,7 +119,7 @@ def clean_interp_method(method: str, **kwargs) -> str:
     return method
 
 
-def find_valid_index(values, how: str):
+def find_valid_index(values: ArrayLike, how: str) -> Optional[int]:
     """
     Retrieves the index of the first valid value.
 
@@ -165,7 +167,7 @@ def interpolate_1d(
     bounds_error: bool = False,
     order: Optional[int] = None,
     **kwargs,
-):
+) -> np.ndarray:
     """
     Logic for the 1-d interpolation.  The result should be 1-d, inputs
     xvalues and yvalues will each be 1-d arrays of the same length.
@@ -218,8 +220,13 @@ def interpolate_1d(
 
     # These are sets of index pointers to invalid values... i.e. {0, 1, etc...
     all_nans = set(np.flatnonzero(invalid))
-    start_nans = set(range(find_valid_index(yvalues, "first")))
-    end_nans = set(range(1 + find_valid_index(yvalues, "last"), len(valid)))
+
+    start_nan_idx = find_valid_index(yvalues, "first")
+    start_nans = set() if start_nan_idx is None else set(range(start_nan_idx))
+
+    end_nan_idx = find_valid_index(yvalues, "last")
+    end_nans = set() if end_nan_idx is None else set(range(1 + end_nan_idx, len(valid)))
+
     mid_nans = all_nans - start_nans - end_nans
 
     # Like the sets above, preserve_nans contains indices of invalid values,
@@ -294,8 +301,15 @@ def interpolate_1d(
 
 
 def _interpolate_scipy_wrapper(
-    x, y, new_x, method, fill_value=None, bounds_error=False, order=None, **kwargs
-):
+    x,
+    y,
+    new_x,
+    method: Optional[str],
+    fill_value: Optional[Scalar] = None,
+    bounds_error: bool = False,
+    order: Optional[int] = None,
+    **kwargs,
+) -> np.ndarray:
     """
     Passed off to scipy.interpolate.interp1d. method is scipy's kind.
     Returns an array interpolated at new_x.  Add any new methods to
@@ -326,7 +340,7 @@ def _interpolate_scipy_wrapper(
     elif method == "cubicspline":
         alt_methods["cubicspline"] = _cubicspline_interpolate
 
-    interp1d_methods = [
+    interp1d_methods: List[str] = [
         "nearest",
         "zero",
         "slinear",
@@ -335,18 +349,18 @@ def _interpolate_scipy_wrapper(
         "polynomial",
     ]
     if method in interp1d_methods:
-        if method == "polynomial":
-            method = order
+        kind = order if method == "polynomial" else method
         terp = interpolate.interp1d(
-            x, y, kind=method, fill_value=fill_value, bounds_error=bounds_error
+            x, y, kind=kind, fill_value=fill_value, bounds_error=bounds_error
         )
         new_y = terp(new_x)
     elif method == "spline":
         # GH #10633, #24014
-        if isna(order) or (order <= 0):
-            raise ValueError(
-                f"order needs to be specified and greater than 0; got order: {order}"
-            )
+        if isna(order):
+            raise ValueError(f"order needs to be specified; got order: {order}")
+        assert isinstance(order, int)
+        if order <= 0:
+            raise ValueError(f"order needs to be greater than 0; got order: {order}")
         terp = interpolate.UnivariateSpline(x, y, k=order, **kwargs)
         new_y = terp(new_x)
     else:
@@ -358,12 +372,21 @@ def _interpolate_scipy_wrapper(
             y = y.copy()
         if not new_x.flags.writeable:
             new_x = new_x.copy()
-        method = alt_methods[method]
-        new_y = method(x, y, new_x, **kwargs)
+
+        assert isinstance(method, str)
+        alt_method = alt_methods[method]
+        new_y = alt_method(x, y, new_x, **kwargs)
     return new_y
 
 
-def _from_derivatives(xi, yi, x, order=None, der=0, extrapolate=False):
+def _from_derivatives(
+    xi: np.ndarray,
+    yi: np.ndarray,
+    x: Union[Scalar, ArrayLike],
+    order: Optional[Union[int, List[int]]] = None,
+    der: Union[int, List[int]] = 0,
+    extrapolate: bool = False,
+) -> np.ndarray:
     """
     Convenience function for interpolate.BPoly.from_derivatives.
 
