Hi @favcau, thank you so much for your very detailed feedback and I'm happy with the score. So don't get me wrong and  I just like to add some comments.

I agree in general with @crokkon that "added this to the library itself (transactionbuilder.py) is not a good idea.", since that file is a sort-of base library (shared with steem-python), so it might be better to be untouched although that's why I also submitted my PR for the official library.

But I actually don't agree with "The implementation (transactionbuilder.py) can potentially break a lot of other things." :) If you can read the code, you can see that it's actually safe, and if it's unsafe, then the existing and related codes have the serious problem. Even if a wrong private key is provided somehow, the error should be handled properly, and of course I've also already checked that.

While I also agree with that it's better to change more high-level files to add this feature, I chose a more easier (but again still safe and legit) way as an external contributor. One lesson I learned from other contributions are if a PR is complicated, then it tends to be ignored :) So that's why I always prefer a very simple fix so that PO can easily know that it's working without a problem :)

But @holger80 is quite active in dev, so maybe more complicated one might have been okay :) I'll change the PR or just close it depending on his response. 

Many thanks everybody!