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In the documentation it is written that PyUnicode_FromObject() is a shortcut for PyUnicode_FromEncodedObject(). But PyUnicode_FromObject() is not call PyUnicode_FromEncodedObject() direct nor indirect. PyUnicode_FromObject() works only with unicode and unicode subclass objects, PyUnicode_FromEncodedObject() is not works with unicode objects.

This is a bug of 2 -> 3 transition.

I made a change to the documentation to reflect PyUnicode_FromObject()'s change in implementation details. Let me know if the wording is off or more information is needed. Thanks!

In the "Otherwise it coerces" sentence, obj should probably be ``obj``.

So (speaking from C API ignorance here), if you pass it a unicode subclass you get back an instance of the base unicode type? Is that what coercion means here?

Thanks for the quick responses.
Brian: Nice catch, I'll add the ``obj`` shortly.
R. David: You're correct, that's exactly what happens, and what coercion means here. The language is almost the same as PyUnicode_FromEncodedObject()'s documentation, but if it's unclear I don't mind changing both. The code's comment offers another way of describing the "type modification":
/* For a Unicode subtype that's not a Unicode object, return a true Unicode object with the same data. */

Perhaps we should correct a documentation for PyUnicode_FromEncodedObject() too. Coercing doesn't look as right term for decoding.

I've uploaded a new patch with the obj argument properly marked up. Brian, I think *obj* is what we want based on other examples in that file. It looks like italics is typically used when discussing parameters since each parameter is italicized in the signature.
R.David and Serhiy, I thought about the use of "coercion" some more and I think the current wording is fine. "Type coercion/conversion" is a commonly used phrase with some languages, so I think the phrase is applicable here as well (for a python example: http://docs.python.org/release/2.5.2/ref/coercion-rules.html). Let me know if you'd still like to see it changed. Thanks.

Well, while 'coercion' does refer to changing from one type to another, and technically we are doing that here, in OO we generally think of subclasses as more-or-less being of the same type as the superclass. So I think it would be clearer to spell out that we are changing the object type to be that of the superclass.

In other languages usually the world "coercion" is used for implicit conversion, i.e. int->long, int->float, float->complex. str->unicode in Python 2 (that's what PyUnicode_FromObject() does). But the last conversion is not supported in Python 3. The term "coercion" has also been used in Python 2 in the narrow sense (see the __coerce__() method), and in this sense Python 3 does not support "coercion". Therefore, I believe that it is better to avoid the use of this term.

Here is a modified patch that avoids "coercion" and is hopefully more explicit. I also fixed the comment in Include/unicodeobject.h.

Added a comment on Rietveld.

Here is a new version where I use the phrase "true Unicode object".

LGTM. Thank you Martin for this improvement.

from_object_v4.patch LGTM, nice enhancement.

New changeset af655e73f7bd by Martin Panter in branch '3.5':
Issue #15984: Correct PyUnicode_FromObject() and _FromEncodedObject() docs
https://hg.python.org/cpython/rev/af655e73f7bd
New changeset 570ada02d0f0 by Martin Panter in branch 'default':
Issue #15984: Merge PyUnicode doc from 3.5
https://hg.python.org/cpython/rev/570ada02d0f0 

I also tweaked the PyUnicode_FromEncodedObject() documentation to avoid the word "coerce" and to fix up outdated stuff.