When trying to add type annotations to a project (https://github.com/DCSO/flor), mypy would complain that bytes was not a valid type, which was quite puzzling to say the least. After some head scratching and searching in the issue log I was able to reproduce the problem with a simple module (Python 3.5.2+ with mypy 0.511):

class Foo(object):

    def __init__(self) -> None:
        self.bytes = bytearray(b"foo")

    def bar(self, f : bytes):
        return f + b"test"

foo = Foo()

foo.bar(b"test")

Running mypy example.py returns the following error messages:

example.py:6: error: Invalid type "bytes"
example.py:7: error: Unsupported left operand type for + (object)

When renaming self.bytes to self._bytes, no issues are reported. It seems that mypy mistakes self.bytes for a class variable, although it is an instance variable and not visible at the declaration time of function bar.