It may make sense to implement the extractors that can be implemented using python bindings only or purely in python that way

[KOLANICH]

It would avoid a possible security issue with calling a subprocess and also may allow progress reporting (i.e. https://github.com/prebuilder/fetchers.py/blob/master/fetchers/unpackers/archives/tar.py#L24).

[pombredanne]

@KOLANICH Thank you for the report. We only use 7zip as a subprocess and there is no easy way to use this as a library AFAIK. In general we are using either the Python stdlib, pure python code or a libarchive Ctypes binding AND 7zip.
There are archives that can be extracted reliably only with 7zip so I am not really inclined to drop it as at the scale this tool is being used it would be a problem (there are billions of archives that have been extracted with this code)

That said, we could have a degraded mode where 7zip and/or libarchive are optional and not always installed