@@ -406,7 +429,13 @@ def _from_derivatives(xi, yi, x, order=None, der=0, extrapolate=False):
     return m(x)
 
 
-def _akima_interpolate(xi, yi, x, der=0, axis=0):
+def _akima_interpolate(
+    xi: ArrayLike,
+    yi: ArrayLike,
+    x: Union[Scalar, ArrayLike],
+    der: Optional[int] = 0,
+    axis: Optional[int] = 0,
+) -> Union[Scalar, ArrayLike]:
     """
     Convenience function for akima interpolation.
     xi and yi are arrays of values used to approximate some function f,
@@ -449,7 +478,14 @@ def _akima_interpolate(xi, yi, x, der=0, axis=0):
     return P(x, nu=der)
 
 
-def _cubicspline_interpolate(xi, yi, x, axis=0, bc_type="not-a-knot", extrapolate=None):
+def _cubicspline_interpolate(
+    xi: ArrayLike,
+    yi: ArrayLike,
+    x: Union[ArrayLike, Scalar],
+    axis: Optional[int] = 0,
+    bc_type: Union[str, Tuple] = "not-a-knot",
+    extrapolate: Optional[Union[bool, str]] = None,
+) -> Union[ArrayLike, Scalar]:
     """
     Convenience function for cubic spline data interpolator.
 
@@ -557,6 +593,8 @@ def _interpolate_with_limit_area(
         first = find_valid_index(values, "first")
         last = find_valid_index(values, "last")
 
+        assert first is not None and last is not None
+
         values = interpolate_2d(
             values,
             method=method,
@@ -574,12 +612,12 @@ def _interpolate_with_limit_area(
 
 
 def interpolate_2d(
-    values,
+    values: np.ndarray,
     method: str = "pad",
     axis: Axis = 0,
     limit: Optional[int] = None,
     limit_area: Optional[str] = None,
-):
+) -> np.ndarray:
     """
     Perform an actual interpolation of values, values will be make 2-d if
     needed fills inplace, returns the result.
@@ -625,7 +663,10 @@ def interpolate_2d(
             raise AssertionError("cannot interpolate on a ndim == 1 with axis != 0")
         values = values.reshape(tuple((1,) + values.shape))
 
-    method = clean_fill_method(method)
+    method_cleaned = clean_fill_method(method)
+    assert isinstance(method_cleaned, str)
+    method = method_cleaned
+
     tvalues = transf(values)
     if method == "pad":
         result = _pad_2d(tvalues, limit=limit)
@@ -644,7 +685,9 @@ def interpolate_2d(
     return result
 
 
-def _cast_values_for_fillna(values, dtype: DtypeObj, has_mask: bool):
+def _cast_values_for_fillna(
+    values: ArrayLike, dtype: DtypeObj, has_mask: bool
+) -> ArrayLike:
     """
     Cast values to a dtype that algos.pad and algos.backfill can handle.
     """
@@ -663,34 +706,41 @@ def _cast_values_for_fillna(values, dtype: DtypeObj, has_mask: bool):
     return values
 
 
-def _fillna_prep(values, mask=None):
+def _fillna_prep(
+    values: np.ndarray, mask: Optional[np.ndarray] = None
+) -> Tuple[np.ndarray, np.ndarray]:
     # boilerplate for _pad_1d, _backfill_1d, _pad_2d, _backfill_2d
-    dtype = values.dtype
 
     has_mask = mask is not None
-    if not has_mask:
-        # This needs to occur before datetime/timedeltas are cast to int64
-        mask = isna(values)
 
-    values = _cast_values_for_fillna(values, dtype, has_mask)
+    # This needs to occur before datetime/timedeltas are cast to int64
+    mask = isna(values) if mask is None else mask
 
+    values = _cast_values_for_fillna(values, values.dtype, has_mask)
     mask = mask.view(np.uint8)
+
     return values, mask
 
 
-def _pad_1d(values, limit=None, mask=None):
+def _pad_1d(
+    values: np.ndarray, limit: Optional[int] = None, mask: Optional[np.ndarray] = None
+) -> np.ndarray:
     values, mask = _fillna_prep(values, mask)
     algos.pad_inplace(values, mask, limit=limit)
     return values
 
