Emacs 24.3 python: Can't guess python-indent-offset, using defaults 4

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).

If all you want is to silence the warnings, while letting emacs still guess the offset as juanleon's answer explains, you can switch the python-indent-guess-indent-offset-verbose variable off.

(setq python-indent-guess-indent-offset t)  
(setq python-indent-guess-indent-offset-verbose nil)

Per this comprehensive answer on the emacs SE.