Fix str unpack type propagation, use dedicated error code (#20325)

Resolves #13823. Supersedes #15511. See also #6406

To avail of this, try `--disable-error-code str-unpack`

Author: hauntsaninja

docs/source/error_code_list.rst

@@ -1151,6 +1151,25 @@ Warn about cases where a bytes object may be converted to a string in an unexpec
     print(f"The alphabet starts with {b!r}")  # The alphabet starts with b'abc'
     print(f"The alphabet starts with {b.decode('utf-8')}")  # The alphabet starts with abc
 
+.. _code-str-unpack:
+
+Check that ``str`` is not unpacked [str-unpack]
+---------------------------------------------------------
+
+It can sometimes be surprising that ``str`` is iterable, especially when unpacking
+in an assignment.
+
+Example:
+
+.. code-block:: python
+
+    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)
+
 .. _code-overload-overlap:
 
 Check that overloaded functions don't overlap [overload-overlap]

mypy/checker.py

@@ -4115,9 +4115,9 @@ 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":
-            self.msg.unpacking_strings_disallowed(context)
         else:
+            if isinstance(rvalue_type, Instance) and rvalue_type.type.fullname == "builtins.str":
+                self.msg.unpacking_strings_disallowed(context)
             self.check_multi_assignment_from_iterable(
                 lvalues, rvalue_type, context, infer_lvalue_type
             )

mypy/errorcodes.py

@@ -215,6 +215,9 @@ def __hash__(self) -> int:
     "General",
     default_enabled=False,
 )
+STR_UNPACK: Final[ErrorCode] = ErrorCode(
+    "str-unpack", "Warn about expressions that unpack str", "General"
+)
 NAME_MATCH: Final = ErrorCode(
     "name-match", "Check that type definition has consistent naming", "General"
 )

mypy/messages.py

@@ -1144,7 +1144,7 @@ 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_UNPACK)
 
     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

@@ -2492,3 +2492,16 @@ x + T  # E: Unsupported left operand type for + ("int")
 T()  # E: "TypeVar" not callable
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testStringDisallowedUnpacking]
+d: dict[str, str]
+
+for a1, b1 in d: # E: Unpacking a string is disallowed
+    reveal_type(a1)  # N: Revealed type is "builtins.str"
+    reveal_type(b1)  # N: Revealed type is "builtins.str"
+
+s = "foo"
+a2, b2 = s # E: Unpacking a string is disallowed
+reveal_type(a2)  # N: Revealed type is "builtins.str"
+reveal_type(b2)  # N: Revealed type is "builtins.str"
+[builtins fixtures/primitives.pyi]

test-data/unit/check-unions.test

@@ -725,23 +725,10 @@ bad: Union[int, str]
 
 x, y = bad # E: "int" object is not iterable \
            # E: Unpacking a string is disallowed
-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
+reveal_type(x) # N: Revealed type is "Union[Any, builtins.str]"
+reveal_type(y) # N: Revealed type is "Union[Any, builtins.str]"
+[builtins fixtures/primitives.pyi]
 
-[builtins fixtures/dict.pyi]
-[out]
 
 [case testUnionAlwaysTooMany]
 from typing import Union, Tuple