Make str unpacking check opt-in

Fixes python#13823. See also python#6406

docs/source/error_code_list2.rst

@@ -416,3 +416,24 @@ Example:
         # The following will not generate an error on either
         # Python 3.8, or Python 3.9
         42 + "testing..."  # type: ignore
+
+
+.. _code-str-unpacking:
+
+Check that ``str`` is explicitly unpacked [str-unpacking]
+---------------------------------------------------------
+
+It can sometimes be surprising that ``str`` is iterable, especially when unpacking.
+
+Example:
+
+.. code-block:: python
+
+    # Use "mypy --enable-error-code str-unpacking ..."
+
+    def print_dict(d: dict[str, str]) -> int:
+        # We meant to do d.items(), but instead we're unpacking the str keys of d
+
+        # Error: Unpacking a string is disallowed
+        for k, v in d:
+            print(k, v)

mypy/checker.py

@@ -3491,7 +3491,11 @@ def check_multi_assignment(
             self.check_multi_assignment_from_union(
                 lvalues, rvalue, rvalue_type, context, infer_lvalue_type
             )
-        elif isinstance(rvalue_type, Instance) and rvalue_type.type.fullname == "builtins.str":
+        elif (
+            self.msg.errors.is_error_code_enabled(codes.STR_UNPACKING)
+            and isinstance(rvalue_type, Instance)
+            and rvalue_type.type.fullname == "builtins.str"
+        ):
             self.msg.unpacking_strings_disallowed(context)
         else:
             self.check_multi_assignment_from_iterable(

mypy/errorcodes.py

@@ -203,6 +203,9 @@ def __hash__(self) -> int:
     "General",
     default_enabled=False,
 )
+STR_UNPACKING: Final[ErrorCode] = ErrorCode(
+    "str-unpacking", "Warn about expressions that unpack str", "General", default_enabled=False
+)
 NAME_MATCH: Final = ErrorCode(
     "name-match", "Check that type definition has consistent naming", "General"
 )

mypy/messages.py

@@ -1108,7 +1108,11 @@ def wrong_number_values_to_unpack(
             )
 
     def unpacking_strings_disallowed(self, context: Context) -> None:
-        self.fail("Unpacking a string is disallowed", context)
+        self.fail(
+            "Unpacking a string is disallowed",
+            context,
+            code=codes.STR_UNPACKING,
+        )
 
     def type_not_iterable(self, type: Type, context: Context) -> None:
         self.fail(f"{format_type(type, self.options)} object is not iterable", context)

test-data/unit/check-expressions.test

@@ -2330,3 +2330,16 @@ T = TypeVar("T")
 x: int
 x + T  # E: Unsupported operand types for + ("int" and "object")
 T()  # E: "object" not callable
+
+[case testStringDisallowedUnpacking]
+# flags: --enable-error-code str-unpacking
+from typing import Dict
+
+d: Dict[str, str]
+
+for a, b in d: # E: Unpacking a string is disallowed
+    pass
+
+s = "foo"
+a, b = s # E: Unpacking a string is disallowed
+[builtins fixtures/dict.pyi]

test-data/unit/check-unions.test

@@ -721,26 +721,11 @@ reveal_type(d) # N: Revealed type is "builtins.list[builtins.int]"
 from typing import Union
 bad: Union[int, str]
 
-x, y = bad # E: "int" object is not iterable \
-           # E: Unpacking a string is disallowed
+x, y = bad # E: "int" object is not iterable
 reveal_type(x) # N: Revealed type is "Any"
 reveal_type(y) # N: Revealed type is "Any"
 [out]
 
-[case testStringDisallowedUnpacking]
-from typing import Dict
-
-d: Dict[str, str]
-
-for a, b in d: # E: Unpacking a string is disallowed
-    pass
-
-s = "foo"
-a, b = s # E: Unpacking a string is disallowed
-
-[builtins fixtures/dict.pyi]
-[out]
-
 [case testUnionAlwaysTooMany]
 from typing import Union, Tuple
 bad: Union[Tuple[int, int, int], Tuple[str, str, str]]