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> On encoding the utf-8-sig codec will write 0xef, 0xbb, 0xbf as the first three bytes to the file

https://docs.python.org/3/library/codecs.html

The codec you're imagining would also make reading a file and writing it back change the file if it contains a BOM.

Indeed it would, but since codecs are only used for files that are semantically text, and in such files BOM is basically a legacy no-op marker, it's not actually a problem. Naive code using text I/O APIs would also have this issue with line endings, for example, so there's precedent for not providing the perfect roundtrip experience (that's what bytes I/O is for).

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