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Now type(None) is more consistent, refs #11550 #11552

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Now type(None) is more consistent, refs #11550 #11552

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c5e3ef3
Now `type(None)` is more consistent, refs #11550
sobolevn Nov 14, 2021
28b3930
Fixes `bool.pyi` fixture
sobolevn Nov 14, 2021
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5 changes: 5 additions & 0 deletions mypy/checkexpr.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3220,6 +3220,11 @@ class LongName(Generic[T]): ...
return type_object_type(tuple_fallback(item).type, self.named_type)
elif isinstance(item, AnyType):
return AnyType(TypeOfAny.from_another_any, source_any=item)
elif isinstance(item, NoneType):
# This is a special case, `x = type(None)` turns into
# `x = TypeAliasType(None)` during semantic analysis.
# So, we need to show the real type here. Which is `Type[None]`.
return TypeType(item)
else:
if alias_definition:
return AnyType(TypeOfAny.special_form)
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2 changes: 2 additions & 0 deletions mypy/semanal.py
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Expand Up @@ -2686,6 +2686,8 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:

res: Optional[Type] = None
if self.is_none_alias(rvalue):
# This is really `Type[None]`, not just `None`, but we use
# this as a backwards compatible solution. See #11550
res = NoneType()
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@sobolevn sobolevn Nov 14, 2021 •
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This can still cause issues, for example:

none = type(None)

print(none())  # E: error: Cannot instantiate type "Type[None]"

But, I've decided to keep this as-is (and do not change this to res = TypeType(NoneType())), because I'm affraid this can cause issues to our old users, who use type(None) for annotations.

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If we decide to change this, we can try to make Type[None] and None compatible. So, this will work:

n = type(None)
var: n = None  # will still be ok!

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I think types that are none should be annotated None. If someone tries to annotate something that is none as type(None) we should give an error and tell them to annotate it as : None (though I also expect this to be rather rare).

alias_tvars, depends_on, qualified_tvars = \
[], set(), [] # type: List[str], Set[str], List[str]
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39 changes: 39 additions & 0 deletions test-data/unit/check-type-aliases.test
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Expand Up @@ -305,6 +305,45 @@ x = y # E: Incompatible types in assignment (expression has type "Optional[int]
y = z
[builtins fixtures/bool.pyi]

[case testTypeNoneAliases]
from typing import Type

a = type(None)
b: Type[None]
c: Type[None] = type(None)
d: None
e = None

reveal_type(a) # N: Revealed type is "Type[None]"
reveal_type(b) # N: Revealed type is "Type[None]"
reveal_type(c) # N: Revealed type is "Type[None]"
reveal_type(d) # N: Revealed type is "None"
reveal_type(e) # N: Revealed type is "None"

reveal_type(type(None)) # N: Revealed type is "Type[None]"
reveal_type(None) # N: Revealed type is "None"
[builtins fixtures/bool.pyi]

[case testTypeNoneAliasesStrict]
# flags: --strict-optional
from typing import Type

a = type(None)
b: Type[None]
c: Type[None] = type(None)
d: None
e = None

reveal_type(a) # N: Revealed type is "Type[None]"
reveal_type(b) # N: Revealed type is "Type[None]"
reveal_type(c) # N: Revealed type is "Type[None]"
reveal_type(d) # N: Revealed type is "None"
reveal_type(e) # N: Revealed type is "None"

reveal_type(type(None)) # N: Revealed type is "Type[None]"
reveal_type(None) # N: Revealed type is "None"
[builtins fixtures/bool.pyi]

[case testAliasToTupleAndCallable]
from typing import Callable, Tuple
C = Callable
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5 changes: 3 additions & 2 deletions test-data/unit/fixtures/bool.pyi
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@@ -1,13 +1,14 @@
# builtins stub used in boolean-related test cases.
from typing import Generic, TypeVar
from typing import Generic, TypeVar, Union
T = TypeVar('T')

class object:
def __init__(self) -> None: pass
def __eq__(self, other: object) -> bool: pass
def __ne__(self, other: object) -> bool: pass

class type: pass
class type:
def __init__(self, obj: Union[object, None]) -> None: pass
class tuple(Generic[T]): pass
class function: pass
class int: pass
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