TypedDict incompatible type message more understandable (#10326)

Added a section to the incompatible_argument function in mypy.messages that is executed when trying to add an incompatible type to a TypedDict. Required adding some imports from mypy.nodes to allow for the isinstance to work.

Fixes #10253

Author: Axeinator

mypy/messages.py

@@ -31,7 +31,7 @@
     FuncDef, reverse_builtin_aliases,
     ARG_POS, ARG_OPT, ARG_NAMED, ARG_NAMED_OPT, ARG_STAR, ARG_STAR2,
     ReturnStmt, NameExpr, Var, CONTRAVARIANT, COVARIANT, SymbolNode,
-    CallExpr, SymbolTable, TempNode
+    CallExpr, IndexExpr, StrExpr, SymbolTable, TempNode
 )
 from mypy.subtypes import (
     is_subtype, find_member, get_member_flags,
@@ -536,7 +536,6 @@ def incompatible_argument(self,
                 arg_name = outer_context.arg_names[n - 1]
                 if arg_name is not None:
                     arg_label = '"{}"'.format(arg_name)
-
             if (arg_kind == ARG_STAR2
                     and isinstance(arg_type, TypedDictType)
                     and m <= len(callee.arg_names)
@@ -549,9 +548,14 @@ def incompatible_argument(self,
                     expected_type,
                     bare=True)
                 arg_label = '"{}"'.format(arg_name)
-            msg = 'Argument {} {}has incompatible type {}; expected {}'.format(
-                arg_label, target, quote_type_string(arg_type_str),
-                quote_type_string(expected_type_str))
+            if isinstance(outer_context, IndexExpr) and isinstance(outer_context.index, StrExpr):
+                msg = 'Value of "{}" has incompatible type {}; expected {}' .format(
+                    outer_context.index.value, quote_type_string(arg_type_str),
+                    quote_type_string(expected_type_str))
+            else:
+                msg = 'Argument {} {}has incompatible type {}; expected {}'.format(
+                    arg_label, target, quote_type_string(arg_type_str),
+                    quote_type_string(expected_type_str))
             code = codes.ARG_TYPE
             expected_type = get_proper_type(expected_type)
             if isinstance(expected_type, UnionType):

test-data/unit/check-typeddict.test

@@ -730,7 +730,7 @@ p['x'] = 1
 from mypy_extensions import TypedDict
 TaggedPoint = TypedDict('TaggedPoint', {'type': str, 'x': int, 'y': int})
 p = TaggedPoint(type='2d', x=42, y=1337)
-p['x'] = 'y'  # E: Argument 2 has incompatible type "str"; expected "int"
+p['x'] = 'y'  # E: Value of "x" has incompatible type "str"; expected "int"
 [builtins fixtures/dict.pyi]
 
 [case testCannotSetItemOfTypedDictWithInvalidStringLiteralKey]