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json.dumps() documentation is slightly incorrect.
http://docs.python.org/library/json.html#json.dumps
Reads:
 "If ensure_ascii is False, then the return value will be a unicode instance."
Should read:
 "If ensure_ascii is False, then the return value MAY BE be a unicode instance."
(Without the caps of course)
bash: python
Python 2.7.1 (r271:86832, Jun 16 2011, 16:59:05) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 23351500)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> type(json.dumps({'a': 1}, ensure_ascii=False))
<type 'str'>
Tested against 2.6 and 2.7.

Entry for dump says
"If ensure_ascii is False (default: True), then some chunks written to fp may be unicode instances,"
Entry for dumps says
"If ensure_ascii is False, then the return value will be a unicode instance."
Entry for JSONEncoder says
"If ensure_ascii is True (the default), the output is guaranteed to be str objects with all incoming unicode characters escaped. If ensure_ascii is False, the output will be a unicode object."
I suspect the latter two meant to say something like
"If ensure_ascii is False and any chunk would be unicode, then the whole output is unicode."
And should 'all incoming unicode characters' either be or include 'all non-ascii chars (as in 3.x)?
---
In 3.x, ensure_ascii apparently has a different meaning.
"If ensure_ascii is True (the default), the output is guaranteed to have all incoming non-ASCII characters escaped. If ensure_ascii is False, these characters will be output as-is."
Unlike other params used in multiple json functions, ensure_ascii is only defined once, under json.JSONEncoder and not under dump and dumps. Should the JSONEncoder entry to copied to dump and dumps?

See #13770 and #13769.

This is a documentation bug for 2.7. Setting as duplicate of #13769.

Fixed, see #13769.