[Twisted-Python] Private positional argument API anti-pattern

[exarkun at twistedmatrix.com]

Hello,

I've noticed there are a couple public APIs in Twisted now that accept a 
"private" positional argument.  I use scare quotes because I don't see 
how it's actually possible to make a positional argument private.

Given a function defined like this:

    def foo(_bar=None):
        ...

There is no indication that a use like this is problematic:

    foo(3)

Perhaps the situation will be different with keyword-only arguments, but 
for positional arguments I think it's unproductive and perhaps even 
confusing to name arguments this way.

I suggest we fix the couple places where this has been done so far and 
we avoid introducing any new instances.

FWIW, I found two examples of this in Twisted using:

    $ grep -E '\W_reactor' twisted/ -r --include '*.py' | grep -E 'def\W'

Jean-Paul