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 bpo-791968.

If you are the same 'ariel' as on SO, thank you for the detective work and the informative report here. Without it this 2.7-only bug would be low priority and might have sat open until we stopped patching 2.7.

The underlying issue is that a) 2.x had the 'feature' of allowing tuples of parameter names to signal automatic unpacking of tuples of arguments, and b) the unnamed tuple got a pseudoname of the form '.n' in the list of argument names, which calltips uses.

>>> def f((a,b), (c,d)): pass
>>> f.func_code.co_varnames
('.0', '.1', 'a', 'b', 'c', 'd')

Issue bpo-791968 added the following line to change '.n' to '<tuple>'.
arg_text = "(%s)" % re.sub("\.\d+", "<tuple>", arg_text)
But this also changes float strings of form 'm.n' to 'm<tuple>'. The solution is to recognize the difference between the absence and presence of a preceding digit and not do the substitution in the latter case. Adding the negative lookbehind assertion (?<![0-9]) makes
def f((a,b), c=0.0):
produce (<tuple>, c=0.0). I am preparing a patch, including the test.

New changeset 3236709650b0 by Terry Jan Reedy in branch '2.7':
Issue bpo-18539: Calltips now work for float default arguments.
http://hg.python.org/cpython/rev/3236709650b0

Superb, and thanks for doing the actual work on fixing it.