Allow returning Literals in __new__

[Gobot1234]

Unblocks python/typeshed#10465

[sobolevn]

Please, add tests to this change :)

[sobolevn]

Please, also add an @overload case you are proposing to typeshed

[sobolevn]

Let's ask for an extra pair of eyes.
@ilevkivskyi would you, please? :)

[ilevkivskyi]

Looks good, added a suggestion to improve the tests.

[Gobot1234]

Somehow reveal_type(bool.__new__(bool, Falsy())) is Literal[False] but bool(Falsy()) isn't. Is there some special casing I'm not seeing here?

[ilevkivskyi]

@Gobot1234 Yes, I think you likely need to add Literal somewhere here

mypy/mypy/typeops.py

Lines 195 to 207 in cfd01d9

	if (
	isinstance(explicit_type, (Instance, TupleType, UninhabitedType))
	# We have to skip protocols, because it can be a subtype of a return type
	# by accident. Like `Hashable` is a subtype of `object`. See #11799
	and isinstance(default_ret_type, Instance)
	and not default_ret_type.type.is_protocol
	# Only use the declared return type from __new__ or declared self in __init__
	# if it is actually returning a subtype of what we would return otherwise.
	and is_subtype(explicit_type, default_ret_type, ignore_type_params=True)
	):
	ret_type: Type = explicit_type
	else:
	ret_type = default_ret_type

[Gobot1234]

@Gobot1234 Yes, I think you likely need to add Literal somewhere here

mypy/mypy/typeops.py

Lines 195 to 207 in cfd01d9

	if (
	isinstance(explicit_type, (Instance, TupleType, UninhabitedType))
	# We have to skip protocols, because it can be a subtype of a return type
	# by accident. Like `Hashable` is a subtype of `object`. See #11799
	and isinstance(default_ret_type, Instance)
	and not default_ret_type.type.is_protocol
	# Only use the declared return type from __new__ or declared self in __init__
	# if it is actually returning a subtype of what we would return otherwise.
	and is_subtype(explicit_type, default_ret_type, ignore_type_params=True)
	):
	ret_type: Type = explicit_type
	else:
	ret_type = default_ret_type

Thank you, never would have spotted that! Everything should work now

[Gobot1234]

Not entirely sure why CI is failing from my testing this seems to work fine


are the fixtures somehow causing this to fail?

[ilevkivskyi]

FWIW you are doing something weird with the fixtures. Try placing your updated definitions of bool and int in the fixture, and then add reveal_type()s directly in the test case body (not in [file builtins.py]).

[Gobot1234]

Like this?

[case testOverride__new__WithLiteralReturnPassing]
from typing import Literal

class Falsy:
    def __bool__(self) -> Literal[False]: pass

reveal_type(bool(Falsy()))  # N: Revealed type is "Literal[False]"
reveal_type(int())  # N: Revealed type is "Literal[0]"

[file builtins.py]
from typing import Literal, Protocol, overload

class str: pass
class dict: pass
class float: pass
class int:
    def __new__(cls) -> Literal[0]: pass

class _Truthy(Protocol):
    def __bool__(self) -> Literal[True]: pass

class _Falsy(Protocol):
    def __bool__(self) -> Literal[False]: pass

class bool(int):
    @overload
    def __new__(cls, __o: _Truthy) -> Literal[True]: pass
    @overload
    def __new__(cls, __o: _Falsy) -> Literal[False]: pass
    def __new__(cls, __o: object):
        pass
[typing fixtures/typing-medium.pyi]

It doesn't seem to work

[ilevkivskyi]

No, move all that stuff from [file builtins.py] into actual fixture, e.g. in test-data/unit/fixtures/__new__.pyi

[Gobot1234]

No, move all that stuff from [file builtins.py] into actual fixture, e.g. in test-data/unit/fixtures/__new__.pyi

Doing so doesn't seem to have changed anything, sorry if I'm misunderstanding

[ilevkivskyi]

Hm, to debug this, try adding reveal_type(bool) to figure out when the problem happens, during definition, or when calling it.

[Gobot1234]

bool is Any

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