Improve message when an annotation is expected (python#6782)

Fixes python#2463

Improve error message when partial types are found for built-in types `Dict`, `List`, `Set`, `FrozenSet`.

Messages used to be like:
```
main:3: error: Need type annotation for 'x'
```
This PR modifies it to look like:
```
main:3: error: Need type annotation for 'x' (hint: "x: List[<type>] = ...")
```

In case the variable is not of a built-in type the hints are not shown.

mypy/checker.py

@@ -2496,13 +2496,13 @@ def infer_variable_type(self, name: Var, lvalue: Lvalue,
             # partial type which will be made more specific later. A partial type
             # gets generated in assignment like 'x = []' where item type is not known.
             if not self.infer_partial_type(name, lvalue, init_type):
-                self.msg.need_annotation_for_var(name, context)
+                self.msg.need_annotation_for_var(name, context, self.options.python_version)
                 self.set_inference_error_fallback_type(name, lvalue, init_type, context)
         elif (isinstance(lvalue, MemberExpr) and self.inferred_attribute_types is not None
               and lvalue.def_var and lvalue.def_var in self.inferred_attribute_types
               and not is_same_type(self.inferred_attribute_types[lvalue.def_var], init_type)):
             # Multiple, inconsistent types inferred for an attribute.
-            self.msg.need_annotation_for_var(name, context)
+            self.msg.need_annotation_for_var(name, context, self.options.python_version)
             name.type = AnyType(TypeOfAny.from_error)
         else:
             # Infer type of the target.
@@ -3724,7 +3724,7 @@ def enter_partial_types(self, *, is_function: bool = False,
                     var.type = NoneType()
                 else:
                     if var not in self.partial_reported and not permissive:
-                        self.msg.need_annotation_for_var(var, context)
+                        self.msg.need_annotation_for_var(var, context, self.options.python_version)
                         self.partial_reported.add(var)
                     if var.type:
                         var.type = self.fixup_partial_type(var.type)
@@ -3748,7 +3748,8 @@ def handle_partial_var_type(
                 if in_scope:
                     context = partial_types[node]
                     if is_local or not self.options.allow_untyped_globals:
-                        self.msg.need_annotation_for_var(node, context)
+                        self.msg.need_annotation_for_var(node, context,
+                                                         self.options.python_version)
                 else:
                     # Defer the node -- we might get a better type in the outer scope
                     self.handle_cannot_determine_type(node.name(), context)

mypy/messages.py

@@ -21,14 +21,15 @@
 from mypy.types import (
     Type, CallableType, Instance, TypeVarType, TupleType, TypedDictType, LiteralType,
     UnionType, NoneType, AnyType, Overloaded, FunctionLike, DeletedType, TypeType,
-    UninhabitedType, TypeOfAny, ForwardRef, UnboundType
+    UninhabitedType, TypeOfAny, ForwardRef, UnboundType, PartialType
 )
 from mypy.nodes import (
     TypeInfo, Context, MypyFile, op_methods, FuncDef, reverse_builtin_aliases,
     ARG_POS, ARG_OPT, ARG_NAMED, ARG_NAMED_OPT, ARG_STAR, ARG_STAR2,
     ReturnStmt, NameExpr, Var, CONTRAVARIANT, COVARIANT, SymbolNode,
     CallExpr
 )
+from mypy.defaults import PYTHON3_VERSION
 from mypy.util import unmangle
 from mypy import message_registry
 
@@ -1077,8 +1078,22 @@ def unimported_type_becomes_any(self, prefix: str, typ: Type, ctx: Context) -> N
         self.fail("{} becomes {} due to an unfollowed import".format(prefix, self.format(typ)),
                   ctx)
 
-    def need_annotation_for_var(self, node: SymbolNode, context: Context) -> None:
-        self.fail("Need type annotation for '{}'".format(unmangle(node.name())), context)
+    def need_annotation_for_var(self, node: SymbolNode, context: Context,
+                                python_version: Optional[Tuple[int, int]] = None) -> None:
+        hint = ''
+        # 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]
+            type_dec = '<type>'
+            if alias == 'Dict':
+                type_dec = '{}, {}'.format(type_dec, type_dec)
+            if python_version < (3, 6):
+                hint = ' (hint: "{} = ...  # type: {}[{}]")'.format(node.name(), alias, type_dec)
+            else:
+                hint = ' (hint: "{}: {}[{}] = ...")'.format(node.name(), alias, type_dec)
+        self.fail("Need type annotation for '{}'{}".format(unmangle(node.name()), hint), context)
 
     def explicit_any(self, ctx: Context) -> None:
         self.fail('Explicit "Any" is not allowed', ctx)

