Unexpected PendingDeprecationWarning

[Nathan Ernst]

I'm using Python 3.5.2, and the following code (when invoked) causes a
PendingDeprecationWarning when used in a unit test:

def identity(x):
  return x

def adjacent_difference(seq, selector=identity):
  i = iter(seq)
  l = selector(next(i))
  while True:
    r = selector(next(i))
    yield r - l
    l = r

I wrote this to mimic the C++ std algorithm (defined here:
http://en.cppreference.com/w/cpp/algorithm/adjacent_difference).

What I don't understand is why I get this warning.

The exact error message I get from unittest is:
PendingDeprecationWarning: generator 'adjacent_difference' raised
StopIteration

I'd appreciate any insight into what is causing this deprecation warning,
as I am stumped.

Regards,
Nate