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Atomic File Write With Temp File And Rename

An atomic file writer that stages to a temporary file in the same directory, fsyncs, and renames so readers never observe partial content.

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Code

import os
import tempfile
from pathlib import Path
from typing import Union


class AtomicWriteError(RuntimeError):
    """Raised when the atomic write could not be completed."""


def atomic_write(path: Union[str, Path], payload: Union[bytes, str], mode: int = 0o644) -> None:
    """Write payload to path atomically via same-directory temp file and rename."""
    dest = Path(path)
    parent = dest.parent
    if not parent.is_dir():
        raise AtomicWriteError(f"destination directory does not exist: {parent}")
    data = payload.encode("utf-8") if isinstance(payload, str) else payload
    fd, tmp_name = tempfile.mkstemp(dir=str(parent), prefix=f".{dest.name}.", suffix=".tmp")
    tmp = Path(tmp_name)
    try:
        with os.fdopen(fd, "wb") as handle:
            handle.write(data)
            handle.flush()
            os.fsync(handle.fileno())
        os.chmod(tmp, mode)
        os.replace(tmp, dest)
    except OSError as exc:
        tmp.unlink(missing_ok=True)
        raise AtomicWriteError(f"atomic write failed for {dest}: {exc}") from exc

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