TextIOWrapper doesn't check the codec type early the way the convenience methods now do, so the open() method currently still suffers from the problems Victor described in bpo-19619 for str.encode() and bytes.decode():

>>> with open("hex.txt", 'w') as f:
...     f.write("aabbccddeeff")
... 
12
>>> print(open('hex.txt').read())
aabbccddeeff
>>> print(open('hex.txt', encoding='hex').read())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: decoder should return a string result, not 'bytes'

These codecs are never going to work correctly with TextIOWrapper, so we should add the appropriate compatibility check in the constructor.

I also created bpo-20405 as an RFE for 3.5, since it seems there is a possible gap in capability relative to Python 2 here.

Attached patch checks the requested encoding is a valid text encoding in TextIOWrapper.__init__.

Two additional test changes were needed:

• the one that checks handling of non-text-encodings at runtime now tweaks quopri to lie about being a text encoding when creating the stream

• the one that checks the interpreter doesn't crash at shutdown needed to be adjusted to handle the fact that _pyio now also fails in that situation, but with a different error (it can't find the ascii codec because the codec machinery is mostly gone)

Currently, this adds a third lookup of the encoding name to the process of creating a TextIOWrapper instance. This could be reduced to just one by changing the retrieval of the encoder and decoder to look in the retrieved codec info tuple, rather than doing the lookup by name again.

Revised patch that avoids doing multiple lookups of the same codec name while creating the stream.

Absent any comments, I'll commit this version with appropriate NEWS and What's New updates tomorrow.

Ah, just noticed the test case is still using the overly specific check for the exception wording. I'll fix that, too.

The _PyCodec_GetIncrementalDecoder name looks too similar to PyCodec_IncrementalDecoder. It would be better to use more different name.

And please note my minor comments on Rietveld.

v3:

• prefix for internal helper APIs is now _PyCodecInfo_ to better distinguish them from the ones that take an encoding name
• error check is now just for "is not a text encoding"
• tweaked the name and comment in the error test to be clear that it is codecs that are not marked as text encodings that are rejected, not just binary transforms
• fixed indentation of multi-line expression in _pyio.py

LGTM.

New changeset f3ec00d2b75e by Nick Coghlan in branch 'default':
Close bpo-20404: blacklist non-text encodings in io.TextIOWrapper
http://hg.python.org/cpython/rev/f3ec00d2b75e

Here is backported to 3.3 patch.

New changeset 140a69d950eb by Georg Brandl in branch '3.3':
Issue bpo-20404: reject non-text encodings early in TextIOWrapper.
http://hg.python.org/cpython/rev/140a69d950eb

New changeset cf6e782a7f94 by Serhiy Storchaka in branch '2.7':
Issue bpo-19543: Emit deprecation warning for known non-text encodings.
https://hg.python.org/cpython/rev/cf6e782a7f94