 
-def _backfill_1d(values, limit=None, mask=None):
+def _backfill_1d(
+    values: np.ndarray, limit: Optional[int] = None, mask: Optional[np.ndarray] = None
+) -> np.ndarray:
     values, mask = _fillna_prep(values, mask)
     algos.backfill_inplace(values, mask, limit=limit)
     return values
 
 
-def _pad_2d(values, limit=None, mask=None):
+def _pad_2d(
+    values: np.ndarray, limit: Optional[int] = None, mask: Optional[np.ndarray] = None
+) -> np.ndarray:
     values, mask = _fillna_prep(values, mask)
 
     if np.all(values.shape):
@@ -701,7 +751,9 @@ def _pad_2d(values, limit=None, mask=None):
     return values
 
 
-def _backfill_2d(values, limit=None, mask=None):
+def _backfill_2d(
+    values: np.ndarray, limit: Optional[int] = None, mask: Optional[np.ndarray] = None
+) -> np.ndarray:
     values, mask = _fillna_prep(values, mask)
 
     if np.all(values.shape):
@@ -715,16 +767,19 @@ def _backfill_2d(values, limit=None, mask=None):
 _fill_methods = {"pad": _pad_1d, "backfill": _backfill_1d}
 
 
-def get_fill_func(method):
-    method = clean_fill_method(method)
-    return _fill_methods[method]
+def get_fill_func(method: str) -> Callable:
+    method_cleaned = clean_fill_method(method)
+    assert isinstance(method_cleaned, str)
+    return _fill_methods[method_cleaned]
 
 
-def clean_reindex_fill_method(method):
+def clean_reindex_fill_method(method: str) -> Optional[str]:
     return clean_fill_method(method, allow_nearest=True)
 
 
-def _interp_limit(invalid, fw_limit, bw_limit):
+def _interp_limit(
+    invalid: np.ndarray, fw_limit: Optional[int], bw_limit: Optional[int]
+) -> Set[IndexLabel]:
     """
     Get indexers of values that won't be filled
     because they exceed the limits.
@@ -759,7 +814,7 @@ def _interp_limit(invalid, fw_limit, bw_limit):
     f_idx = set()
     b_idx = set()
 
-    def inner(invalid, limit):
+    def inner(invalid: np.ndarray, limit: int) -> Set[IndexLabel]:
         limit = min(limit, N)
         windowed = _rolling_window(invalid, limit + 1).all(1)
         idx = set(np.where(windowed)[0] + limit) | set(
@@ -789,7 +844,7 @@ def inner(invalid, limit):
     return f_idx & b_idx
 
 
-def _rolling_window(a: np.ndarray, window: int):
+def _rolling_window(a: np.ndarray, window: int) -> np.ndarray:
     """
     [True, True, False, True, False], 2 ->
 

From 605dc3ce03c2bbdf6c336a97b241e3da430fc9a7 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 7 Dec 2020 11:13:16 -0500
Subject: [PATCH 02/21] review: remove assert

---
 pandas/core/missing.py | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 7246af42e3098..9c14a75be8753 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -356,11 +356,10 @@ def _interpolate_scipy_wrapper(
         new_y = terp(new_x)
     elif method == "spline":
         # GH #10633, #24014
-        if isna(order):
-            raise ValueError(f"order needs to be specified; got order: {order}")
-        assert isinstance(order, int)
-        if order <= 0:
-            raise ValueError(f"order needs to be greater than 0; got order: {order}")
+        if order is not None and order >= 0:
+            raise ValueError(
+                f"order needs to be specified and greater than 0; got order: {order}"
+            )
         terp = interpolate.UnivariateSpline(x, y, k=order, **kwargs)
         new_y = terp(new_x)
     else:

From f2d5ec44f968debff3a288369223e2ef3ad5110a Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 7 Dec 2020 13:22:13 -0500
Subject: [PATCH 03/21] typo

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

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 57598c3197cb4..bbb213ee5a043 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -354,7 +354,7 @@ def _interpolate_scipy_wrapper(
         new_y = terp(new_x)
     elif method == "spline":
         # GH #10633, #24014
-        if order is not None and order >= 0:
+        if order is None or order <= 0:
             raise ValueError(
                 f"order needs to be specified and greater than 0; got order: {order}"
             )

