From 1be9f585a928006e6397fde17d6dce7de8011daf Mon Sep 17 00:00:00 2001
From: Omer Ozarslan <omerfaruko@gmail.com>
Date: Thu, 22 Apr 2021 06:51:57 -0500
Subject: [PATCH 1/2] TYP: Fix some typehints for ExtensionDtype

---
 pandas/core/dtypes/base.py | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/pandas/core/dtypes/base.py b/pandas/core/dtypes/base.py
index 8104b0170fbe2..81b81b0893c24 100644
--- a/pandas/core/dtypes/base.py
+++ b/pandas/core/dtypes/base.py
@@ -7,6 +7,7 @@
 from typing import (
     TYPE_CHECKING,
     Any,
+    TypeVar,
 )
 
 import numpy as np
@@ -26,6 +27,9 @@
 if TYPE_CHECKING:
     from pandas.core.arrays import ExtensionArray
 
+    # To parameterize on same ExtensionDtype
+    E = TypeVar("E", bound="ExtensionDtype")
+
 
 class ExtensionDtype:
     """
@@ -151,7 +155,7 @@ def na_value(self) -> object:
         return np.nan
 
     @property
-    def type(self) -> type[Any]:
+    def type(self) -> type_t[Any]:
         """
         The scalar type for the array, e.g. ``int``
 
@@ -209,7 +213,7 @@ def construct_array_type(cls) -> type_t[ExtensionArray]:
         raise NotImplementedError
 
     @classmethod
-    def construct_from_string(cls, string: str):
+    def construct_from_string(cls, string: str) -> ExtensionDtype:
         r"""
         Construct this type from a string.
 
@@ -364,7 +368,7 @@ def _get_common_dtype(self, dtypes: list[DtypeObj]) -> DtypeObj | None:
             return None
 
 
-def register_extension_dtype(cls: type[ExtensionDtype]) -> type[ExtensionDtype]:
+def register_extension_dtype(cls: type[E]) -> type[E]:
     """
     Register an ExtensionType with pandas as class decorator.
 
@@ -420,7 +424,7 @@ def register(self, dtype: type[ExtensionDtype]) -> None:
 
         self.dtypes.append(dtype)
 
-    def find(self, dtype: type[ExtensionDtype] | str) -> type[ExtensionDtype] | None:
+    def find(self, dtype: type[E] | E | str) -> type[E] | E | ExtensionDtype | None:
         """
         Parameters
         ----------
@@ -431,8 +435,9 @@ def find(self, dtype: type[ExtensionDtype] | str) -> type[ExtensionDtype] | None
         return the first matching dtype, otherwise return None
         """
         if not isinstance(dtype, str):
-            dtype_type = dtype
-            if not isinstance(dtype, type):
+            if isinstance(dtype, type):
+                dtype_type = dtype
+            else:
                 dtype_type = type(dtype)
             if issubclass(dtype_type, ExtensionDtype):
                 return dtype

From ac2e3e6570d19c31041b75244f6b3a9853d15f9e Mon Sep 17 00:00:00 2001
From: Omer Ozarslan <omerfaruko@gmail.com>
Date: Thu, 22 Apr 2021 08:57:58 -0500
Subject: [PATCH 2/2] Revert minor changes causing mypy issues

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

diff --git a/pandas/core/dtypes/base.py b/pandas/core/dtypes/base.py
index 81b81b0893c24..9671c340a0a92 100644
--- a/pandas/core/dtypes/base.py
+++ b/pandas/core/dtypes/base.py
@@ -213,7 +213,7 @@ def construct_array_type(cls) -> type_t[ExtensionArray]:
         raise NotImplementedError
 
     @classmethod
-    def construct_from_string(cls, string: str) -> ExtensionDtype:
+    def construct_from_string(cls, string: str):
         r"""
         Construct this type from a string.
 
@@ -424,7 +424,7 @@ def register(self, dtype: type[ExtensionDtype]) -> None:
 
         self.dtypes.append(dtype)
 
-    def find(self, dtype: type[E] | E | str) -> type[E] | E | ExtensionDtype | None:
+    def find(self, dtype: type[ExtensionDtype] | str) -> type[ExtensionDtype] | None:
         """
         Parameters
         ----------
@@ -435,9 +435,8 @@ def find(self, dtype: type[E] | E | str) -> type[E] | E | ExtensionDtype | None:
         return the first matching dtype, otherwise return None
         """
         if not isinstance(dtype, str):
-            if isinstance(dtype, type):
-                dtype_type = dtype
-            else:
+            dtype_type = dtype
+            if not isinstance(dtype, type):
                 dtype_type = type(dtype)
             if issubclass(dtype_type, ExtensionDtype):
                 return dtype