Clean up and fix email message types (#13532)

* Unify the `_MessageT` type var in `email._policybase`.
* Use explicit type arguments for `Message` type in `_MessageT` type var.
  In particular, change bound from `Message[str, str]` to `Message[Any, Any]`.
* Change `__init__()` overloads of `Parser` and `BytesParser` to accept
  `Message` objects that are not `Message[str, str]` if `_class` is not also given.

stdlib/@tests/test_cases/email/check_parser.py

@@ -0,0 +1,16 @@
+import email.policy
+from email.message import EmailMessage, Message
+from email.parser import BytesParser, Parser
+from typing_extensions import assert_type
+
+p1 = Parser()
+p2 = Parser(policy=email.policy.default)
+
+assert_type(p1, Parser[Message[str, str]])
+assert_type(p2, Parser[EmailMessage])
+
+bp1 = BytesParser()
+bp2 = BytesParser(policy=email.policy.default)
+
+assert_type(bp1, BytesParser[Message[str, str]])
+assert_type(bp2, BytesParser[EmailMessage])

stdlib/email/__init__.pyi

@@ -1,6 +1,7 @@
 from collections.abc import Callable
+from email._policybase import _MessageT
 from email.message import Message
-from email.policy import Policy, _MessageT
+from email.policy import Policy
 from typing import IO, overload
 from typing_extensions import TypeAlias
 

stdlib/email/_policybase.pyi

@@ -2,12 +2,12 @@ from abc import ABCMeta, abstractmethod
 from email.errors import MessageDefect
 from email.header import Header
 from email.message import Message
-from typing import Generic, Protocol, TypeVar, type_check_only
+from typing import Any, Generic, Protocol, TypeVar, type_check_only
 from typing_extensions import Self
 
 __all__ = ["Policy", "Compat32", "compat32"]
 
-_MessageT = TypeVar("_MessageT", bound=Message, default=Message)
+_MessageT = TypeVar("_MessageT", bound=Message[Any, Any], default=Message[str, str])
 
 @type_check_only
 class _MessageFactory(Protocol[_MessageT]):
@@ -74,4 +74,4 @@ class Compat32(Policy[_MessageT]):
     def fold(self, name: str, value: str) -> str: ...
     def fold_binary(self, name: str, value: str) -> bytes: ...
 
-compat32: Compat32[Message]
+compat32: Compat32[Message[str, str]]

stdlib/email/feedparser.pyi

@@ -1,12 +1,11 @@
 from collections.abc import Callable
+from email._policybase import _MessageT
 from email.message import Message
 from email.policy import Policy
-from typing import Generic, TypeVar, overload
+from typing import Generic, overload
 
 __all__ = ["FeedParser", "BytesFeedParser"]
 
-_MessageT = TypeVar("_MessageT", bound=Message, default=Message)
-
 class FeedParser(Generic[_MessageT]):
     @overload
     def __init__(self: FeedParser[Message], _factory: None = None, *, policy: Policy[Message] = ...) -> None: ...

stdlib/email/generator.pyi

@@ -7,7 +7,7 @@ from typing_extensions import Self
 __all__ = ["Generator", "DecodedGenerator", "BytesGenerator"]
 
 # By default, generators do not have a message policy.
-_MessageT = TypeVar("_MessageT", bound=Message, default=Any)
+_MessageT = TypeVar("_MessageT", bound=Message[Any, Any], default=Any)
 
 class Generator(Generic[_MessageT]):
     maxheaderlen: int | None

stdlib/email/mime/message.pyi

@@ -1,5 +1,6 @@
+from email._policybase import _MessageT
 from email.mime.nonmultipart import MIMENonMultipart
-from email.policy import Policy, _MessageT
+from email.policy import Policy
 
 __all__ = ["MIMEMessage"]
 

stdlib/email/mime/multipart.pyi

@@ -1,7 +1,8 @@
 from collections.abc import Sequence
 from email import _ParamsType
+from email._policybase import _MessageT
 from email.mime.base import MIMEBase
-from email.policy import Policy, _MessageT
+from email.policy import Policy
 
 __all__ = ["MIMEMultipart"]
 

stdlib/email/parser.pyi

@@ -1,20 +1,21 @@
 from _typeshed import SupportsRead
 from collections.abc import Callable
+from email._policybase import _MessageT
 from email.feedparser import BytesFeedParser as BytesFeedParser, FeedParser as FeedParser
 from email.message import Message
 from email.policy import Policy
 from io import _WrappedBuffer
-from typing import Generic, TypeVar, overload
+from typing import Generic, overload
 