From e83904fdcd182d14318735d184853f87e3fed27d Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 7 Dec 2020 14:08:26 -0500
Subject: [PATCH 04/21] add isna check

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

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index bbb213ee5a043..ffd4c1b810486 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -354,7 +354,7 @@ def _interpolate_scipy_wrapper(
         new_y = terp(new_x)
     elif method == "spline":
         # GH #10633, #24014
-        if order is None or order <= 0:
+        if order is None or isna(order) or order <= 0:
             raise ValueError(
                 f"order needs to be specified and greater than 0; got order: {order}"
             )

From 71caeeb618a0ba6c6f641c6330c256568d9f0f79 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 7 Dec 2020 16:46:57 -0500
Subject: [PATCH 05/21] better error msg when interp method not string

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

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index ffd4c1b810486..85ccfbeda4075 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -370,9 +370,11 @@ def _interpolate_scipy_wrapper(
         if not new_x.flags.writeable:
             new_x = new_x.copy()
 
-        assert isinstance(method, str)
-        alt_method = alt_methods[method]
-        new_y = alt_method(x, y, new_x, **kwargs)
+        if isinstance(method, str):
+            alt_method = alt_methods[method]
+            new_y = alt_method(x, y, new_x, **kwargs)
+        else:
+            raise ValueError(f"{method} is not a valid interp method")
     return new_y
 
 

From 8fbbd477ed5c93e2f3c1648252148adfd14bcdbe Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 7 Dec 2020 16:54:39 -0500
Subject: [PATCH 06/21] improve docstring

---
 pandas/core/missing.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 85ccfbeda4075..bc518f8e1fdbe 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -398,15 +398,16 @@ def _from_derivatives(
         sorted 1D array of x-coordinates
     yi : array_like or list of array-likes
         yi[i][j] is the j-th derivative known at xi[i]
-    order: None or int or array_like of ints. Default: None.
+    x : scalar or array_like
+    order: None or int or array_like of ints, default: None
         Specifies the degree of local polynomials. If not None, some
         derivatives are ignored.
-    der : int or list
+    der : int or list, default: 0
         How many derivatives to extract; None for all potentially nonzero
         derivatives (that is a number equal to the number of points), or a
         list of derivatives to extract. This number includes the function
         value as 0th derivative.
-     extrapolate : bool, optional
+     extrapolate : bool, default False
         Whether to extrapolate to ouf-of-bounds points based on first and last
         intervals, or to return NaNs. Default: True.
 

From 575c22764f89bbfc0cecb058ca2cd3b5896f7e47 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 7 Dec 2020 16:59:41 -0500
Subject: [PATCH 07/21] remove Optional

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

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index bc518f8e1fdbe..13370e2e33fa7 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -482,7 +482,7 @@ def _cubicspline_interpolate(
     xi: ArrayLike,
     yi: ArrayLike,
     x: Union[ArrayLike, Scalar],
-    axis: Optional[int] = 0,
+    axis: Axis = 0,
     bc_type: Union[str, Tuple] = "not-a-knot",
     extrapolate: Optional[Union[bool, str]] = None,
 ) -> Union[ArrayLike, Scalar]:

From b19896b42a76ac74724c31ea9e75fe15eb410246 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 7 Dec 2020 17:02:35 -0500
Subject: [PATCH 08/21] use Axis TypeVar

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

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 13370e2e33fa7..662185539a390 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -434,7 +434,7 @@ def _akima_interpolate(
     yi: ArrayLike,
     x: Union[Scalar, ArrayLike],
     der: Optional[int] = 0,
-    axis: Optional[int] = 0,
+    axis: Axis = 0,
 ) -> Union[Scalar, ArrayLike]:
     """
     Convenience function for akima interpolation.

