Use X | Y union syntax in error messages

[omarsilva1]

This should fix #15082:
If the python version is set to 3.10 or later union error reports are going to be displayed using the X | Y syntax.
If None is found in a union it is shown as the last element in order to improve visibility (Union[bool, None, str] becomes "bool | str | None".
To achieve this an option use_or_syntax() is created that is set to true if the python version is 3.10 or later.

For testing a hidden flag --force-union-syntax is used that sets the option to false.

[JukkaL]

(The close/reopen was so that Black would run and hopefully fix the failing build.)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

aioredis (https://github.com/aio-libs/aioredis)
- aioredis/connection.py:455: error: Type of variable becomes "Optional[Any]" due to an unfollowed import  [no-any-unimported]
+ aioredis/connection.py:455: error: Type of variable becomes "Any | None" due to an unfollowed import  [no-any-unimported]
- aioredis/connection.py:933: error: Unsupported right operand type for in ("Union[bytes, memoryview, int]")  [operator]
+ aioredis/connection.py:933: error: Unsupported right operand type for in ("bytes | memoryview | int")  [operator]
- aioredis/connection.py:934: error: Item "memoryview" of "Union[bytes, memoryview, int]" has no attribute "split"  [union-attr]
+ aioredis/connection.py:934: error: Item "memoryview" of "bytes | memoryview | int" has no attribute "split"  [union-attr]
- aioredis/connection.py:934: error: Item "int" of "Union[bytes, memoryview, int]" has no attribute "split"  [union-attr]
+ aioredis/connection.py:934: error: Item "int" of "bytes | memoryview | int" has no attribute "split"  [union-attr]
- aioredis/client.py:4114: error: Incompatible types in assignment (expression has type "dict[Union[bytes, str, memoryview], Optional[Any]]", variable has type "dict[Union[bytes, str, memoryview], Callable[[dict[str, str]], Awaitable[None]]]")  [assignment]
+ aioredis/client.py:4114: error: Incompatible types in assignment (expression has type "dict[bytes | str | memoryview, Any | None]", variable has type "dict[bytes | str | memoryview, Callable[[dict[str, str]], Awaitable[None]]]")  [assignment]
- aioredis/client.py:4158: error: Argument 1 to "update" of "MutableMapping" has incompatible type "dict[Union[bytes, str, memoryview], Optional[Any]]"; expected "SupportsKeysAndGetItem[Union[bytes, str, memoryview], Callable[[dict[str, str]], Awaitable[None]]]"  [arg-type]
+ aioredis/client.py:4158: error: Argument 1 to "update" of "MutableMapping" has incompatible type "dict[bytes | str | memoryview, Any | None]"; expected "SupportsKeysAndGetItem[bytes | str | memoryview, Callable[[dict[str, str]], Awaitable[None]]]"  [arg-type]
- aioredis/client.py:4172: error: Incompatible types in assignment (expression has type "dict[Union[bytes, str, memoryview], Callable[[dict[str, str]], Awaitable[None]]]", variable has type "dict[Any, Optional[Any]]")  [assignment]
+ aioredis/client.py:4172: error: Incompatible types in assignment (expression has type "dict[bytes | str | memoryview, Callable[[dict[str, str]], Awaitable[None]]]", variable has type "dict[Any, Any | None]")  [assignment]

pylox (https://github.com/sco1/pylox)
- pylox/containers/array.py:84: note:     def [_T] pop(self, Any, Union[Any, _T], /) -> Union[Any, _T]
+ pylox/containers/array.py:84: note:     def [_T] pop(self, Any, Any | _T, /) -> Any | _T
- pylox/interpreter.py:160: error: Incompatible types in assignment (expression has type "Optional[Environment]", variable has type "Environment")  [assignment]
+ pylox/interpreter.py:160: error: Incompatible types in assignment (expression has type "Environment | None", variable has type "Environment")  [assignment]
- pylox/interpreter.py:235: error: Returning Any from function declared to return "Union[float, bool]"  [no-any-return]
+ pylox/interpreter.py:235: error: Returning Any from function declared to return "float | bool"  [no-any-return]
- pylox/interpreter.py:266: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:266: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:282: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:282: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:286: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:286: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:292: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:292: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:297: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:297: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:300: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:300: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:303: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:303: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:306: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:306: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:309: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:309: error: Returning Any from function declared to return "float | str | None"  [no-any-return]
- pylox/interpreter.py:312: error: Returning Any from function declared to return "Union[float, str, None]"  [no-any-return]
+ pylox/interpreter.py:312: error: Returning Any from function declared to return "float | str | None"  [no-any-return]