 __all__ = ["Parser", "HeaderParser", "BytesParser", "BytesHeaderParser", "FeedParser", "BytesFeedParser"]
 
-_MessageT = TypeVar("_MessageT", bound=Message, default=Message)
-
 class Parser(Generic[_MessageT]):
     @overload
-    def __init__(self: Parser[Message[str, str]], _class: None = None, *, policy: Policy[Message[str, str]] = ...) -> None: ...
+    def __init__(self: Parser[Message[str, str]], _class: None = None) -> None: ...
     @overload
-    def __init__(self, _class: Callable[[], _MessageT], *, policy: Policy[_MessageT] = ...) -> None: ...
+    def __init__(self, _class: None = None, *, policy: Policy[_MessageT]) -> None: ...
+    @overload
+    def __init__(self, _class: Callable[[], _MessageT] | None, *, policy: Policy[_MessageT] = ...) -> None: ...
     def parse(self, fp: SupportsRead[str], headersonly: bool = False) -> _MessageT: ...
     def parsestr(self, text: str, headersonly: bool = False) -> _MessageT: ...
 
@@ -25,9 +26,9 @@ class HeaderParser(Parser[_MessageT]):
 class BytesParser(Generic[_MessageT]):
     parser: Parser[_MessageT]
     @overload
-    def __init__(
-        self: BytesParser[Message[str, str]], _class: None = None, *, policy: Policy[Message[str, str]] = ...
-    ) -> None: ...
+    def __init__(self: BytesParser[Message[str, str]], _class: None = None) -> None: ...
+    @overload
+    def __init__(self, _class: None = None, *, policy: Policy[_MessageT]) -> None: ...
     @overload
     def __init__(self, _class: Callable[[], _MessageT], *, policy: Policy[_MessageT] = ...) -> None: ...
     def parse(self, fp: _WrappedBuffer, headersonly: bool = False) -> _MessageT: ...

stdlib/email/policy.pyi

@@ -1,14 +1,12 @@
 from collections.abc import Callable
-from email._policybase import Compat32 as Compat32, Policy as Policy, _MessageFactory, compat32 as compat32
+from email._policybase import Compat32 as Compat32, Policy as Policy, _MessageFactory, _MessageT, compat32 as compat32
 from email.contentmanager import ContentManager
-from email.message import EmailMessage, Message
-from typing import Any, TypeVar, overload
+from email.message import EmailMessage
+from typing import Any, overload
 from typing_extensions import Self
 
 __all__ = ["Compat32", "compat32", "Policy", "EmailPolicy", "default", "strict", "SMTP", "HTTP"]
 
-_MessageT = TypeVar("_MessageT", bound=Message, default=Message)
-
 class EmailPolicy(Policy[_MessageT]):
     utf8: bool
     refold_source: str

stdlib/http/client.pyi

@@ -5,6 +5,7 @@ import sys
 import types
 from _typeshed import MaybeNone, ReadableBuffer, SupportsRead, SupportsReadline, WriteableBuffer
 from collections.abc import Callable, Iterable, Iterator, Mapping
+from email._policybase import _MessageT
 from socket import socket
 from typing import BinaryIO, Literal, TypeVar, overload
 from typing_extensions import Self, TypeAlias
@@ -33,7 +34,6 @@ __all__ = [
 
 _DataType: TypeAlias = SupportsRead[bytes] | Iterable[ReadableBuffer] | ReadableBuffer
 _T = TypeVar("_T")
-_MessageT = TypeVar("_MessageT", bound=email.message.Message)
 _HeaderValue: TypeAlias = ReadableBuffer | str | int
 
 HTTP_PORT: int

stdlib/mailbox.pyi

@@ -4,6 +4,7 @@ import sys
 from _typeshed import StrPath, SupportsNoArgReadline, SupportsRead
 from abc import ABCMeta, abstractmethod
 from collections.abc import Callable, Iterable, Iterator, Mapping, Sequence
+from email._policybase import _MessageT
 from types import GenericAlias, TracebackType
 from typing import IO, Any, AnyStr, Generic, Literal, Protocol, TypeVar, overload
 from typing_extensions import Self, TypeAlias
@@ -29,7 +30,6 @@ __all__ = [
 ]
 
 _T = TypeVar("_T")
-_MessageT = TypeVar("_MessageT", bound=Message)
 
 class _SupportsReadAndReadline(SupportsRead[bytes], SupportsNoArgReadline[bytes], Protocol): ...