From 5036ee189bd9da394b5850d7dd4e239244fda827 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Tue, 8 Dec 2020 13:14:04 -0500
Subject: [PATCH 09/21] more hints

---
 pandas/core/missing.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 662185539a390..9f0f3ba6aee7a 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -299,9 +299,9 @@ def interpolate_1d(
 
 
 def _interpolate_scipy_wrapper(
-    x,
-    y,
-    new_x,
+    x: np.ndarray,
+    y: np.ndarray,
+    new_x: Union[Scalar, np.ndarray],
     method: Optional[str],
     fill_value: Optional[Scalar] = None,
     bounds_error: bool = False,

From 2a318234b09961605bbcae97e4bec0059502c67f Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Wed, 9 Dec 2020 12:52:07 -0500
Subject: [PATCH 10/21] review comments

---
 pandas/core/missing.py | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 9f0f3ba6aee7a..cd9ac081cd839 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -7,7 +7,7 @@
 import numpy as np
 
 from pandas._libs import algos, lib
-from pandas._typing import ArrayLike, Axis, DtypeObj, IndexLabel, Scalar
+from pandas._typing import ArrayLike, DtypeObj, IndexLabel, Scalar
 from pandas.compat._optional import import_optional_dependency
 
 from pandas.core.dtypes.cast import infer_dtype_from_array
@@ -63,7 +63,9 @@ def mask_missing(
     return mask
 
 
-def clean_fill_method(method: str, allow_nearest: bool = False) -> Optional[str]:
+def clean_fill_method(
+    method: Optional[str], allow_nearest: bool = False
+) -> Optional[str]:
     # asfreq is compat for resampling
     if method in [None, "asfreq"]:
         return None
@@ -162,7 +164,7 @@ def find_valid_index(values: ArrayLike, how: str) -> Optional[int]:
 def interpolate_1d(
     xvalues: "Index",
     yvalues: np.ndarray,
-    method: Optional[str] = "linear",
+    method: str = "linear",
     limit: Optional[int] = None,
     limit_direction: str = "forward",
     limit_area: Optional[str] = None,
@@ -302,7 +304,7 @@ def _interpolate_scipy_wrapper(
     x: np.ndarray,
     y: np.ndarray,
     new_x: Union[Scalar, np.ndarray],
-    method: Optional[str],
+    method: str,
     fill_value: Optional[Scalar] = None,
     bounds_error: bool = False,
     order: Optional[int] = None,
@@ -338,7 +340,7 @@ def _interpolate_scipy_wrapper(
     elif method == "cubicspline":
         alt_methods["cubicspline"] = _cubicspline_interpolate
 
-    interp1d_methods: List[str] = [
+    interp1d_methods = [
         "nearest",
         "zero",
         "slinear",
@@ -430,11 +432,11 @@ def _from_derivatives(
 
 
 def _akima_interpolate(
-    xi: ArrayLike,
-    yi: ArrayLike,
+    xi: np.ndarray,
+    yi: np.ndarray,
     x: Union[Scalar, ArrayLike],
-    der: Optional[int] = 0,
-    axis: Axis = 0,
+    der: int = 0,
+    axis: int = 0,
 ) -> Union[Scalar, ArrayLike]:
     """
     Convenience function for akima interpolation.
@@ -445,9 +447,9 @@ def _akima_interpolate(
 
     Parameters
     ----------
-    xi : array_like
+    xi : np.ndarray
         A sorted list of x-coordinates, of length N.
-    yi : array_like
+    yi : np.ndarray
         A 1-D array of real values.  `yi`'s length along the interpolation
         axis must be equal to the length of `xi`. If N-D array, use axis
         parameter to select correct axis.
@@ -482,7 +484,7 @@ def _cubicspline_interpolate(
     xi: ArrayLike,
     yi: ArrayLike,
     x: Union[ArrayLike, Scalar],
-    axis: Axis = 0,
+    axis: int = 0,
     bc_type: Union[str, Tuple] = "not-a-knot",
     extrapolate: Optional[Union[bool, str]] = None,
 ) -> Union[ArrayLike, Scalar]:
@@ -614,7 +616,7 @@ def _interpolate_with_limit_area(
 def interpolate_2d(
     values: np.ndarray,
     method: str = "pad",
-    axis: Axis = 0,
+    axis: int = 0,
     limit: Optional[int] = None,
     limit_area: Optional[str] = None,
 ) -> np.ndarray:

From 4aeec703118d7eba81c621b3131075670e89a1c5 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Thu, 10 Dec 2020 13:22:57 -0500
Subject: [PATCH 11/21] review comment

