Please consider the following Python 3.6 code:

from io import BytesIO


class NamedBytesIO(BytesIO):

    def __init__(self, content: bytes, name: str) -> None:
        super().__init__(content)
        self.name = name

mypy 0.560 complains:

test.py:8: error: Property "name" defined in "NamedBytesIO" is read-only

This is due to the fact that in typeshed BytesIO derives from BinaryIO, which defines the name property. The actual BytesIO implementation does not have such an attribute:

>>> from io import BytesIO
>>> BytesIO(b"").name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: '_io.BytesIO' object has no attribute 'name'

Interestingly, StringIO does not have this problem:

from io import StringIO


class NamedStringIO(StringIO):

    def __init__(self, content: str, name: str) -> None:
        super().__init__(content)
        self.name = name

This checks fine with mypy, since the StringIO stub includes a name field, which also does not match the implementation:

>>> from io import StringIO
>>> StringIO("").name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: '_io.StringIO' object has no attribute 'name'