mypy/test/data.py

@@ -513,12 +513,14 @@ def expand_errors(input: List[str], output: List[str], fnam: str) -> None:
                 elif m.group(1) == 'W':
                     severity = 'warning'
                 col = m.group('col')
+                message = m.group('message')
+                message = message.replace('\\#', '#')  # adds back escaped # character
                 if col is None:
                     output.append(
-                        '{}:{}: {}: {}'.format(fnam, i + 1, severity, m.group('message')))
+                        '{}:{}: {}: {}'.format(fnam, i + 1, severity, message))
                 else:
                     output.append('{}:{}:{}: {}: {}'.format(
-                        fnam, i + 1, col, severity, m.group('message')))
+                        fnam, i + 1, col, severity, message))
 
 
 def fix_win_path(line: str) -> str:

runtests.py

@@ -34,7 +34,7 @@
                 STUBGEN_PY]
 
 # We split the pytest run into three parts to improve test
-# parallelization. Each run should have tests that each take a roughly similiar
+# parallelization. Each run should have tests that each take a roughly similar
 # time to run.
 cmds = {
     # Self type check

test-data/unit/README.md

@@ -30,6 +30,8 @@ Add the test in this format anywhere in the file:
 with text "abc..."
 - note a space after `E:` and `flags:`
 - `# E:12` adds column number to the expected error
+- use `\` to escape the `#` character and indicate that the rest of the line is part of 
+the error message
 - repeating `# E: ` several times in one line indicates multiple expected errors in one line
 - `W: ...` and `N: ...` works exactly like `E:`, but report a warning and a note respectively
 - lines that don't contain the above should cause no type check errors

test-data/unit/check-classes.test

@@ -912,7 +912,7 @@ class C:
 x = C.x
 [builtins fixtures/list.pyi]
 [out]
-main:2: error: Need type annotation for 'x'
+main:2: error: Need type annotation for 'x' (hint: "x: List[<type>] = ...")
 
 [case testAccessingGenericClassAttribute]
 from typing import Generic, TypeVar

test-data/unit/check-expressions.test

@@ -1714,7 +1714,7 @@ d5 = dict(a=1, b='') # type: Dict[str, Any]
 
 [case testDictWithoutKeywordArgs]
 from typing import Dict
-d = dict() # E: Need type annotation for 'd'
+d = dict() # E: Need type annotation for 'd' (hint: "d: Dict[<type>, <type>] = ...")
 d2 = dict() # type: Dict[int, str]
 dict(undefined) # E: Name 'undefined' is not defined
 [builtins fixtures/dict.pyi]
@@ -1953,8 +1953,8 @@ a.__pow__() # E: Too few arguments for "__pow__" of "int"
 [builtins fixtures/ops.pyi]
 
 [case testTypeAnnotationNeededMultipleAssignment]
-x, y = [], [] # E: Need type annotation for 'x' \
-            # E: Need type annotation for 'y'
+x, y = [], [] # E: Need type annotation for 'x' (hint: "x: List[<type>] = ...") \
+            # E: Need type annotation for 'y' (hint: "y: List[<type>] = ...")
 [builtins fixtures/list.pyi]
 
 [case testStrictEqualityEq]

test-data/unit/check-functions.test

@@ -1514,7 +1514,7 @@ if g(C()):
     def f(x: B) -> B: pass
 
 [case testRedefineFunctionDefinedAsVariableInitializedToEmptyList]
-f = [] # E: Need type annotation for 'f'
+f = [] # E: Need type annotation for 'f' (hint: "f: List[<type>] = ...")
 if object():
     def f(): pass # E: Incompatible redefinition
 f()  # E: "List[Any]" not callable
@@ -2320,7 +2320,7 @@ def make_list() -> List[T]: pass
 
 l: List[int] = make_list()
 
-bad = make_list()  # E: Need type annotation for 'bad'
+bad = make_list()  # E: Need type annotation for 'bad' (hint: "bad: List[<type>] = ...")
 [builtins fixtures/list.pyi]
 
 [case testAnonymousArgumentError]

test-data/unit/check-inference-context.test

@@ -425,7 +425,7 @@ class B(A): pass
 [case testLocalVariableInferenceFromEmptyList]
 import typing
 def f() -> None:
-    a = []     # E: Need type annotation for 'a'
+    a = []     # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
     b = [None]
     c = [B()]
     if int():

test-data/unit/check-inference.test

@@ -1375,20 +1375,20 @@ a.append(0)  # E: Argument 1 to "append" of "list" has incompatible type "int";
 [out]
 
 [case testInferListInitializedToEmptyAndNotAnnotated]
-a = []  # E: Need type annotation for 'a'
+a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 [builtins fixtures/list.pyi]
 [out]
 
 [case testInferListInitializedToEmptyAndReadBeforeAppend]
-a = []  # E: Need type annotation for 'a'
+a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 if a: pass
 a.xyz  # E: "List[Any]" has no attribute "xyz"
 a.append('')
 [builtins fixtures/list.pyi]
 [out]
 
