When you open a python file, emacs guesses the indentation offset (number of spaces to indent) based on that file style.
When you create a file (the case you describe), emacs cannot guess (file is empty) so it uses your default (4) and notifies the user.
In other words: tt is a harmless warning; if you find this is a bug please report it as such.
If you don't like emacs guessing the offset, customize the variable python-indent-guess-indent-offset to nil,
and then emacs will use always your default (very unsafe in python,
where indentation has meaning and you could be editing a file created by somebody else with other defaults).
