post_processing_function is documented as

A function name of the replacement linear class that is called after processing.

but the lookup targets the module being replaced:

if post_processing_function is not None:
    func = getattr(module, post_processing_function, None)
    if func is not None:
        func(module)

module there is still the original torch.nn.Linear. It never carries the hook, so getattr(..., None) returns None and the block does nothing:

replace_linear(model, MyLinear, post_processing_function="post_init")
# fc replaced with MyLinear, post_init calls: []

The parameter is a no-op for every caller, and silently so — the replacement itself works, only the hook is skipped.

Two problems in those three lines: the wrong object, and func(module) passing an extra positional argument to what getattr has already bound.

The change

Look the hook up on model._modules[name] (the instance just constructed) and call it with no argument.

After:

replace_linear(model, MyLinear, post_processing_function="post_init")
# fc and block[0] replaced, post_init calls: ['4x8', '8x16']
# lm_head untouched via skip_modules

A replacement class that does not define the method still returns None from getattr and passes through, so this cannot start raising for existing callers. Nothing in the repo calls replace_linear with the hook, so the change is only visible to downstream users, for whom it goes from "silently ignored" to "runs".

Tests

New tests/test_utils.py:

• the hook fires once per replaced module, with skip_modules still honoured
• omitting the hook changes nothing
• a replacement class without the method does not raise
• copy_weights=True still carries the original weight

Reverting only bitsandbytes/utils.py:

tests/test_utils.py::test_replace_linear_runs_the_post_processing_hook FAILED
1 failed, 3 passed

and with the change, 4 passed. ruff check and ruff format --check clean.