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Merged
JukkaL merged 3 commits into from
Dec 11, 2025
Merged

Implement bytes.startswith in mypyc #20387

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1185d2f
Implement bytes.startswith in mypyc
esarp Dec 9, 2025
5dbd6a9
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Dec 9, 2025
69fbc68
Optimize checks; add bytearray tests
esarp Dec 10, 2025
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Optimize checks; add bytearray tests
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@esarp
esarp committed Dec 10, 2025
commit 69fbc684df64fbb7fea0ecdbda636ff6c24302bd
9 changes: 4 additions & 5 deletions mypyc/lib-rt/bytes_ops.c
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Expand Up @@ -178,20 +178,19 @@ int CPyBytes_Startswith(PyObject *self, PyObject *subobj) {
return 1;
}

Py_ssize_t self_len = PyBytes_GET_SIZE(self);
Py_ssize_t subobj_len = PyBytes_GET_SIZE(subobj);
if (subobj_len == 0) {
return 1;
}

Py_ssize_t self_len = PyBytes_GET_SIZE(self);
if (subobj_len > self_len) {
return 0;
}

const char *self_buf = PyBytes_AS_STRING(self);
const char *subobj_buf = PyBytes_AS_STRING(subobj);

if (subobj_len == 0) {
return 1;
}

return memcmp(self_buf, subobj_buf, (size_t)subobj_len) == 0 ? 1 : 0;
}
_Py_IDENTIFIER(startswith);
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1 change: 1 addition & 0 deletions mypyc/test-data/fixtures/ir.py
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Expand Up @@ -193,6 +193,7 @@ def __add__(self, s: bytes) -> bytearray: ...
def __setitem__(self, i: int, o: int) -> None: ...
def __getitem__(self, i: int) -> int: ...
def decode(self, x: str = ..., y: str = ...) -> str: ...
def startswith(self, t: bytes) -> bool: ...

class bool(int):
def __init__(self, o: object = ...) -> None: ...
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8 changes: 8 additions & 0 deletions mypyc/test-data/run-bytes.test
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Expand Up @@ -213,6 +213,14 @@ def test_startswith() -> None:
assert b''.startswith(b'')
assert not b''.startswith(test)

@JukkaL JukkaL Dec 10, 2025

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Test with bytearray 1) as the receiver object and 2) the argument. This way we will also test the slow path.

@esarp esarp Dec 10, 2025

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Added a few checks to cover those as well


# Test bytearray to verify slow paths
assert test.startswith(bytearray(b'some'))
assert not test.startswith(bytearray(b'other'))

test = bytearray(b'some string')
assert test.startswith(b'some')
assert not test.startswith(b'other')

[case testBytesSlicing]
def test_bytes_slicing() -> None:
b = b'abcdefg'
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