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The long discussion in issue 7475 and some subsequent discussions I had with Armin Ronacher have made it clear to me that the key distinction between the codec systems in Python 2 and Python 3 is the following differences in type signatures of various operations:
Python 2 (8 bit str):
 codecs module: object <-> object
 convenience methods: basestring <-> basestring
 available codecs: unicode <-> str, str <-> str, unicode <-> unicode
Python 3 (Unicode str):
 codecs module: object <-> object
 convenience methods: str <-> bytes
 available codecs: str <-> bytes, bytes <-> bytes, str <-> str
The significant distinction is the fact that, in Python 2, the convenience methods covered all standard library codecs, but for Python 3, the codecs module needs to be used directly for the bytes <-> bytes codecs and the one str <-> str codec (since those codecs no longer satisfy the constraints of the text model related convenience methods).
After attempting to implement a 2to3 fixer for these non-Unicode codecs in issue 17823, I realised that wouldn't really work properly (since it's a data driven error based on the behaviour of the named codec), so I'm rejecting that proposal and replacing it with this one for additional Py3k warnings in Python 2.7.7.
My proposal is to take the following cases and make them produce warnings under Python 2.7.7 when Py3k warnings are enabled (remember, these are the 2.7 types, not the 3.x ones):
- the str.encode method is called (redirect to codecs.encode to handle arbitrary input types in a forward compatible way)
- the unicode.decode method is called (redirect to codecs.decode to handle arbitrary input types)
- PyUnicode_AsEncodedString produces something other than an 8-bit string (redirect to codecs.encode for arbitrary output types)
- PyUnicode_Decode produces something other than a unicode string (redirect to codecs.decode for arbitrary output types)
For the latter two cases, issue 17828 includes updates to the Python 3 error messages to similarly redirect to the convenience functions in the codecs module. However, the removed convenience methods will continue to simply trigger AttributeError in Python 3 with no special casing.

Just thinking the first case might get quite a few false positives. Maybe that would still be acceptable, I dunno.
> - the str.encode method is called (redirect to codecs.encode to handle arbitrary input types in a forward compatible way)
I guess you are trying to catch cases like this, which I have come across quite a few times:
data.encode("hex") # data is a byte string
But I think you would also catch cases that depend on Python 2 "str" objects automatically converting to Unicode. Here are some examples taken from real code:
file_name.encode("utf-8") # File name parameter may be str or unicode
# Code meant to be compatible with both Python 2 and 3:
"""<?xml . . . encoding="iso-8859-1"?>""".encode("iso-8859-1")
("data %s\n" % len(...)).encode("ascii")

On 10.11.2013 10:20, Nick Coghlan wrote:
> 
> The long discussion in issue 7475 and some subsequent discussions I had with Armin Ronacher have made it clear to me that the key distinction between the codec systems in Python 2 and Python 3 is the following differences in type signatures of various operations:
> 
> Python 2 (8 bit str):
> 
> codecs module: object <-> object
> convenience methods: basestring <-> basestring
> available codecs: unicode <-> str, str <-> str, unicode <-> unicode
> 
> Python 3 (Unicode str):
> 
> codecs module: object <-> object
> convenience methods: str <-> bytes
> available codecs: str <-> bytes, bytes <-> bytes, str <-> str
> 
> The significant distinction is the fact that, in Python 2, the convenience methods covered all standard library codecs, but for Python 3, the codecs module needs to be used directly for the bytes <-> bytes codecs and the one str <-> str codec (since those codecs no longer satisfy the constraints of the text model related convenience methods).
Please remember that the codec sub-system is extensible. It's
easily possible to add more codecs via registered codec
search functions.
Whatever you add as warning has to be aware of the fact that
there may be codecs in the system that are not part of the
stdlib and which can potentially implement codecs that use
other type combinations that the ones you listed above.