Auto-Split (https://github.com/Avasam/Auto-Split)
- src/capture_method/__init__.py:87: note:          def get(self, CaptureMethodEnum, /) -> Optional[Type[CaptureMethodBase]]
+ src/capture_method/__init__.py:87: note:          def get(self, CaptureMethodEnum, /) -> Type[CaptureMethodBase] | None
- src/capture_method/__init__.py:87: note:          def [_T] get(self, CaptureMethodEnum, Union[Type[CaptureMethodBase], _T], /) -> Union[Type[CaptureMethodBase], _T]
+ src/capture_method/__init__.py:87: note:          def [_T] get(self, CaptureMethodEnum, Type[CaptureMethodBase] | _T, /) -> Type[CaptureMethodBase] | _T
- src/capture_method/__init__.py:87: note:          def get(self, CaptureMethodEnum, /) -> Optional[Type[CaptureMethodBase]]
+ src/capture_method/__init__.py:87: note:          def get(self, CaptureMethodEnum, /) -> Type[CaptureMethodBase] | None
- src/capture_method/__init__.py:87: note:          def [_T] get(self, CaptureMethodEnum, /, default: Union[Type[CaptureMethodBase], _T]) -> Union[Type[CaptureMethodBase], _T]
+ src/capture_method/__init__.py:87: note:          def [_T] get(self, CaptureMethodEnum, /, default: Type[CaptureMethodBase] | _T) -> Type[CaptureMethodBase] | _T

poetry (https://github.com/python-poetry/poetry)
- src/poetry/utils/password_manager.py:69: error: Returning Any from function declared to return "Optional[str]"  [no-any-return]
+ src/poetry/utils/password_manager.py:69: error: Returning Any from function declared to return "str | None"  [no-any-return]
- src/poetry/mixology/version_solver.py:426: error: Returning Any from function declared to return "Optional[str]"  [no-any-return]
+ src/poetry/mixology/version_solver.py:426: error: Returning Any from function declared to return "str | None"  [no-any-return]
- src/poetry/mixology/version_solver.py:455: error: Returning Any from function declared to return "Optional[str]"  [no-any-return]
+ src/poetry/mixology/version_solver.py:455: error: Returning Any from function declared to return "str | None"  [no-any-return]
- src/poetry/packages/locker.py:362: error: Returning Any from function declared to return "Union[SupportsDunderLT[Any], SupportsDunderGT[Any]]"  [no-any-return]
+ src/poetry/packages/locker.py:362: error: Returning Any from function declared to return "SupportsDunderLT[Any] | SupportsDunderGT[Any]"  [no-any-return]
- src/poetry/utils/env.py:1016: error: Argument 1 to "generate_env_name" of "EnvManager" has incompatible type "Optional[str]"; expected "str"  [arg-type]
+ src/poetry/utils/env.py:1016: error: Argument 1 to "generate_env_name" of "EnvManager" has incompatible type "str | None"; expected "str"  [arg-type]
- src/poetry/puzzle/provider.py:169: error: Returning Any from function declared to return "Union[SupportsDunderLT[Any], SupportsDunderGT[Any]]"  [no-any-return]
+ src/poetry/puzzle/provider.py:169: error: Returning Any from function declared to return "SupportsDunderLT[Any] | SupportsDunderGT[Any]"  [no-any-return]
- src/poetry/console/commands/show.py:435: error: Returning Any from function declared to return "Union[SupportsDunderLT[Any], SupportsDunderGT[Any]]"  [no-any-return]
+ src/poetry/console/commands/show.py:435: error: Returning Any from function declared to return "SupportsDunderLT[Any] | SupportsDunderGT[Any]"  [no-any-return]
- src/poetry/console/commands/show.py:484: error: Returning Any from function declared to return "Union[SupportsDunderLT[Any], SupportsDunderGT[Any]]"  [no-any-return]
+ src/poetry/console/commands/show.py:484: error: Returning Any from function declared to return "SupportsDunderLT[Any] | SupportsDunderGT[Any]"  [no-any-return]

