Use mutagen for embedding thumbnails

[tripulse]

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Question

youtube-dl uses external binaries to embed thumbnails into MP4/MP3 files. My question is that why use FFmpeg or AtomicParsley for embedding thumbnails.

Because, there is a library named mutagen which can handle ID3 tags and MP4 metadata atoms.
And does in-place manipulation without generating a temporary file. And, the biggest perk is that it is made in native Python.

Then what is the point of using AtomicParsley for the sole purpose of embedding thumbnails into a MP4 (MPEG-4 Part-II) file?

[dstftw]

1. Because this is what implemented.
2. Inplace manipulation is a very bad idea.

[tripulse]

Explain, why inplace manipulation is a very bad idea?