---
 pandas/core/missing.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index cd9ac081cd839..cdbd7e1840666 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -481,8 +481,8 @@ def _akima_interpolate(
 
 
 def _cubicspline_interpolate(
-    xi: ArrayLike,
-    yi: ArrayLike,
+    xi: np.ndarray,
+    yi: np.ndarray,
     x: Union[ArrayLike, Scalar],
     axis: int = 0,
     bc_type: Union[str, Tuple] = "not-a-knot",

From d67977dc2436ca7a42cfc453db956f52add33acb Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Thu, 10 Dec 2020 14:28:33 -0500
Subject: [PATCH 12/21] review comment: values_to_mask

---
 pandas/core/dtypes/cast.py | 13 ++++++++++---
 pandas/core/missing.py     |  4 +---
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 165e63e23d60e..ce25f6da94363 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -33,7 +33,14 @@
     ints_to_pytimedelta,
 )
 from pandas._libs.tslibs.timezones import tz_compare
-from pandas._typing import AnyArrayLike, ArrayLike, Dtype, DtypeObj, Scalar
+from pandas._typing import (
+    AnyArrayLike,
+    ArrayLike,
+    Dtype,
+    DtypeObj,
+    PandasScalar,
+    Scalar,
+)
 from pandas.util._validators import validate_bool_kwarg
 
 from pandas.core.dtypes.common import (
@@ -834,8 +841,8 @@ def dict_compat(d: Dict[Scalar, Scalar]) -> Dict[Scalar, Scalar]:
 
 
 def infer_dtype_from_array(
-    arr, pandas_dtype: bool = False
-) -> Tuple[DtypeObj, ArrayLike]:
+    arr: Union[ArrayLike, PandasScalar], pandas_dtype: bool = False
+) -> Tuple[Dtype, ArrayLike]:
     """
     Infer the dtype from an array.
 
diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index cdbd7e1840666..e3bb3e149ea4d 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -23,9 +23,7 @@
     from pandas import Index
 
 
-def mask_missing(
-    arr: ArrayLike, values_to_mask: Union[List, Tuple, Scalar]
-) -> np.ndarray:
+def mask_missing(arr: ArrayLike, values_to_mask: ArrayLike) -> np.ndarray:
     """
     Return a masking array of same size/shape as arr
     with entries equaling any member of values_to_mask set to True

From 24f418af0df0a5a55ef0b2681fb8227f5b7715c0 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Fri, 11 Dec 2020 01:23:19 -0500
Subject: [PATCH 13/21] review comments: mask_missing/infer_dtype_from_array

---
 pandas/core/dtypes/cast.py | 6 +++---
 pandas/core/missing.py     | 6 ++++--
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index ce25f6da94363..046c8b3a10e4c 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -841,8 +841,8 @@ def dict_compat(d: Dict[Scalar, Scalar]) -> Dict[Scalar, Scalar]:
 
 
 def infer_dtype_from_array(
-    arr: Union[ArrayLike, PandasScalar], pandas_dtype: bool = False
-) -> Tuple[Dtype, ArrayLike]:
+    arr: Union[ArrayLike, Series, PandasScalar], pandas_dtype: bool = False
+) -> Tuple[DtypeObj, Union[ArrayLike, Series]]:
     """
     Infer the dtype from an array.
 
@@ -890,7 +890,7 @@ def infer_dtype_from_array(
     # don't force numpy coerce with nan's
     inferred = lib.infer_dtype(arr, skipna=False)
     if inferred in ["string", "bytes", "mixed", "mixed-integer"]:
-        return (np.dtype(np.object_), arr)
+        return np.dtype(np.object_), arr
 
     arr = np.asarray(arr)
     return arr.dtype, arr
diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index e3bb3e149ea4d..342f0a583f17d 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -20,10 +20,12 @@
 from pandas.core.dtypes.missing import isna
 
 if TYPE_CHECKING:
-    from pandas import Index
+    from pandas import Index, Series
 
 
-def mask_missing(arr: ArrayLike, values_to_mask: ArrayLike) -> np.ndarray:
+def mask_missing(
+    arr: ArrayLike, values_to_mask: "Union[ArrayLike, Scalar, Series]"
+) -> np.ndarray:
     """
     Return a masking array of same size/shape as arr
     with entries equaling any member of values_to_mask set to True

