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

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Merged
hauntsaninja merged 4 commits into from
Dec 2, 2025
Merged

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

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ea73237
error code
hauntsaninja Nov 29, 2025
e3063bf
preserve types
hauntsaninja Nov 29, 2025
778f8b7
fix fixtures
hauntsaninja Nov 29, 2025
e680011
add docs
hauntsaninja Nov 29, 2025
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error code
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@hauntsaninja
hauntsaninja committed Nov 29, 2025
commit ea7323771d478374cbe011cf22af8ce7ab013a17
3 changes: 3 additions & 0 deletions mypy/errorcodes.py
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Expand Up @@ -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"
)
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2 changes: 1 addition & 1 deletion mypy/messages.py
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Expand Up @@ -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)
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17 changes: 17 additions & 0 deletions test-data/unit/check-expressions.test
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Expand Up @@ -2492,3 +2492,20 @@ 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) # E: Cannot determine type of "a1" \
# N: Revealed type is "Any"
reveal_type(b1) # E: Cannot determine type of "b1" \
# N: Revealed type is "Any"

s = "foo"
a2, b2 = s # E: Unpacking a string is disallowed
reveal_type(a2) # E: Cannot determine type of "a2" \
# N: Revealed type is "Any"
reveal_type(b2) # E: Cannot determine type of "b2" \
# N: Revealed type is "Any"
[builtins fixtures/dict.pyi]
14 changes: 0 additions & 14 deletions test-data/unit/check-unions.test
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Expand Up @@ -729,20 +729,6 @@ 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]]
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