Efficiently copying a file

TLDR: Use shutil.copyfileobj(fsrc, fdst)

I'm writing a Datasette plugin that handles an uploaded file, borrowing the Starlette mechanism for handling file uploads, documented here.

Starlette uploads result in a SpooledTemporaryFile file-like object. These look very much like a file, with one frustrating limitation: they don't have a defined, stable path on disk.

I thought that this meant that you couldn't easily copy it, and ended up coming up with this recipe based on this code I spotted in the BáiZé framework:

from shutil import COPY_BUFSIZE

with open(new_filepath, "wb+") as target_file:
    source_read = source_file.read
    target_write = target_file.write
    while True:
        buf = source_read(COPY_BUFSIZE)
        if not buf:

COPY_BUFSIZE defined by Python here - it handles the difference in ideal buffer size between Windows and other operating systems:

COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 64 * 1024

But then I sat down to write this TIL, and stumbled across shutil.copyfileobj(fsrc, fdst) in the standard library which implements the exact same pattern!

def copyfileobj(fsrc, fdst, length=0):
    """copy data from file-like object fsrc to file-like object fdst"""
    # Localize variable access to minimize overhead.
    if not length:
        length = COPY_BUFSIZE
    fsrc_read = fsrc.read
    fdst_write = fdst.write
    while True:
        buf = fsrc_read(length)
        if not buf:

So you should use that instead.

Created 2022-05-13T17:22:54-07:00 · Edit