The typing.AnyStr documentation:

https://docs.python.org/3/library/typing.html#typing.AnyStr

It gives some examples using u-strings (u'foo') but doesn't make explicit some subtleties about behaviour with Python 2. Specifically, with Python 2 all the given examples work, and even this works:

concat("foo", u"bar")

Which seems contrary to the goal of AnyStr being "used for functions that may accept any kind of string without allowing different kinds of strings to mix".

I think the documentation should call out that for Python 2, AnyStr doesn't distinguish between str & unicode, and mention that in python 2, b'str' is equivalent to 'str' (I know this is mentioned elsewhere, but it seems useful to repeat it here).