This happens in IDLE on Windows 2000, from 2.3 EXE 
installer.  Type the following:

>>> def f((a,b), c):
	print a, b, c

>>> f(

The tooltip shows up containing the following exact 
string (but the triple quotes):
"""(.0, c)"""

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Quite fast, Neal! :)
It seems it's not exactly an IDLE bug; check ToolTip.py, line 
134 (vanilla 2.3); it's the f.func_code.co_varnames that 
returns this tuple:
('.0', 'c', 'a', 'b')

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I tried this:

def f((a,b), c, (d,e)): pass

and f.func_code.co_varnames is

('.0', 'c', '.4', 'a', 'b', 'd', 'e')

That means .0 and .4 are dummy placeholders for the 
argument tuples.  I couldn't find a direct way to know the 
length of each tuple, though --unless one analyzes the first 
UNPACK_SEQUENCE bytecodes of fob.func_code.co_code, 
and then uses the tuple fob.func_code.co_varnames
[fob.func_code.co_argcount:] to recreate the tuples; should 
we get into this trouble, or just do a regular expression 
replace a la:

argText = "(%s)" % re.sub("\.\d+", "<tuple>", argText)

at line 144?

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Problem still exists in 2.5 as of Rev 50739.

Used your idea of displaying "<tuple>'

Rev 53641

It seems like the solution has caused a different issue: now, 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>.

Ariel, if you think there is a problem, please open a new issue with all the pertinent information.  This issue was closed long ago.

I will, sorry. I thought it's possible reopen an issue. Since all the relevant information is already contained here, and since the problem is with the fix to the issue, I figured it's best to reopen this one.

If someone pushes a patch and closes the issue and the buildbots break, reopening the issue is the appropriate response. Too much later and the pool of potentially interested people will have changed. Anyway, the new issue is #18539.