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Short term fix for bytes narrowing by hauntsaninja · Pull Request #20704 · python/mypy · GitHub

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Short term fix for bytes narrowing #20704

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Merged
JukkaL merged 1 commit into from
Feb 2, 2026
Conversation 4 Commits 1 Checks 22 Files changed
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Short term fix for bytes narrowing #20704

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Add tests for bytes narrowing
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hauntsaninja committed Jan 30, 2026
commit aaa0f5f82201f2f4944f73d05961731f99856420
1 change: 0 additions & 1 deletion mypy/checker.py
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Expand Up @@ -8630,7 +8630,6 @@ def reduce_and_conditional_type_maps(ms: list[TypeMap], *, use_meet: bool) -> Ty


BUILTINS_CUSTOM_EQ_CHECKS: Final = {
"builtins.bytes",
"builtins.bytearray",
"builtins.memoryview",
"builtins.list",
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77 changes: 77 additions & 0 deletions test-data/unit/check-narrowing.test
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Expand Up @@ -3389,3 +3389,80 @@ def foo(x: str | array) -> str:
return result
raise
[builtins fixtures/tuple.pyi]

[case testNarrowingOptionalBytes]
from __future__ import annotations
def f(x: bytes | None):
if x == b"asdf":
reveal_type(x) # N: Revealed type is "builtins.bytes"
else:
reveal_type(x) # N: Revealed type is "builtins.bytes | None"
[builtins fixtures/primitives.pyi]

[case testNarrowingBytesLikeWithPromotion]
# flags: --strict-equality --warn-unreachable --strict-bytes
from __future__ import annotations

def check_test(x: bytes) -> None: ...
check_test(bytearray(b"asdf")) # E: Argument 1 to "check_test" has incompatible type "bytearray"; expected "bytes"

def main(
v_bytes: bytes,
v_bytearray: bytearray,
v_memoryview: memoryview,
v_all: bytes | bytearray | memoryview,
) -> None:
if v_bytes == v_bytearray:
reveal_type(v_bytes) # N: Revealed type is "builtins.bytes"
reveal_type(v_bytearray) # N: Revealed type is "builtins.bytearray"
if v_bytes == v_memoryview:
reveal_type(v_bytes) # N: Revealed type is "builtins.bytes"
reveal_type(v_memoryview) # N: Revealed type is "builtins.memoryview"
if v_bytearray == v_memoryview:
reveal_type(v_bytearray) # N: Revealed type is "builtins.bytearray"
reveal_type(v_memoryview) # N: Revealed type is "builtins.memoryview"

if v_all == v_bytes:
reveal_type(v_all) # N: Revealed type is "builtins.bytes"
reveal_type(v_bytes) # N: Revealed type is "builtins.bytes"
if v_all == v_bytearray:
reveal_type(v_all) # N: Revealed type is "builtins.bytes | builtins.bytearray | builtins.memoryview"
reveal_type(v_bytearray) # N: Revealed type is "builtins.bytearray"
if v_all == v_memoryview:
reveal_type(v_all) # N: Revealed type is "builtins.bytes | builtins.bytearray | builtins.memoryview"
reveal_type(v_memoryview) # N: Revealed type is "builtins.memoryview"
[builtins fixtures/primitives.pyi]

[case testNarrowingBytesLikeNoPromotion]
# flags: --strict-equality --warn-unreachable --no-strict-bytes
from __future__ import annotations

def check_test(x: bytes) -> None: ...
check_test(bytearray(b"asdf"))

def main(
v_bytes: bytes,
v_bytearray: bytearray,
v_memoryview: memoryview,
v_all: bytes | bytearray | memoryview,
) -> None:
if v_bytes == v_bytearray:
reveal_type(v_bytes) # N: Revealed type is "builtins.bytes"
reveal_type(v_bytearray) # N: Revealed type is "builtins.bytearray"
if v_bytes == v_memoryview:
reveal_type(v_bytes) # N: Revealed type is "builtins.bytes"
reveal_type(v_memoryview) # N: Revealed type is "builtins.memoryview"
if v_bytearray == v_memoryview:
reveal_type(v_bytearray) # N: Revealed type is "builtins.bytearray"
reveal_type(v_memoryview) # N: Revealed type is "builtins.memoryview"

if v_all == v_bytes:
reveal_type(v_all) # N: Revealed type is "builtins.bytes"
reveal_type(v_bytes) # N: Revealed type is "builtins.bytes"
if v_all == v_bytearray:
reveal_type(v_all) # N: Revealed type is "builtins.bytes | builtins.bytearray | builtins.memoryview"
reveal_type(v_bytearray) # N: Revealed type is "builtins.bytearray"
if v_all == v_memoryview:
reveal_type(v_all) # N: Revealed type is "builtins.bytes | builtins.bytearray | builtins.memoryview"
reveal_type(v_memoryview) # N: Revealed type is "builtins.memoryview"
[builtins fixtures/primitives.pyi]
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