I'm struggling to understand this bug. I've tried idle and plain cPython and neither exhibit it. I suspect thats due to how readline is itself tokenizing things.

Python 3.6.0a0 (default:ef5a2ba9df62, Jul 28 2015, 15:48:19) 
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> readline.parse_and_bind("tab: complete")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'readline' is not defined
>>> import readline
>>> readline.parse_and_bind("tab: complete")
>>> class A:
...  def foo(self):pass
...  def foobar(self):pass
... 
>>> A.foo
A.foo(     A.foobar(  
>>> A.foo(      

Whats probably not obvious there is that TAB at the ( indented a tab, rather than offering any completions.

I'm hesitant to apply this without a deeper understanding of where its used, in case of side effects.