Make Mapping.get(default) more constrained

[ndmitchell]

Follow up to #14347, with all the tests now passing. Does the same thing as #10294 but using the pattern from #10501 everywhere. Means that the type of Mapping.get now has a type that very clearly returns the default, rather than potentially returning a fresh type that has never been seen before. Makes Pyrefly give more precise types for os.environ.get("X", "Y").

Assuming these commits are squash merged, but can squash in advance if required (the commit history involved a lot of experimenting with CI).

[github-actions]

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

operator (https://github.com/canonical/operator)
+ ops/model.py:1025: note:          def get(self, str, /, default: Binding) -> Binding
+ ops/model.py:1025: note:          @overload
- ops/model.py:1025: note:          def [_T] get(self, str, /, default: Binding | _T) -> Binding | _T
+ ops/model.py:1025: note:          def [_T] get(self, str, /, default: _T) -> Binding | _T

starlette (https://github.com/encode/starlette)
+ starlette/requests.py:137: error: Returning Any from function declared to return "dict[str, Any]"  [no-any-return]

django-stubs (https://github.com/typeddjango/django-stubs)
+ django-stubs/utils/datastructures.pyi:65: error: Unused "type: ignore" comment  [unused-ignore]

pyodide (https://github.com/pyodide/pyodide)
+ pyodide-build/pyodide_build/xbuildenv_releases.py:220: note:     def get(self, str, /, default: str) -> str
- pyodide-build/pyodide_build/xbuildenv_releases.py:220: note:     def [_T] get(self, str, /, default: str | _T) -> str | _T
+ pyodide-build/pyodide_build/xbuildenv_releases.py:220: note:     def [_T] get(self, str, /, default: _T) -> str | _T

discord.py (https://github.com/Rapptz/discord.py)
- ...typeshed_to_test/stdlib/typing.pyi:1013: note: "update" of "TypedDict" defined here
+ ...typeshed_to_test/stdlib/typing.pyi:1015: note: "update" of "TypedDict" defined here
- ...typeshed_to_test/stdlib/typing.pyi:1013: note: "update" of "TypedDict" defined here
+ ...typeshed_to_test/stdlib/typing.pyi:1015: note: "update" of "TypedDict" defined here

core (https://github.com/home-assistant/core)
+ homeassistant/components/transmission/coordinator.py:63: error: Returning Any from function declared to return "int"  [no-any-return]
+ homeassistant/components/transmission/coordinator.py:68: error: Returning Any from function declared to return "str"  [no-any-return]
+ homeassistant/components/islamic_prayer_times/coordinator.py:57: error: Returning Any from function declared to return "str"  [no-any-return]
+ homeassistant/components/islamic_prayer_times/coordinator.py:71: error: Returning Any from function declared to return "str"  [no-any-return]
+ homeassistant/components/islamic_prayer_times/coordinator.py:76: error: Returning Any from function declared to return "str"  [no-any-return]
+ homeassistant/components/bthome/coordinator.py:48: error: Returning Any from function declared to return "bool"  [no-any-return]
+ homeassistant/components/androidtv_remote/helpers.py:28: error: Returning Any from function declared to return "bool"  [no-any-return]
+ homeassistant/components/xiaomi_ble/coordinator.py:70: error: Returning Any from function declared to return "bool"  [no-any-return]
+ homeassistant/components/unifiprotect/data.py:96: error: Returning Any from function declared to return "bool"  [no-any-return]
+ homeassistant/components/unifiprotect/data.py:101: error: Returning Any from function declared to return "int"  [no-any-return]
+ homeassistant/components/shelly/utils.py:454: error: Returning Any from function declared to return "int"  [no-any-return]
+ homeassistant/components/mikrotik/coordinator.py:86: error: Returning Any from function declared to return "bool"  [no-any-return]
+ homeassistant/components/mikrotik/coordinator.py:91: error: Returning Any from function declared to return "bool"  [no-any-return]
+ homeassistant/components/bthome/device_trigger.py:73: error: Returning Any from function declared to return "list[str]"  [no-any-return]
+ homeassistant/components/squeezebox/media_player.py:292: error: Returning Any from function declared to return "int"  [no-any-return]
+ homeassistant/components/shelly/coordinator.py:166: error: Returning Any from function declared to return "int"  [no-any-return]

steam.py (https://github.com/Gobot1234/steam.py)
+ steam/ext/commands/utils.py:43: note:          def get(self, str, /, default: _VT) -> _VT
+ steam/ext/commands/utils.py:43: note:          @overload
- steam/ext/commands/utils.py:43: note:          def [_T] get(self, str, /, default: _VT | _T) -> _VT | _T
+ steam/ext/commands/utils.py:43: note:          def [_T] get(self, str, /, default: _T) -> _VT | _T

ignite (https://github.com/pytorch/ignite)
+ ignite/handlers/visdom_logger.py:182: error: Redundant cast to "str"  [redundant-cast]

[ndmitchell]

Looking at home-assistant/core and encode/starlette these are both approximately:

def f(x: dict[str, Any]) -> int:
    return d.get("field", 0)

The error is no-any-return which seems correct - the return type should be inferred to be Any | int and mypy now complains about that.

[hauntsaninja]

Thanks, this looks good to me!

[AlexWaygood]

Thank you!