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Change idlelib.CallTips.get_argspec to use inspect.signature, new in 3.3, instead of inspect.getfullargspec and inspect.formatargspec. One thing it handles better is a namedtuple class, which has a C-coded __init__ inherited from object a Python-coded __new__ written by namedtuple. Signature() will also handle C-coded functions if and when Argument Clinic (new in 3.4) adds signature information.
from collections import namedtuple
from inspect import signature
Point = namedtuple('Point', 'x y')
print(signature(Point.__new__))
# '(_cls, x, y)'
print(signature(Point))
# '(x, y)'
Note that str (called by print) is needed to get the desired string form.
There are tests in CallTips.py, but they should be converted to unittest, moved to idle_test/test_calltips.py, changed to match new output detail, and expanded to include new examples.