In Python 3.2 and earlier:

>>> def f(): pass
...
>>> f.__name__ = 'g'
>>> f
<function g at 0x100487628>

However in Python 3.3 and later, the last line shows 'f' instead of 'g'.

I guess this is a direct consequence of PEP 3155 [1].  From the PEP:

"""
The repr() and str() of functions and classes is modified to use __qualname__ rather than __name__.
"""

[1] http://www.python.org/dev/peps/pep-3155/

Python 3.4.0a0 (default:62658d9d8926+, Aug  1 2013, 23:05:18) 
[GCC 4.4.1 20090725 (Red Hat 4.4.1-2)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def func(): pass
... 
>>> func
<function func at 0xb7698a54>

>>> func.__qualname__="PEP 3155"
>>> func
<function PEP 3155 at 0xb7698a54>

Yup, here are the relevant lines of the diff for PEP 3155:

@@ -568,7 +607,7 @@
 func_repr(PyFunctionObject *op)
 {
     return PyUnicode_FromFormat("<function %U at %p>",
-                               op->func_name, op);
+                               op->func_qualname, op);
 }

Sorry. The actual problem reported was that the traceback uses the wrong name. I can still reproduce this in 3.4; attached is a repro.

That's because argument error messages use the code's co_name rather than the functions. (This is a dupe of another bug about this that I can't find.)

http://bugs.python.org/issue4322 ?

Exactly