pip (https://github.com/pypa/pip)
- src/pip/_internal/metadata/_json.py:67: error: Item "None" of "Optional[list[Any]]" has no attribute "__iter__" (not iterable)  [union-attr]
+ src/pip/_internal/metadata/_json.py:67: error: Item "None" of "list[Any] | None" has no attribute "__iter__" (not iterable)  [union-attr]
- src/pip/_internal/metadata/_json.py:70: error: Argument 1 to "sanitise_header" has incompatible type "Optional[Any]"; expected "Union[Header, str]"  [arg-type]
+ src/pip/_internal/metadata/_json.py:70: error: Argument 1 to "sanitise_header" has incompatible type "Any | None"; expected "Header | str"  [arg-type]
- src/pip/_internal/network/xmlrpc.py:36: error: Argument 3 of "request" is incompatible with supertype "Transport"; supertype defines the argument type as "Union[bytes, Union[bytearray, memoryview, array[Any], mmap, _CData, PickleBuffer]]"  [override]
+ src/pip/_internal/network/xmlrpc.py:36: error: Argument 3 of "request" is incompatible with supertype "Transport"; supertype defines the argument type as "bytes | bytearray | memoryview | array[Any] | mmap | _CData | PickleBuffer"  [override]

alerta (https://github.com/alerta/alerta)
- alerta/models/key.py:53: error: Argument "expire_time" to "ApiKey" has incompatible type "Optional[datetime]"; expected "datetime"  [arg-type]
+ alerta/models/key.py:53: error: Argument "expire_time" to "ApiKey" has incompatible type "datetime | None"; expected "datetime"  [arg-type]
- alerta/models/heartbeat.py:88: error: Argument "create_time" to "Heartbeat" has incompatible type "Optional[datetime]"; expected "datetime"  [arg-type]
+ alerta/models/heartbeat.py:88: error: Argument "create_time" to "Heartbeat" has incompatible type "datetime | None"; expected "datetime"  [arg-type]
- alerta/tasks.py:23: error: Argument 4 to "process_action" has incompatible type "Optional[int]"; expected "int"  [arg-type]
+ alerta/tasks.py:23: error: Argument 4 to "process_action" has incompatible type "int | None"; expected "int"  [arg-type]
- alerta/tasks.py:24: error: Argument 3 to "from_action" of "Alert" has incompatible type "Optional[int]"; expected "int"  [arg-type]
+ alerta/tasks.py:24: error: Argument 3 to "from_action" of "Alert" has incompatible type "int | None"; expected "int"  [arg-type]
- alerta/tasks.py:38: error: Argument 3 to "from_status" of "Alert" has incompatible type "Optional[int]"; expected "int"  [arg-type]
+ alerta/tasks.py:38: error: Argument 3 to "from_status" of "Alert" has incompatible type "int | None"; expected "int"  [arg-type]

aiohttp (https://github.com/aio-libs/aiohttp)
- aiohttp/helpers.py:771: error: Value of type "Optional[list[tuple[str, str]]]" is not indexable  [index]
+ aiohttp/helpers.py:771: error: Value of type "list[tuple[str, str]] | None" is not indexable  [index]

jax (https://github.com/google/jax)
- jax/_src/numpy/linalg.py:641: error: Incompatible types in assignment (expression has type "Array", variable has type "Optional[float]")  [assignment]
+ jax/_src/numpy/linalg.py:641: error: Incompatible types in assignment (expression has type "Array", variable has type "float | None")  [assignment]

