Improve error message for partial Nones

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>].

Author: pranavrajpal

mypy/messages.py

@@ -1193,18 +1193,26 @@ def need_annotation_for_var(self, node: SymbolNode, context: Context,
                                 python_version: Optional[Tuple[int, int]] = None) -> None:
         hint = ''
         has_variable_annotations = not python_version or python_version >= (3, 6)
+        # 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
+                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

@@ -2394,7 +2394,7 @@ 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
@@ -2405,7 +2405,7 @@ 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
@@ -2454,7 +2454,7 @@ 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
@@ -2637,7 +2637,7 @@ 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)
@@ -2647,15 +2647,15 @@ 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"