Exploring generic Policy.header_fetch_parse for python#2333

stdlib/email/message.pyi

@@ -13,6 +13,7 @@ _T = TypeVar("_T")
 
 _PayloadType: TypeAlias = list[Message] | str | bytes | bytearray
 _CharsetType: TypeAlias = Charset | str | None
+# Type returned by Policy.header_fetch_parse, AnyOf[str | Header]
 _HeaderType: TypeAlias = Any
 
 class Message:
@@ -24,14 +25,15 @@ class Message:
     def set_unixfrom(self, unixfrom: str) -> None: ...
     def get_unixfrom(self) -> str | None: ...
     def attach(self, payload: Message) -> None: ...
-    def get_payload(self, i: int | None = None, decode: bool = False) -> Any: ...  # returns _PayloadType | None
+    def get_payload(self, i: int | None = None, decode: bool = False) -> Any: ...  # AnyOf[_PayloadType, None]
     def set_payload(self, payload: _PayloadType, charset: _CharsetType = None) -> None: ...
     def set_charset(self, charset: _CharsetType) -> None: ...
     def get_charset(self) -> _CharsetType: ...
     def __len__(self) -> int: ...
     def __contains__(self, name: str) -> bool: ...
     def __iter__(self) -> Iterator[str]: ...
-    def __getitem__(self, name: str) -> _HeaderType: ...
+    # Same as get with failobj=None
+    def __getitem__(self, name: str) -> _HeaderType | None: ...
     def __setitem__(self, name: str, val: _HeaderType) -> None: ...
     def __delitem__(self, name: str) -> None: ...
     def keys(self) -> list[str]: ...

stdlib/email/policy.pyi

@@ -1,14 +1,23 @@
+from _typeshed import Unused
 from abc import ABCMeta, abstractmethod
 from collections.abc import Callable
 from email.contentmanager import ContentManager
 from email.errors import MessageDefect
 from email.header import Header
+from email.headerregistry import BaseHeader
 from email.message import Message
-from typing import Any
+from typing import Any, Protocol, TypeVar, overload
 from typing_extensions import Self
 
 __all__ = ["Compat32", "compat32", "Policy", "EmailPolicy", "default", "strict", "SMTP", "HTTP"]
 
+class _HasName(Protocol):
+    name: str
+
+_HasNameT = TypeVar("_HasNameT", bound=_HasName)
+_HeaderT = TypeVar("_HeaderT", bound=Header)
+_StrBaseHeader = TypeVar("_StrBaseHeader", bound=str)  # BaseHeader matches this bound
+
 class Policy(metaclass=ABCMeta):
     max_line_length: int | None
     linesep: str
@@ -35,7 +44,7 @@ class Policy(metaclass=ABCMeta):
     @abstractmethod
     def header_store_parse(self, name: str, value: str) -> tuple[str, str]: ...
     @abstractmethod
-    def header_fetch_parse(self, name: str, value: str) -> str: ...
+    def header_fetch_parse(self, name: str, value: str) -> str | Header: ...
     @abstractmethod
     def fold(self, name: str, value: str) -> str: ...
     @abstractmethod
@@ -44,7 +53,10 @@ class Policy(metaclass=ABCMeta):
 class Compat32(Policy):
     def header_source_parse(self, sourcelines: list[str]) -> tuple[str, str]: ...
     def header_store_parse(self, name: str, value: str) -> tuple[str, str]: ...
-    def header_fetch_parse(self, name: str, value: str) -> str | Header: ...  # type: ignore[override]
+    @overload
+    def header_fetch_parse(self, name: Unused, value: _HeaderT) -> _HeaderT: ...
+    @overload
+    def header_fetch_parse(self, name: str, value: _StrBaseHeader) -> _StrBaseHeader | Header: ...
     def fold(self, name: str, value: str) -> str: ...
     def fold_binary(self, name: str, value: str) -> bytes: ...
 
@@ -71,7 +83,10 @@ class EmailPolicy(Policy):
     ) -> None: ...
     def header_source_parse(self, sourcelines: list[str]) -> tuple[str, str]: ...
     def header_store_parse(self, name: str, value: Any) -> tuple[str, Any]: ...
-    def header_fetch_parse(self, name: str, value: str) -> Any: ...
+    @overload
+    def header_fetch_parse(self, name: Unused, value: _HasNameT) -> _HasNameT: ...
+    @overload
+    def header_fetch_parse(self, name: str, value: str) -> BaseHeader: ...
     def fold(self, name: str, value: str) -> Any: ...
     def fold_binary(self, name: str, value: str) -> bytes: ...