steam.py (https://github.com/Gobot1234/steam.py)
- steam/_const.py:114: error: Incompatible default for argument "unpacker" (default has type "Callable[[Union[bytes, Union[bytearray, memoryview, array[Any], mmap, _CData, PickleBuffer]], int], tuple[Any, ...]]", argument has type "Callable[[bytes], tuple[int]]")  [assignment]
+ steam/_const.py:114: error: Incompatible default for argument "unpacker" (default has type "Callable[[bytes | bytearray | memoryview | array[Any] | mmap | _CData | PickleBuffer, int], tuple[Any, ...]]", argument has type "Callable[[bytes], tuple[int]]")  [assignment]
- steam/id.py:94: error: Incompatible types in assignment (expression has type "int", variable has type "Optional[Type]")  [assignment]
+ steam/id.py:94: error: Incompatible types in assignment (expression has type "int", variable has type "Type | None")  [assignment]
- steam/id.py:95: error: Incompatible types in assignment (expression has type "int", variable has type "Optional[Universe]")  [assignment]
+ steam/id.py:95: error: Incompatible types in assignment (expression has type "int", variable has type "Universe | None")  [assignment]
- steam/id.py:98: error: Incompatible types in assignment (expression has type "int", variable has type "Optional[Instance]")  [assignment]
+ steam/id.py:98: error: Incompatible types in assignment (expression has type "int", variable has type "Instance | None")  [assignment]
- steam/id.py:100: note: Right operand is of type "Optional[Universe]"
+ steam/id.py:100: note: Right operand is of type "Universe | None"
- steam/id.py:100: note: Left operand is of type "Optional[Universe]"
+ steam/id.py:100: note: Left operand is of type "Universe | None"
- steam/id.py:102: note: Right operand is of type "Optional[Type]"
+ steam/id.py:102: note: Right operand is of type "Type | None"
- steam/id.py:102: note: Left operand is of type "Optional[Type]"
+ steam/id.py:102: note: Left operand is of type "Type | None"
- steam/id.py:104: note: Right operand is of type "Optional[Instance]"
+ steam/id.py:104: note: Right operand is of type "Instance | None"
- steam/id.py:104: note: Left operand is of type "Optional[Instance]"
+ steam/id.py:104: note: Left operand is of type "Instance | None"
- steam/id.py:114: note: Left operand is of type "Optional[Universe]"
+ steam/id.py:114: note: Left operand is of type "Universe | None"
- steam/id.py:114: note: Left operand is of type "Optional[Type]"
+ steam/id.py:114: note: Left operand is of type "Type | None"
- steam/id.py:114: note: Left operand is of type "Optional[Instance]"
+ steam/id.py:114: note: Left operand is of type "Instance | None"
- steam/id.py:507: error: Argument "instance" to "ID" has incompatible type "int"; expected "Optional[Instance]"  [arg-type]
+ steam/id.py:507: error: Argument "instance" to "ID" has incompatible type "int"; expected "Instance | None"  [arg-type]
- steam/id.py:558: error: Argument "universe" to "ID" has incompatible type "int"; expected "Optional[Universe]"  [arg-type]
+ steam/id.py:558: error: Argument "universe" to "ID" has incompatible type "int"; expected "Universe | None"  [arg-type]
- steam/id.py:558: error: Argument "instance" to "ID" has incompatible type "int"; expected "Optional[Instance]"  [arg-type]
+ steam/id.py:558: error: Argument "instance" to "ID" has incompatible type "int"; expected "Instance | None"  [arg-type]
- steam/models.py:156: error: Value of type "Optional[Match[str]]" is not indexable  [index]
+ steam/models.py:156: error: Value of type "Match[str] | None" is not indexable  [index]
- steam/models.py:157: error: Value of type "Optional[Match[str]]" is not indexable  [index]
+ steam/models.py:157: error: Value of type "Match[str] | None" is not indexable  [index]
- steam/package.py:321: error: Value of type "Optional[Match[str]]" is not indexable  [index]
+ steam/package.py:321: error: Value of type "Match[str] | None" is not indexable  [index]
- steam/game_server.py:146: error: Argument "type" to "Query" has incompatible type "Type[list[Any]]"; expected "Union[Type[list[str]], tuple[Type[list[str]], ...], None]"  [arg-type]
+ steam/game_server.py:146: error: Argument "type" to "Query" has incompatible type "Type[list[Any]]"; expected "Type[list[str]] | tuple[Type[list[str]], ...] | None"  [arg-type]
- steam/game_server.py:151: error: Argument "type" to "Query" has incompatible type "Type[list[Any]]"; expected "Union[Type[list[str]], tuple[Type[list[str]], ...], None]"  [arg-type]
+ steam/game_server.py:151: error: Argument "type" to "Query" has incompatible type "Type[list[Any]]"; expected "Type[list[str]] | tuple[Type[list[str]], ...] | None"  [arg-type]
- steam/game_server.py:156: error: Argument "type" to "Query" has incompatible type "Type[list[Any]]"; expected "Union[Type[list[str]], tuple[Type[list[str]], ...], None]"  [arg-type]
+ steam/game_server.py:156: error: Argument "type" to "Query" has incompatible type "Type[list[Any]]"; expected "Type[list[str]] | tuple[Type[list[str]], ...] | None"  [arg-type]
