When defining a function with factional default arguments, the <tuple> text replaces the correct values in the tooltip.

Here's an example -

>>> def f(a=0.5):
        pass
>>> f(

tooltip shows - "(a=0<tuple>)", i.e. replaces the ".5" with <tuple>.

This was found to happen on IDLE with python 2.7.3 and 2.7.5 (the latter was on 64 bit python installation on a windows 7 machine).

The problem was discussed here - 
http://stackoverflow.com/questions/17053492/pythons-idle-behavior-while-defining-fractional-default-values-to-function-para
and the problem was found to stem from the CallTips.py file (exact code lines in the link).

I believe this is the result of a fix to issue791968.