We know from release notes, that "A backslash-character pair that is not a valid escape sequence now generates a DeprecationWarning".  Sometimes it's true:
$ python -W error
Python 3.6.0b1+ (default, Oct  4 2016, 08:47:51) 
[GCC 4.9.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> "xxx" != "hello \world"
DeprecationWarning: invalid escape sequence '\w'

But shouldn't DeprecationWarning be in the following case as well?

$ cat a.py 
def f(s):
    return s != "hello \world"
$ cat b.py 
import a
print(a.f("xxx"))
$ python b.py 
True
$ python -W error b.py 
True