> It looks like the module has never set the file mode, at least since it was added to Python in 1994. I suggest just remove the dead code and fix the documentation.

You cannot just remove the mode parameter without a deprecation period. Right now, the flag is documented:

https://docs.python.org/dev/library/uu.html
"mode is used to set the permission bits if the file must be created"

I agree with Martin that I am not comfortable to fix the bug in stable branches (2.7, 3.6, 3.7).

I'm +0 to fix the uu module in master (future 3.8).

If we fix the bug in master, we should document that mode is ignored in 2.7, 3.6 and 3.7 (update their documentation).