- steam/game_server.py:259: error: Argument "callback" to "Query" has incompatible type "Optional[Callable[[Any], Any]]"; expected "Callable[[T_co], Any]"  [arg-type]
+ steam/game_server.py:259: error: Argument "callback" to "Query" has incompatible type "Callable[[Any], Any] | None"; expected "Callable[[T_co], Any]"  [arg-type]
- steam/game_server.py:278: error: Incompatible return value type (got "Union[Query[Any], str]", expected "str")  [return-value]
+ steam/game_server.py:278: error: Incompatible return value type (got "Query[Any] | str", expected "str")  [return-value]
- steam/abc.py:196: error: Incompatible types in assignment (expression has type "steam.comment.Comment", variable has type "Optional[steam.protobufs.comments.GetCommentThreadResponse.Comment]")  [assignment]
+ steam/abc.py:196: error: Incompatible types in assignment (expression has type "steam.comment.Comment", variable has type "steam.protobufs.comments.GetCommentThreadResponse.Comment | None")  [assignment]
- steam/abc.py:556: note: Left operand is of type "Optional[datetime]"
+ steam/abc.py:556: note: Left operand is of type "datetime | None"
- steam/abc.py:556: note: Right operand is of type "Optional[datetime]"
+ steam/abc.py:556: note: Right operand is of type "datetime | None"
- steam/abc.py:749: error: No overload variant of "fetch_server" of "Client" matches argument types "Union[IPv4Address, IPv6Address]", "object"  [call-overload]
+ steam/abc.py:749: error: No overload variant of "fetch_server" of "Client" matches argument types "IPv4Address | IPv6Address", "object"  [call-overload]
- steam/abc.py:749: note:     def fetch_server(self, *, id: Union[SupportsInt, SupportsIndex, str, bytes]) -> Coroutine[Any, Any, Optional[GameServer]]
+ steam/abc.py:749: note:     def fetch_server(self, *, id: SupportsInt | SupportsIndex | str | bytes) -> Coroutine[Any, Any, GameServer | None]
- steam/abc.py:749: note:     def fetch_server(self, *, ip: Union[IPv4Address, str], port: int = ...) -> Coroutine[Any, Any, Optional[GameServer]]
+ steam/abc.py:749: note:     def fetch_server(self, *, ip: IPv4Address | str, port: int = ...) -> Coroutine[Any, Any, GameServer | None]
- steam/user.py:378: error: Argument "type" to "parse_id64" has incompatible type "int"; expected "Optional[Type]"  [arg-type]
+ steam/user.py:378: error: Argument "type" to "parse_id64" has incompatible type "int"; expected "Type | None"  [arg-type]
- steam/published_file.py:506: error: List comprehension has incompatible type List[Optional[Friend]]; expected List[Friend]  [misc]
+ steam/published_file.py:506: error: List comprehension has incompatible type List[Friend | None]; expected List[Friend]  [misc]
- steam/event.py:271: error: Incompatible types in assignment (expression has type "Union[IPv4Address, str, None]", variable has type "Optional[IPv4Address]")  [assignment]
+ steam/event.py:271: error: Incompatible types in assignment (expression has type "IPv4Address | str | None", variable has type "IPv4Address | None")  [assignment]
- steam/http.py:465: error: Argument "type" to "parse_id64" has incompatible type "int"; expected "Optional[Type]"  [arg-type]
+ steam/http.py:465: error: Argument "type" to "parse_id64" has incompatible type "int"; expected "Type | None"  [arg-type]
- steam/http.py:589: error: Item "None" of "Union[timedelta, None, Any]" has no attribute "total_seconds"  [union-attr]
+ steam/http.py:589: error: Item "None" of "timedelta | Any | None" has no attribute "total_seconds"  [union-attr]
- steam/http.py:589: error: Item "None" of "Optional[tzinfo]" has no attribute "utcoffset"  [union-attr]
+ steam/http.py:589: error: Item "None" of "tzinfo | None" has no attribute "utcoffset"  [union-attr]
- steam/http.py:599: error: Item "None" of "Optional[tzinfo]" has no attribute "utcoffset"  [union-attr]
+ steam/http.py:599: error: Item "None" of "tzinfo | None" has no attribute "utcoffset"  [union-attr]
- steam/http.py:600: error: Item "None" of "Union[timedelta, None, Any]" has no attribute "total_seconds"  [union-attr]
+ steam/http.py:600: error: Item "None" of "timedelta | Any | None" has no attribute "total_seconds"  [union-attr]
- steam/http.py:875: error: Value of type "Optional[Match[str]]" is not indexable  [index]
+ steam/http.py:875: error: Value of type "Match[str] | None" is not indexable  [index]
- steam/gateway.py:490: error: Argument "type" to "parse_id64" has incompatible type "int"; expected "Optional[Type]"  [arg-type]
+ steam/gateway.py:490: error: Argument "type" to "parse_id64" has incompatible type "int"; expected "Type | None"  [arg-type]
- steam/gateway.py:490: error: Argument "universe" to "parse_id64" has incompatible type "int"; expected "Optional[Universe]"  [arg-type]
+ steam/gateway.py:490: error: Argument "universe" to "parse_id64" has incompatible type "int"; expected "Universe | None"  [arg-type]

