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).

The ``typing`` module doesn't exist in Python 2.7. 

All code samples provided in the docs *work* since no type-checking is performed by Python. That is, no enforcing of the types provided is made, that's for 3rd party packages to supply.

Unless of course you mean pip installing typing for Py2 and then using ``# type``  comments to provide the types. 

Even in that case, I don't really think the documentation for Python 3.5 should be mentioning types in 2.7, that'd get confusing.

I agree that it would just be confusing if the Python 3 docs were to explain Python 2 specific things.  Maybe this should be explained in the mypy docs (or maybe it's really a mypy bug).  Can you open an issue there?