From aeb0b829b205ad17c4ecd50fdeed2283097aca4a Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Fri, 11 Dec 2020 01:38:04 -0500
Subject: [PATCH 14/21] typo

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

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 046c8b3a10e4c..24dbb3e24930f 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -842,7 +842,7 @@ def dict_compat(d: Dict[Scalar, Scalar]) -> Dict[Scalar, Scalar]:
 
 def infer_dtype_from_array(
     arr: Union[ArrayLike, Series, PandasScalar], pandas_dtype: bool = False
-) -> Tuple[DtypeObj, Union[ArrayLike, Series]]:
+) -> "Tuple[DtypeObj, Union[ArrayLike, Series]]":
     """
     Infer the dtype from an array.
 

From 25d00518de485b525574bd375b0f829b36642159 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Fri, 11 Dec 2020 02:03:03 -0500
Subject: [PATCH 15/21] typo

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

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 24dbb3e24930f..1d5a489e1b1c1 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -841,7 +841,7 @@ def dict_compat(d: Dict[Scalar, Scalar]) -> Dict[Scalar, Scalar]:
 
 
 def infer_dtype_from_array(
-    arr: Union[ArrayLike, Series, PandasScalar], pandas_dtype: bool = False
+    arr: "Union[ArrayLike, Series, PandasScalar]", pandas_dtype: bool = False
 ) -> "Tuple[DtypeObj, Union[ArrayLike, Series]]":
     """
     Infer the dtype from an array.

From bbd25ed2f6a97f8fa9a3c945a4ea4da943edfefd Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Fri, 11 Dec 2020 12:26:55 -0500
Subject: [PATCH 16/21] review comment

---
 pandas/core/missing.py | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 342f0a583f17d..a4d6d79ba3de7 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -2,12 +2,22 @@
 Routines for filling missing data.
 """
 from functools import partial
-from typing import TYPE_CHECKING, Any, Callable, List, Optional, Set, Tuple, Union
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Callable,
+    List,
+    Optional,
+    Sequence,
+    Set,
+    Tuple,
+    Union,
+)
 
 import numpy as np
 
 from pandas._libs import algos, lib
-from pandas._typing import ArrayLike, DtypeObj, IndexLabel, Scalar
+from pandas._typing import AnyArrayLike, ArrayLike, DtypeObj, IndexLabel, Scalar
 from pandas.compat._optional import import_optional_dependency
 
 from pandas.core.dtypes.cast import infer_dtype_from_array
@@ -20,11 +30,11 @@
 from pandas.core.dtypes.missing import isna
 
 if TYPE_CHECKING:
-    from pandas import Index, Series
+    from pandas import Index
 
 
 def mask_missing(
-    arr: ArrayLike, values_to_mask: "Union[ArrayLike, Scalar, Series]"
+    arr: ArrayLike, values_to_mask: Union[AnyArrayLike, Scalar, Sequence[Any]]
 ) -> np.ndarray:
     """
     Return a masking array of same size/shape as arr

From b505de510c0785bd6b46ad5598b6c6b4228e33ee Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 14 Dec 2020 17:14:13 -0500
Subject: [PATCH 17/21] review comment

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

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index a4d6d79ba3de7..69be8b4740c38 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -34,7 +34,7 @@
 
 
 def mask_missing(
-    arr: ArrayLike, values_to_mask: Union[AnyArrayLike, Scalar, Sequence[Any]]
+    arr: AnyArrayLike, values_to_mask: Union[AnyArrayLike, Scalar, Sequence[Any]]
 ) -> np.ndarray:
     """
     Return a masking array of same size/shape as arr

From e39c152534166c3c372a76fe503cd57d7e77c4a2 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 14 Dec 2020 17:16:21 -0500
Subject: [PATCH 18/21] docstring fix

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

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 69be8b4740c38..785538d52306b 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -419,7 +419,7 @@ def _from_derivatives(
         derivatives (that is a number equal to the number of points), or a
         list of derivatives to extract. This number includes the function
         value as 0th derivative.
-     extrapolate : bool, default False
+    extrapolate : bool, default False
         Whether to extrapolate to ouf-of-bounds points based on first and last
         intervals, or to return NaNs. Default: True.
 

