I see the following kind of exceptions when running test_memoryio:

Exception ignored in: <function IOBase.__del__ at 0x7f7cd44f49b0>
Traceback (most recent call last):
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 409, in __del__
    self.close()
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2152, in close
    if self.buffer is not None and not self.closed:
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2093, in buffer
    return self._buffer
AttributeError: 'StringIO' object has no attribute '_buffer'
Exception ignored in: <function IOBase.__del__ at 0x7f7cd44f49b0>
Traceback (most recent call last):
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 409, in __del__
    self.close()
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2152, in close
    if self.buffer is not None and not self.closed:
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2093, in buffer
    return self._buffer
AttributeError: 'StringIO' object has no attribute '_buffer'
ok

It seems this could be related to issue18748.

Multiple tests log an "Exception ignored", but all of them come from the Python implementation.

The problem is that _pyio.BytesIO and _pyio.TextIOWrapper initialize their self._buffer and self._seekable attribute "later" in the constructor, but then expect these attribute to exist in __del__().

Example:

>>> import _pyio; _pyio.BytesIO(b'data', foo=b'fat')

Exception ignored in: <function IOBase.__del__ at 0x7f1874c5c550>
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/_pyio.py", line 409, in __del__
    self.close()
  File "/home/vstinner/prog/python/master/Lib/_pyio.py", line 903, in close
    self._buffer.clear()
AttributeError: 'BytesIO' object has no attribute '_buffer'

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() got an unexpected keyword argument 'foo'

BytesIO.__init__() is not even called.

An easy fix would be to do nothing in __del__() if hasattr(self, '_buffer') if false.

Or to add `_buffer = None` at the class level.

Antoine:
> Or to add `_buffer = None` at the class level.

Ok, I wrote bpo-13601 to implement this idea and fix test_memoryio warnings.

New changeset a3568417c49f36860393075b21c93996a5f6799b by Victor Stinner in branch 'master':
bpo-37054, _pyio: Fix BytesIO and TextIOWrapper __del__() (GH-13601)
https://github.com/python/cpython/commit/a3568417c49f36860393075b21c93996a5f6799b

New changeset 0f352d44e7c14c1c93e3999402c85512b9d5a6ca by Miss Islington (bot) in branch '3.7':
bpo-37054, _pyio: Fix BytesIO and TextIOWrapper __del__() (GH-13601)
https://github.com/python/cpython/commit/0f352d44e7c14c1c93e3999402c85512b9d5a6ca

Thanks for the report Antoine. It is now fixed in 3.7 and master branches.