 [case testInferListInitializedToEmptyAndIncompleteTypeInAppend]
-a = [] # E: Need type annotation for 'a'
+a = [] # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 a.append([])
 a()  # E: "List[Any]" not callable
 [builtins fixtures/list.pyi]
@@ -1413,7 +1413,7 @@ def f() -> None:
 
 [case testInferListInitializedToEmptyAndNotAnnotatedInFunction]
 def f() -> None:
-    a = []  # E: Need type annotation for 'a'
+    a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 
 def g() -> None: pass
 
@@ -1424,7 +1424,7 @@ a.append(1)
 
 [case testInferListInitializedToEmptyAndReadBeforeAppendInFunction]
 def f() -> None:
-    a = []  # E: Need type annotation for 'a'
+    a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
     if a: pass
     a.xyz  # E: "List[Any]" has no attribute "xyz"
     a.append('')
@@ -1441,7 +1441,7 @@ class A:
 
 [case testInferListInitializedToEmptyAndNotAnnotatedInClassBody]
 class A:
-    a = []  # E: Need type annotation for 'a'
+    a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 
 class B:
     a = []
@@ -1461,15 +1461,15 @@ class A:
 [case testInferListInitializedToEmptyAndNotAnnotatedInMethod]
 class A:
     def f(self) -> None:
-        a = []  # E: Need type annotation for 'a'
+        a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 [builtins fixtures/list.pyi]
 [out]
 
 [case testInferListInitializedToEmptyInMethodViaAttribute]
 class A:
     def f(self) -> None:
         # Attributes aren't supported right now.
-        self.a = [] # E: Need type annotation for 'a'
+        self.a = [] # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
         self.a.append(1)
         self.a.append('')
 [builtins fixtures/list.pyi]
@@ -1480,7 +1480,7 @@ from typing import List
 
 class A:
     def __init__(self) -> None:
-        self.x = [] # E: Need type annotation for 'x'
+        self.x = [] # E: Need type annotation for 'x' (hint: "x: List[<type>] = ...")
 
 class B(A):
     # TODO?: This error is kind of a false positive, unfortunately
@@ -1526,16 +1526,16 @@ a() # E: "Dict[str, int]" not callable
 [out]
 
 [case testInferDictInitializedToEmptyUsingUpdateError]
-a = {}  # E: Need type annotation for 'a'
+a = {}  # E: Need type annotation for 'a' (hint: "a: Dict[<type>, <type>] = ...")
 a.update([1, 2])  # E: Argument 1 to "update" of "dict" has incompatible type "List[int]"; expected "Mapping[Any, Any]"
 a()  # E: "Dict[Any, Any]" not callable
 [builtins fixtures/dict.pyi]
 [out]
 
 [case testInferDictInitializedToEmptyAndIncompleteTypeInUpdate]
-a = {} # E: Need type annotation for 'a'
+a = {} # E: Need type annotation for 'a' (hint: "a: Dict[<type>, <type>] = ...")
 a[1] = {}
-b = {} # E: Need type annotation for 'b'
+b = {} # E: Need type annotation for 'b' (hint: "b: Dict[<type>, <type>] = ...")
 b[{}] = 1
 [builtins fixtures/dict.pyi]
 [out]
@@ -1555,14 +1555,14 @@ def add():
 [case testSpecialCaseEmptyListInitialization]
 def f(blocks: Any): # E: Name 'Any' is not defined \
                     # N: Did you forget to import it from "typing"? (Suggestion: "from typing import Any")
-    to_process = [] # E: Need type annotation for 'to_process'
+    to_process = [] # E: Need type annotation for 'to_process' (hint: "to_process: List[<type>] = ...")
     to_process = list(blocks)
 [builtins fixtures/list.pyi]
 [out]
 
 [case testSpecialCaseEmptyListInitialization2]
 def f(blocks: object):
-    to_process = [] # E: Need type annotation for 'to_process'
+    to_process = [] # E: Need type annotation for 'to_process' (hint: "to_process: List[<type>] = ...")
     to_process = list(blocks) # E: No overload variant of "list" matches argument type "object" \
                               # N: Possible overload variant: \
                               # N:     def [T] __init__(self, x: Iterable[T]) -> List[T] \
@@ -1621,7 +1621,7 @@ x.append('') # E: Argument 1 to "append" of "list" has incompatible type "str";
 x = None
 if object():
     # Promote from partial None to partial list.
-    x = []  # E: Need type annotation for 'x'
+    x = []  # E: Need type annotation for 'x' (hint: "x: List[<type>] = ...")
     x
 [builtins fixtures/list.pyi]
 
@@ -1630,7 +1630,7 @@ def f() -> None:
     x = None
     if object():
         # Promote from partial None to partial list.
-        x = []  # E: Need type annotation for 'x'
+        x = []  # E: Need type annotation for 'x' (hint: "x: List[<type>] = ...")
 [builtins fixtures/list.pyi]
 [out]
 
@@ -1716,7 +1716,7 @@ class A:
             pass
 [builtins fixtures/for.pyi]
 [out]
-main:3: error: Need type annotation for 'x'
+main:3: error: Need type annotation for 'x' (hint: "x: List[<type>] = ...")
 
