Can Python 3 copy files without overwriting?

Question

In a 2016 blogpost, it is claimed:

def notActuallySafeCopy(srcfile, destfile):
    if os.path.exists(destfile):
        raise IOError('destination already exists')
    
    shutil.copy(srcfile, destfile)

there is a race condition here -- there is a brief window of time after the check in which if a file is created at "destfile", it will be replaced.

... the Python documentation explicitly says that shutil.copy and shutil.copy2 can silently overwrite the destination file if it already exists, and there is no flag to prevent this.

Is this still true? Does Python supply a way to copy a file that does not overwrite a destination file?

Barmar · Accepted Answer · 2024-12-14 06:11:12Z

1

Open the destination with x mode, which fails if the file already exists.

def safe_copy(srcfile, destfile):
    with open(srcfile, "rb") as src, open(destfile, "wxb") as dest:
        dest.write(src.read())

Opening destfile will raise FileExistsError. See What does python3 open "x" mode do?

answered Dec 14, 2024 at 6:11

Barmar

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