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Editorial Team
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Asked: 2026-05-14T16:49:55+00:00

I have a Python script that reads a file (typically from optical media) marking

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I have a Python script that reads a file (typically from optical media) marking the unreadable sectors, to allow a re-attempt to read said unreadable sectors on a different optical reader.

I discovered that my script does not work with block devices (e.g. /dev/sr0), in order to create a copy of the contained ISO9660/UDF filesystem, because os.stat().st_size is zero. The algorithm currently needs to know the filesize in advance; I can change that, but the issue (of knowing the block device size) remains, and it’s not answered here, so I open this question.

I am aware of the following two related SO questions:

Therefore, I’m asking: in Python, how can I get the file size of a block device file?

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1 Answer

  1. Editorial Team

    The “most clean” (i.e. not dependent on external volumes and most reusable) Python solution I’ve reached, is to open the device file and seek at the end, returning the file offset:

    def get_file_size(filename):
    "Get the file size by seeking at end"
    fd= os.open(filename, os.O_RDONLY)
    try:
        return os.lseek(fd, 0, os.SEEK_END)
    finally:
        os.close(fd)
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