reveal_type(type(None)) is object

[sobolevn]

It should be type, or even better NoneType.

Code sample:

reveal_type(type)
none = type(None)
reveal_type(none)

Output:

» mypy out/ex.py --show-traceback --warn-unreachable 
out/ex.py:1: note: Revealed type is "Overload(def (o: builtins.object) -> builtins.type, def (name: builtins.str, bases: builtins.tuple[builtins.type], dict: builtins.dict[builtins.str, Any], **kwds: Any) -> builtins.type)"
out/ex.py:3: note: Revealed type is "builtins.object"

Refs #11539

I will fix this. Not sure where yet: in typeshed or here.

[sobolevn]

Fun thing:

none = type(None)
reveal_type(none)  # N: Revealed type is "builtins.object"

# VS:

reveal_type(type(None))  # N: Revealed type is "Type[None]"

[sobolevn]

This difference happens because the inner mypy AST is different:

MypyFile:1(
  out/ex.py
  AssignmentStmt:1(
    NameExpr(none* [ex.none])
    TypeAliasExpr(None))

  # reveal_type(none)

  ExpressionStmt:2(
    RevealExpr:2(
      NameExpr(none [ex.none])))

  # reveal_type(type(None))

  ExpressionStmt:4(
    RevealExpr:4(
      CallExpr:4(
        NameExpr(type [builtins.type])
        Args(
          NameExpr(None [builtins.None]))))))

For some reason type(None) is turned into TypeAlias to None, which is not 100% correct. Here are some examples, where these two types are different:

>>> NoneType = type(None)

>>> NoneType()
>>> None()
<stdin>:1: SyntaxWarning: 'NoneType' object is not callable; perhaps you missed a comma?
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not callable

>>> NoneType.__bool__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor '__bool__' of 'NoneType' object needs an argument
>>> None.__bool__()
False

[sobolevn]

Here's what we need to change:

mypy/mypy/semanal.py

Lines 2246 to 2260 in f9bf649

	def is_none_alias(self, node: Expression) -> bool:
	"""Is this a r.h.s. for a None alias?
	We special case the assignments like Void = type(None), to allow using
	Void in type annotations.
	"""
	if isinstance(node, CallExpr):
	if (isinstance(node.callee, NameExpr) and len(node.args) == 1 and
	isinstance(node.args[0], NameExpr)):
	call = self.lookup_qualified(node.callee.name, node.callee)
	arg = self.lookup_qualified(node.args[0].name, node.args[0])
	if (call is not None and call.node and call.node.fullname == 'builtins.type' and
	arg is not None and arg.node and arg.node.fullname == 'builtins.None'):
	return True
	return False

[sobolevn]

This can be a breaking change, but I am going to try this anyway.