From cb82c9a6263527fef0a734b959c6daf2f5af1a0f Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Mon, 14 Dec 2020 17:36:02 -0500
Subject: [PATCH 19/21] review comments

---
 pandas/core/missing.py | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/pandas/core/missing.py b/pandas/core/missing.py
index 785538d52306b..c710b45253407 100644
--- a/pandas/core/missing.py
+++ b/pandas/core/missing.py
@@ -393,7 +393,7 @@ def _interpolate_scipy_wrapper(
 def _from_derivatives(
     xi: np.ndarray,
     yi: np.ndarray,
-    x: Union[Scalar, ArrayLike],
+    x: np.ndarray,
     order: Optional[Union[int, List[int]]] = None,
     der: Union[int, List[int]] = 0,
     extrapolate: bool = False,
@@ -444,7 +444,7 @@ def _from_derivatives(
 def _akima_interpolate(
     xi: np.ndarray,
     yi: np.ndarray,
-    x: Union[Scalar, ArrayLike],
+    x: np.ndarray,
     der: int = 0,
     axis: int = 0,
 ) -> Union[Scalar, ArrayLike]:
@@ -463,7 +463,7 @@ def _akima_interpolate(
         A 1-D array of real values.  `yi`'s length along the interpolation
         axis must be equal to the length of `xi`. If N-D array, use axis
         parameter to select correct axis.
-    x : scalar or array_like
+    x : array_like
         Of length M.
     der : int, optional
         How many derivatives to extract; None for all potentially
@@ -493,7 +493,7 @@ def _akima_interpolate(
 def _cubicspline_interpolate(
     xi: np.ndarray,
     yi: np.ndarray,
-    x: Union[ArrayLike, Scalar],
+    x: np.ndarray,
     axis: int = 0,
     bc_type: Union[str, Tuple] = "not-a-knot",
     extrapolate: Optional[Union[bool, str]] = None,
@@ -512,7 +512,7 @@ def _cubicspline_interpolate(
         Array containing values of the dependent variable. It can have
         arbitrary number of dimensions, but the length along ``axis``
         (see below) must match the length of ``x``. Values must be finite.
-    x : scalar or array_like, shape (m,)
+    x : array_like, shape (m,)
     axis : int, optional
         Axis along which `y` is assumed to be varying. Meaning that for
         ``x[i]`` the corresponding values are ``np.take(y, i, axis=axis)``.

From 315822cc352327ebc772d8ca77cbb186082eec28 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Sun, 21 Feb 2021 15:06:47 -0500
Subject: [PATCH 20/21] TYP: infer_dtype_from_array

---
 pandas/core/dtypes/cast.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 733e4fb5e0a8c..0bb39255c1af8 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -48,7 +48,6 @@
     ArrayLike,
     Dtype,
     DtypeObj,
-    PandasScalar,
     Scalar,
 )
 from pandas.util._exceptions import find_stack_level
@@ -850,7 +849,7 @@ def dict_compat(d: Dict[Scalar, Scalar]) -> Dict[Scalar, Scalar]:
 
 
 def infer_dtype_from_array(
-    arr: Union[ArrayLike, Series, PandasScalar], pandas_dtype: bool = False
+    arr: AnyArrayLike, pandas_dtype: bool = False
 ) -> Tuple[DtypeObj, Union[ArrayLike, Series]]:
     """
     Infer the dtype from an array.

From df4b70ad211b896d00f5261dbe0f52271625f511 Mon Sep 17 00:00:00 2001
From: Andrew Wieteska <andrew.r.wieteska@gmail.com>
Date: Sun, 21 Feb 2021 15:07:43 -0500
Subject: [PATCH 21/21] minimize diff

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

diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index 0bb39255c1af8..fb664e4ed657e 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -898,7 +898,7 @@ def infer_dtype_from_array(
     # don't force numpy coerce with nan's
     inferred = lib.infer_dtype(arr, skipna=False)
     if inferred in ["string", "bytes", "mixed", "mixed-integer"]:
-        return np.dtype(np.object_), arr
+        return (np.dtype(np.object_), arr)
 
     arr = np.asarray(arr)
     return arr.dtype, arr