reveal_type(type(None)) is object

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.

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]"

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

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

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

This is much better now:

Revealed type is "type[None]"

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`reveal_type(type(None))` is `object` #11550

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	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

reveal_type(type(None)) is object #11550

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sobolevn commented on Nov 14, 2021

sobolevn commented on Nov 14, 2021

sobolevn commented on Nov 14, 2021

sobolevn commented on Nov 14, 2021

ilevkivskyi commented on Feb 8, 2025

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`reveal_type(type(None))` is `object` #11550