... (truncated 310 lines) ...

operator (https://github.com/canonical/operator)
- ops/lib/__init__.py:262: error: Item "None" of "Optional[Loader]" has no attribute "exec_module"  [union-attr]
+ ops/lib/__init__.py:262: error: Item "None" of "Loader | None" has no attribute "exec_module"  [union-attr]
- ops/pebble.py:878: error: Incompatible types in assignment (expression has type "Union[ServiceStartup, str]", variable has type "ServiceStartup")  [assignment]
+ ops/pebble.py:878: error: Incompatible types in assignment (expression has type "ServiceStartup | str", variable has type "ServiceStartup")  [assignment]
- ops/pebble.py:882: error: Incompatible types in assignment (expression has type "Union[ServiceStatus, str]", variable has type "ServiceStatus")  [assignment]
+ ops/pebble.py:882: error: Incompatible types in assignment (expression has type "ServiceStatus | str", variable has type "ServiceStatus")  [assignment]
- ops/pebble.py:1017: error: Incompatible types in assignment (expression has type "Union[FileType, str]", variable has type "FileType")  [assignment]
+ ops/pebble.py:1017: error: Incompatible types in assignment (expression has type "FileType | str", variable has type "FileType")  [assignment]
- ops/pebble.py:1085: error: Incompatible types in assignment (expression has type "Union[CheckLevel, str, None]", variable has type "CheckLevel")  [assignment]
+ ops/pebble.py:1085: error: Incompatible types in assignment (expression has type "CheckLevel | str | None", variable has type "CheckLevel")  [assignment]
- ops/pebble.py:1089: error: Incompatible types in assignment (expression has type "Union[CheckStatus, str]", variable has type "CheckStatus")  [assignment]
+ ops/pebble.py:1089: error: Incompatible types in assignment (expression has type "CheckStatus | str", variable has type "CheckStatus")  [assignment]
- ops/pebble.py:1238: error: Incompatible types in assignment (expression has type "Optional[BytesIO]", variable has type "Optional[StringIO]")  [assignment]
+ ops/pebble.py:1238: error: Incompatible types in assignment (expression has type "BytesIO | None", variable has type "StringIO | None")  [assignment]
- ops/pebble.py:1343: error: Argument 1 of "write" is incompatible with supertype "BufferedIOBase"; supertype defines the argument type as "Union[bytes, Union[bytearray, memoryview, array[Any], mmap, _CData, PickleBuffer]]"  [override]
+ ops/pebble.py:1343: error: Argument 1 of "write" is incompatible with supertype "BufferedIOBase"; supertype defines the argument type as "bytes | bytearray | memoryview | array[Any] | mmap | _CData | PickleBuffer"  [override]
- ops/pebble.py:1367: error: Return type "Union[str, bytes]" of "read" incompatible with return type "bytes" in supertype "BufferedIOBase"  [override]
+ ops/pebble.py:1367: error: Return type "str | bytes" of "read" incompatible with return type "bytes" in supertype "BufferedIOBase"  [override]
- ops/pebble.py:1367: error: Argument 1 of "read" is incompatible with supertype "BufferedIOBase"; supertype defines the argument type as "Optional[int]"  [override]
+ ops/pebble.py:1367: error: Argument 1 of "read" is incompatible with supertype "BufferedIOBase"; supertype defines the argument type as "int | None"  [override]
- ops/pebble.py:1400: error: Return type "Union[str, bytes]" of "read1" incompatible with return type "bytes" in supertype "BufferedIOBase"  [override]
+ ops/pebble.py:1400: error: Return type "str | bytes" of "read1" incompatible with return type "bytes" in supertype "BufferedIOBase"  [override]
- ops/log.py:70: error: Incompatible types in assignment (expression has type "Callable[[Type[BaseException], BaseException, TracebackType], Any]", variable has type "Callable[[Type[BaseException], BaseException, Optional[TracebackType]], Any]")  [assignment]
