Python Write/Create Files

Python Write & Create Files: Safe Writing & Atomic Operations

Writing files requires careful handling of buffers, file creation flags, and atomic write operations to prevent file corruption during sudden system crashes.

File Creation Modes

• 'w': Overwrite file contents if file exists; create file if missing.
• 'a': Append to end of file; create file if missing.
• 'x': Exclusive creation; raises FileExistsError if file exists.

Practical Code Example

from pathlib import Path
import tempfile
import os

def atomic_write(target_path: Path, content: str) -> None:
    """Safely write data using a temporary file to guarantee atomic writes."""
    dir_path = target_path.parent
    dir_path.mkdir(parents=True, exist_ok=True)

    # Create temporary file in same directory
    with tempfile.NamedTemporaryFile(mode='w', dir=dir_path, delete=False, encoding='utf-8') as tmp_file:
        tmp_file.write(content)
        tmp_path = Path(tmp_file.name)

    # Atomic replace operation
    tmp_path.replace(target_path)

if __name__ == "__main__":
    out_file = Path("safe_output.json")
    atomic_write(out_file, '{"status": "complete"}')
    print(f"Atomically written file exists: {out_file.exists()}")
    print("Content:", out_file.read_text(encoding="utf-8"))
    out_file.unlink()

Best Practices & Gotchas

• Use Atomic Writes for Critical Data: Write to a temporary file in the target directory and use os.replace() to swap files atomically.
• Flush Buffers if Necessary: Call file.flush() or os.fsync(file.fileno()) if critical data must be flushed from OS cache to physical disk immediately.
• Ensure Directory Exists: Call Path(path).parent.mkdir(parents=True, exist_ok=True) before attempting to write to subdirectories.

Self-Check Challenge

Write a function append_log(msg: str) that appends timestamped log lines to app.log.