I have not tried io.open(), nor would I suspect most users would realize that they needed to do so, in order to get the canonical behaviour from an operation called "write" on a file opened in "append" mode.

IMO: If pythons file.write() does not give the guarantee POLA would indicate, it's either a bug or a doc-issue, no matter how many workarounds might exist.

But I have neither a clue to the aspirational goals of python, nor to what it might take to fix this, so it's entirely your call.