Martin: you're right, it wouldn't be feasible to check for the 8-bit str encoding case, since the types of string literals will implicitly change between the two versions. However, the latter three cases would be feasible to check (the unicode.decode one is particularly pernicious, since it's the culprit that can lead to UnicodeEncodeErrors on a decoding operation as Python implicitly tries to encode a Unicode string as ASCII).
MAL: The latter two Py3k warnings would be in the same place as the corresponding output type errors in Python 3 (i.e. all in unicodeobject.c), so they would never trigger for the general codecs machinery.
Python 2 actually already has output type checks in the same place as the proposed warnings, it just only checks for "basestring" rather than anything more specific. Those two warnings would just involve adding the more restrictive Py3k-style check when -3 was enabled.
A Py3k warning for unicode.decode is just a straight "this method won't be there any more in Python 3" warning, since there's no way for the conversion from Python 2 to Python 3 to implicitly replace a Unicode string with 8-bit data the way string literals switch from 8-bit data to Unicode text.

This fixes the four cases Nick referred to, although not in the functions expected:
 - str.encode no longer exists
 - unicode.decode no longer exists
 - encoders used by str.encode must return bytes (does not apply to codecs.encode)
 - decoders used by unicode.decode must return unicode (does not apply to codecs.decode)

For the warnings, it's actually bytes.decode() (et al) that are expected to return str, and str.encode() that's expected to return bytes. The codecs themselves remain free to do what they want (hence the recommendation to use codecs.encode() and codecs.decode() to invoke arbitrary codecs without the type constraints of the builtins). Aside from that, those 2 warnings and the unicode.decode() one look good.
However, the "bytes.encode()" warning is the one we determined couldn't be treated as a warning as "text".encode("ascii") is valid in both Python 2 & 3, it just does a str->str conversion in 2.x, and a str -> bytes conversion in 3.x.

I think we should just backport issue19619 and issue20404.

Here is a patch that backports issue19619 and issue20404 with changing an exception to Py3k warning, and makes necessary changes in other modules and tests.
$ ./python -3
Python 2.7.10rc0 (2.7:4234b0dd2a54+, Apr 28 2015, 16:51:51) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 'abcd'.decode('hex')
__main__:1: DeprecationWarning: 'hex' is not a text encoding; use codecs.decode() to handle arbitrary codecs
'\xab\xcd'

What would you say about this Benjamin?

Nick, Benjamin, could you please look at the patch?

Serhiy's patch looks to me like it would pragmatically cover all the cases most likely to affect porting efforts: using the standard library bytes<->bytes codecs through the convenience methods.
For a Python 2 backport, I'm slightly more concerned with exposing the argument in the constructor signature, but see value in being consistent with Python 3 if anyone decides to use this to check a custom codec.
The main advantage this approach has over the typecheck based approach is that it can correctly warn about "data".encode("hex"), while a typecheck based approach can't distinguish that from "text".encode("ascii")

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

Committed patch covers a large part of this issue, but not all.
Following patch emits py3k warning for unicode.decode(). For now unicode(u'a', 'ascii') is forbidden, but u'a'.decode('ascii') is allowed in 2.7.
The risk of false positive in this patch is lower than in emitting warning on str.encode(), but is larger than in just committed patch.

New changeset 0347f6e14ad6 by Tal Einat in branch '3.5':
Issue #19543: Implementation of isclose as per PEP 485
https://hg.python.org/cpython/rev/0347f6e14ad6 

New changeset 5c8c123943cf by Tal Einat in branch 'default':
Issue #19543: Implementation of isclose as per PEP 485
https://hg.python.org/cpython/rev/5c8c123943cf 

The last two commit notifications were intended for issue #24270.

New changeset c89a0f24d5f6 by Serhiy Storchaka in branch '2.7':
Issue #19543: Added Py3k warning for decoding unicode.
https://hg.python.org/cpython/rev/c89a0f24d5f6 

Is there something left to do with this issue?

I think so - if anyone spots another place a Py3k warning could be usefully emitted, it can be handled as a new issue.