helpful-string improvements

CHANGELOG.md

@@ -8,6 +8,8 @@
 - Enable stub mode within `TYPE_CHECKING` branches (#702)
 - Infer from overloads - add default value in impl (#697)
 - Warn for missing returns with explicit `Any` return types (#715)
+- Added `--helpful-string-allow-none` to allow `None` with `helpful-string` (#717)
+- Enabled `helpful-string` by default (#717)
 ### Fixes
 - positional arguments on overloads break super (#697)
 - positional arguments on overloads duplicate unions (#697)

mypy/checkstrformat.py

@@ -51,6 +51,7 @@
     TypedDictType,
     TypeOfAny,
     TypeStrVisitor,
+    TypeVarLikeType,
     TypeVarTupleType,
     TypeVarType,
     UnionType,
@@ -442,6 +443,13 @@ def check_specs_in_format_call(
     def helpful_check(self, actual_type: ProperType, context: Context) -> bool:
         if isinstance(actual_type, (TupleType, TypedDictType, LiteralType)):
             return True
+        if isinstance(actual_type, TypeVarType):
+            if actual_type.values:
+                for value in actual_type.values:
+                    self.helpful_check(get_proper_type(value), context)
+                return False
+        while isinstance(actual_type, TypeVarLikeType):
+            actual_type = actual_type.upper_bound
         bad_builtin = False
         if isinstance(actual_type, Instance):
             if "dataclass" in actual_type.type.metadata:
@@ -464,13 +472,11 @@ def helpful_check(self, actual_type: ProperType, context: Context) -> bool:
                     if base.module_name == "builtins":
                         return True
         type_string = actual_type.accept(TypeStrVisitor(options=self.chk.options))
-        if (
-            custom_special_method(actual_type, "__format__")
-            or custom_special_method(actual_type, "__str__")
-            or custom_special_method(actual_type, "__repr__")
-        ):
+        if custom_special_method(actual_type, ("__format__", "__str__", "__repr__")):
             return True
-        if bad_builtin or isinstance(actual_type, NoneType):
+        if bad_builtin or (
+            not self.msg.options.helpful_string_allow_none and isinstance(actual_type, NoneType)
+        ):
             self.msg.fail(
                 f'The string for "{type_string}" isn\'t helpful in a user-facing or semantic string',
                 context,

mypy/errorcodes.py

@@ -130,7 +130,7 @@ def __hash__(self) -> int:
     "str-format", "Check that string formatting/interpolation is type-safe", "General"
 )
 HELPFUL_STRING: Final = ErrorCode(
-    "helpful-string", "Check that string conversions are useful", "General", default_enabled=False
+    "helpful-string", "Check that string conversions are useful", "General"
 )
 STR_BYTES_PY3: Final = ErrorCode(
     "str-bytes-safe", "Warn about implicit coercions related to bytes and string types", "General"

mypy/main.py

@@ -619,6 +619,12 @@ def add_invertible_flag(
         "You probably want to set this on a module override",
         group=based_group,
     )
+    add_invertible_flag(
+        "--helpful-string-allow-none",
+        default=False,
+        help="Allow Nones to appear in f-strings",
+        group=based_group,
+    )
     add_invertible_flag(
         "--ide", default=False, help="Best default for IDE integration.", group=based_group
     )
@@ -1351,6 +1357,7 @@ def add_invertible_flag(
             "callable-functiontype",
             "possible-function",
             "bad-cast",
+            "helpful-string",
         }
         if mypy.options._based
         else set()

mypy/options.py

@@ -43,6 +43,7 @@ class BuildType:
     "extra_checks",
     "follow_imports_for_stubs",
     "follow_imports",
+    "helpful_string_allow_none",
     "ignore_errors",
     "ignore_missing_imports",
     "ignore_missing_py_typed",
@@ -175,6 +176,7 @@ def __init__(self) -> None:
         self.bare_literals = True
         self.ignore_missing_py_typed = False
         self.ide = False
+        self.helpful_string_allow_none = False
 
         # disallow_any options
         self.disallow_any_generics = flip_if_not_based(True)

mypy/typeops.py

@@ -1078,11 +1078,13 @@ def visit_type_var_tuple(self, t: TypeVarTupleType) -> list[TypeVarLikeType]:
         return [t] if self.include_all else []
 
 
-def custom_special_method(typ: Type, name: str, check_all: bool = False) -> bool:
+def custom_special_method(typ: Type, name: str | Iterable[str], check_all: bool = False) -> bool:
     """Does this type have a custom special method such as __format__() or __eq__()?
 
     If check_all is True ensure all items of a union have a custom method, not just some.
     """
+    if not isinstance(name, str):
+        return any(custom_special_method(typ, n, check_all) for n in name)
     typ = get_proper_type(typ)
     if isinstance(typ, Instance):
         method = typ.type.get(name)
@@ -1104,6 +1106,8 @@ def custom_special_method(typ: Type, name: str, check_all: bool = False) -> bool
     if isinstance(typ, AnyType):
         # Avoid false positives in uncertain cases.
         return True
+    if isinstance(typ, TypeVarLikeType):
+        return custom_special_method(typ.upper_bound, name, check_all)
     # TODO: support other types (see ExpressionChecker.has_member())?
     return False
 

test-data/unit/check-based-format.test

@@ -129,3 +129,8 @@ class D:
 data: D
 f"{data}"
 [builtins fixtures/primitives.pyi]
+
+
+[case testHelpfulStringAllowNone]
+# flags: --helpful-string-allow-none
+f"{None}"