Improve error message for partial None with --local-partial-types (#…

…12822)

When --local-partial-types is set and we can't infer a complete type for
a type that we initially inferred as partial None, show an error message
that suggests to add a type annotation of the form Optional[<type>].

Co-authored-by: hauntsaninja <hauntsaninja@gmail.com>

mypy/messages.py

@@ -1526,23 +1526,30 @@ def need_annotation_for_var(
  ) -> None:
  hint = ""
  has_variable_annotations = not python_version or python_version >= (3, 6)
+ pep604_supported = not python_version or python_version >= (3, 10)
+ # type to recommend the user adds
+ recommended_type = None
  # Only gives hint if it's a variable declaration and the partial type is a builtin type
- if (
- python_version
- and isinstance(node, Var)
- and isinstance(node.type, PartialType)
- and node.type.type
- and node.type.type.fullname in reverse_builtin_aliases
- ):
- alias = reverse_builtin_aliases[node.type.type.fullname]
- alias = alias.split(".")[-1]
+ if python_version and isinstance(node, Var) and isinstance(node.type, PartialType):
  type_dec = "<type>"
- if alias == "Dict":
- type_dec = f"{type_dec}, {type_dec}"
+ if not node.type.type:
+ # partial None
+ if pep604_supported:
+ recommended_type = f"{type_dec} | None"
+ else:
+ recommended_type = f"Optional[{type_dec}]"
+ elif node.type.type.fullname in reverse_builtin_aliases:
+ # partial types other than partial None
+ alias = reverse_builtin_aliases[node.type.type.fullname]
+ alias = alias.split(".")[-1]
+ if alias == "Dict":
+ type_dec = f"{type_dec}, {type_dec}"
+ recommended_type = f"{alias}[{type_dec}]"
+ if recommended_type is not None:
  if has_variable_annotations:
- hint = f' (hint: "{node.name}: {alias}[{type_dec}] = ...")'
+ hint = f' (hint: "{node.name}: {recommended_type} = ...")'
  else:
- hint = f' (hint: "{node.name} = ... # type: {alias}[{type_dec}]")'
+ hint = f' (hint: "{node.name} = ... # type: {recommended_type}")'
 
  if has_variable_annotations:
  needed = "annotation"

test-data/unit/check-inference.test

@@ -2393,7 +2393,7 @@ if bool():
 
 [case testLocalPartialTypesWithGlobalInitializedToNone]
 # flags: --local-partial-types
-x = None # E: Need type annotation for "x"
+x = None # E: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 
 def f() -> None:
  global x
@@ -2404,7 +2404,7 @@ reveal_type(x) # N: Revealed type is "None"
 
 [case testLocalPartialTypesWithGlobalInitializedToNone2]
 # flags: --local-partial-types
-x = None # E: Need type annotation for "x"
+x = None # E: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 
 def f():
  global x
@@ -2453,7 +2453,7 @@ reveal_type(a) # N: Revealed type is "builtins.str"
 [case testLocalPartialTypesWithClassAttributeInitializedToNone]
 # flags: --local-partial-types
 class A:
- x = None # E: Need type annotation for "x"
+ x = None # E: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 
  def f(self) -> None:
  self.x = 1
@@ -2636,7 +2636,7 @@ from typing import List
 def f(x): pass
 
 class A:
- x = None # E: Need type annotation for "x"
+ x = None # E: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 
  def f(self, p: List[str]) -> None:
  self.x = f(p)
@@ -2646,15 +2646,15 @@ class A:
 [case testLocalPartialTypesAccessPartialNoneAttribute]
 # flags: --local-partial-types
 class C:
- a = None # E: Need type annotation for "a"
+ a = None # E: Need type annotation for "a" (hint: "a: Optional[<type>] = ...")
 
  def f(self, x) -> None:
  C.a.y # E: Item "None" of "Optional[Any]" has no attribute "y"
 
 [case testLocalPartialTypesAccessPartialNoneAttribute2]
 # flags: --local-partial-types
 class C:
- a = None # E: Need type annotation for "a"
+ a = None # E: Need type annotation for "a" (hint: "a: Optional[<type>] = ...")
 
  def f(self, x) -> None:
  self.a.y # E: Item "None" of "Optional[Any]" has no attribute "y"
@@ -3248,6 +3248,10 @@ if x:
  reveal_type(x) # N: Revealed type is "builtins.bytes"
 [builtins fixtures/dict.pyi]
 
+[case testSuggestPep604AnnotationForPartialNone]
+# flags: --local-partial-types --python-version 3.10
+x = None # E: Need type annotation for "x" (hint: "x: <type> | None = ...")
+
 [case testTupleContextFromIterable]
 from typing import TypeVar, Iterable, List, Union
 

test-data/unit/fine-grained.test

@@ -4305,9 +4305,9 @@ y = 0
 [file a.py.2]
 y = ''
 [out]
-main:4: error: Need type annotation for "x"
+main:4: error: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 ==
-main:4: error: Need type annotation for "x"
+main:4: error: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 
 [case testNonePartialType2]
 import a
@@ -4323,9 +4323,9 @@ y = 0
 [file a.py.2]
 y = ''
 [out]
-main:4: error: Need type annotation for "x"
+main:4: error: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 ==
-main:4: error: Need type annotation for "x"
+main:4: error: Need type annotation for "x" (hint: "x: Optional[<type>] = ...")
 
 [case testNonePartialType3]
 import a
@@ -4337,7 +4337,7 @@ def f() -> None:
  y = ''
 [out]
 ==
-a.py:1: error: Need type annotation for "y"
+a.py:1: error: Need type annotation for "y" (hint: "y: Optional[<type>] = ...")
 
 [case testNonePartialType4]
 import a
@@ -4353,7 +4353,7 @@ def f() -> None:
  global y
  y = ''
 [out]
-a.py:1: error: Need type annotation for "y"
+a.py:1: error: Need type annotation for "y" (hint: "y: Optional[<type>] = ...")
 ==
 
 [case testSkippedClass1]