+ ops/log.py:70: error: Incompatible types in assignment (expression has type "Callable[[Type[BaseException], BaseException, TracebackType], Any]", variable has type "Callable[[Type[BaseException], BaseException, TracebackType | None], Any]")  [assignment]
- ops/model.py:783: note:          def get(self, str, /) -> Optional[Binding]
+ ops/model.py:783: note:          def get(self, str, /) -> Binding | None
- ops/model.py:783: note:          def [_T] get(self, str, /, default: Union[Binding, _T]) -> Union[Binding, _T]
+ ops/model.py:783: note:          def [_T] get(self, str, /, default: Binding | _T) -> Binding | _T
- ops/model.py:783: note:          def get(self, binding_key: Union[str, Relation]) -> Binding
+ ops/model.py:783: note:          def get(self, binding_key: str | Relation) -> Binding
- ops/model.py:783: note:          def get(self, str, /) -> Optional[Binding]
+ ops/model.py:783: note:          def get(self, str, /) -> Binding | None
- ops/model.py:783: note:          def [_T] get(self, str, /, default: Union[Binding, _T]) -> Union[Binding, _T]
+ ops/model.py:783: note:          def [_T] get(self, str, /, default: Binding | _T) -> Binding | _T
- ops/model.py:783: note:          def get(self, binding_key: Union[str, Relation]) -> Binding
+ ops/model.py:783: note:          def get(self, binding_key: str | Relation) -> Binding
- ops/model.py:884: error: Incompatible types in assignment (expression has type "Optional[str]", variable has type "str")  [assignment]
+ ops/model.py:884: error: Incompatible types in assignment (expression has type "str | None", variable has type "str")  [assignment]
- ops/model.py:948: error: Incompatible types in assignment (expression has type "Optional[str]", variable has type "str")  [assignment]
+ ops/model.py:948: error: Incompatible types in assignment (expression has type "str | None", variable has type "str")  [assignment]
- ops/model.py:2121: error: Argument 2 to "_list_recursive" of "Container" has incompatible type "Union[str, Path]"; expected "Path"  [arg-type]
+ ops/model.py:2121: error: Argument 2 to "_list_recursive" of "Container" has incompatible type "str | Path"; expected "Path"  [arg-type]
- ops/model.py:2196: error: Argument 2 to "_list_recursive" of "Container" has incompatible type "Union[str, Path]"; expected "Path"  [arg-type]
+ ops/model.py:2196: error: Argument 2 to "_list_recursive" of "Container" has incompatible type "str | Path"; expected "Path"  [arg-type]
- ops/model.py:2616: note:     def run(args: Union[Union[str, bytes, PathLike[str], PathLike[bytes]], Sequence[Union[str, bytes, PathLike[str], PathLike[bytes]]]], bufsize: int = ..., executable: Optional[Union[str, bytes, PathLike[str], PathLike[bytes]]] = ..., stdin: Union[None, int, IO[Any]] = ..., stdout: Union[None, int, IO[Any]] = ..., stderr: Union[None, int, IO[Any]] = ..., preexec_fn: Optional[Callable[[], Any]] = ..., close_fds: bool = ..., shell: bool = ..., cwd: Optional[Union[str, bytes, PathLike[str], PathLike[bytes]]] = ..., env: Optional[Union[Mapping[bytes, Union[str, bytes, PathLike[str], PathLike[bytes]]], Mapping[str, Union[str, bytes, PathLike[str], PathLike[bytes]]]]] = ..., universal_newlines: Optional[bool] = ..., startupinfo: Any = ..., creationflags: int = ..., restore_signals: bool = ..., start_new_session: bool = ..., pass_fds: Collection[int] = ..., *, capture_output: bool = ..., check: bool = ..., encoding: Optional[str] = ..., errors: Optional[str] = ..., input: Optional[str] = ..., text: Literal[True], timeout: Optional[float] = ..., user: Union[str, int, None] = ..., group: Union[str, int, None] = ..., extra_groups: Optional[Iterable[Union[str, int]]] = ..., umask: int = ..., pipesize: int = ...) -> CompletedProcess[str]

... (truncated 588 lines) ...```

[JukkaL]

Thanks for the PR! This looks great, and really improves usability for users targeting Python 3.10 or later.