 [case testPartialTypeErrorSpecialCase3]
 class A:
@@ -1882,9 +1882,9 @@ o = 1
 
 [case testMultipassAndPartialTypesSpecialCase3]
 def f() -> None:
-    x = {} # E: Need type annotation for 'x'
+    x = {} # E: Need type annotation for 'x' (hint: "x: Dict[<type>, <type>] = ...")
     y = o
-    z = {} # E: Need type annotation for 'z'
+    z = {} # E: Need type annotation for 'z' (hint: "z: Dict[<type>, <type>] = ...")
 o = 1
 [builtins fixtures/dict.pyi]
 [out]
@@ -2070,7 +2070,7 @@ main:4: error: Invalid type: try using Literal[0] instead?
 class C:
     x = None
     def __init__(self) -> None:
-        self.x = []  # E: Need type annotation for 'x'
+        self.x = []  # E: Need type annotation for 'x' (hint: "x: List[<type>] = ...")
 [builtins fixtures/list.pyi]
 [out]
 
@@ -2246,7 +2246,7 @@ class A:
 [case testLocalPartialTypesWithClassAttributeInitializedToEmptyDict]
 # flags: --local-partial-types
 class A:
-    x = {}  # E: Need type annotation for 'x'
+    x = {}  # E: Need type annotation for 'x' (hint: "x: Dict[<type>, <type>] = ...")
 
     def f(self) -> None:
         self.x[0] = ''
@@ -2269,7 +2269,7 @@ reveal_type(a) # E: Revealed type is 'builtins.list[builtins.int]'
 
 [case testLocalPartialTypesWithGlobalInitializedToEmptyList2]
 # flags: --local-partial-types
-a = [] # E: Need type annotation for 'a'
+a = [] # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 
 def f() -> None:
     a.append(1)
@@ -2280,7 +2280,7 @@ reveal_type(a) # E: Revealed type is 'builtins.list[Any]'
 
 [case testLocalPartialTypesWithGlobalInitializedToEmptyList3]
 # flags: --local-partial-types
-a = [] # E: Need type annotation for 'a'
+a = [] # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 
 def f():
     a.append(1)
@@ -2302,7 +2302,7 @@ reveal_type(a) # E: Revealed type is 'builtins.dict[builtins.int, builtins.str]'
 
 [case testLocalPartialTypesWithGlobalInitializedToEmptyDict2]
 # flags: --local-partial-types
-a = {} # E: Need type annotation for 'a'
+a = {} # E: Need type annotation for 'a' (hint: "a: Dict[<type>, <type>] = ...")
 
 def f() -> None:
     a[0] = ''
@@ -2313,7 +2313,7 @@ reveal_type(a) # E: Revealed type is 'builtins.dict[Any, Any]'
 
 [case testLocalPartialTypesWithGlobalInitializedToEmptyDict3]
 # flags: --local-partial-types
-a = {} # E: Need type annotation for 'a'
+a = {} # E: Need type annotation for 'a' (hint: "a: Dict[<type>, <type>] = ...")
 
 def f():
     a[0] = ''

test-data/unit/check-python2.test

@@ -363,3 +363,8 @@ def foo(
 [case testNoneHasNoBoolInPython2]
 none = None
 b = none.__bool__() # E: "None" has no attribute "__bool__"
+
+[case testDictWithoutTypeCommentInPython2]
+# flags: --py2
+d = dict() # E: Need type annotation for 'd' (hint: "d = ...  \# type: Dict[<type>, <type>]")
+[builtins_py2 fixtures/floatdict_python2.pyi]

test-data/unit/check-tuples.test

@@ -511,8 +511,8 @@ d, e = f, g, h = 1, 1 # E: Need more than 2 values to unpack (3 expected)
 [case testAssignmentToStarMissingAnnotation]
 from typing import List
 t = 1, 2
-a, b, *c = 1, 2  # E: Need type annotation for 'c'
-aa, bb, *cc = t  # E: Need type annotation for 'cc'
+a, b, *c = 1, 2  # E: Need type annotation for 'c' (hint: "c: List[<type>] = ...")
+aa, bb, *cc = t  # E: Need type annotation for 'cc' (hint: "cc: List[<type>] = ...")
 [builtins fixtures/list.pyi]
 
 [case testAssignmentToStarAnnotation]