The problem actually has to do with trying to use update_wrapper on a
method instead of a function - bound and unbound methods don't have all
the same attributes that actual functions do.

>>> import functools
>>> functools.WRAPPER_ASSIGNMENTS
('__module__', '__name__', '__doc__')
>>> functools.WRAPPER_UPDATES
('__dict__',)
>>> def f(): pass
...
>>> class C:
...   def m(): pass
...
>>> set(dir(f)) - set(dir(C.m))
set(['func_closure', 'func_dict', '__module__', 'func_name',
'func_defaults', '__dict__', '__name__', 'func_code', 'func_doc',
'func_globals'])

Is an exception the right response when encountering a missing
attribute? Given that the error can be explicitly silenced by writing
"functools.update_wrapper(g, str.split,
set(functools.WRAPPER_ASSIGNMENTS) & set(dir(str.split)))", I'm inclined
to think the current behaviour is the correct option.

Since this is an issue that doesn't come up with the main intended use
case for update_wrapper (writing decorators), and since it can be
handled easily by limiting the set of attributes copied to those the
object actually has, I'm converting this tracker item to an enhancement
request asking for a shorter way of spelling "ignore missing attributes"
(e.g. a keyword-only flag).