cfbwrite

Write a complete MS-CFB v3 (Microsoft Compound File Binary) container from Python — including streams that changed size.

olefile reads Compound Files well and can replace a stream only with data of exactly the same length. That is enough to rearrange existing content and not enough to add any, so anything that grows a stream needs the container rebuilt. This library rebuilds it.

from cfbwrite import rebuild

rebuild(src="original.rdb", dst="updated.rdb", replacements={
    ("Relays", "QPC1_TR1", "SET_D1.TXT"): new_bytes,   # any size
})

Two properties it is built around, because it came from a tool that writes protection-relay settings files:

• It verifies its own output. The result is reopened and every stream compared against the source before it is handed over. A bug in the writer surfaces as a failed write, never as a silently corrupt file.
• It writes atomically. The container is built in a temporary file beside the destination and only then os.replaced into place, so a failure leaves the destination exactly as it was.

Extracted from PAC CT.

Status: scaffold. Code lands here per docs/MIGRATION.md §4.1 of PAC CT.

Licence

AGPL-3.0-or-later — see LICENSE.

Install

pip install cfbwrite

Requires Python 3.10+ and olefile.

Why it exists

It was pacct/parsers/ole_rebuild.py inside PAC CT, a commissioning toolkit for protective relays. Those relays keep their settings in .rdb files, which are Compound Files, and editing a DNP3 point map there routinely grows a stream. Nothing in the module knows any of that — it is the container format and nothing else — so it lives here instead.