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http://docs.python.org/2/library/itertools.html#itertools.chain.from_iterable
>>> class A:
... @classmethod
... def from_iterable(iterables):
... for it in iterables:
... for element in it:
... yield element
... 
>>> A.from_iterable(['ABC', 'DEF'])
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: from_iterable() takes 1 positional argument but 2 were given
>>>

http://docs.python.org/3/library/itertools.html#itertools.chain.from_iterable 

It is implemented as a classmethod, but the "equivalent" code doesn't need to be part of the class all. I'm not sure what should be done here (say @staticmethod? Leave the decorator off?). We should probably see what Raymond thinks. I lean toward the latter, that's the way it is in the python2 docs, and it doesn't seem to have caused any confusion.

The 2.7 doc says 'Roughly equivalent to' rather than 'Equivalent to'.
The undecorated Python version of from_iterable actually works as an attribute of the Python version of chain: chain.from_iterable = from_iterable. I would just remove the decorator. The entire itertools doc exploits that fact that generator functions are analogous to classes, but it does not work to mix the two in the way that the 3.x chain entry does.

Perhaps it should be staticmethod, not classmethod.

It should be a classmethod.
>>> import itertools
>>> class C(itertools.chain): pass
... 
>>> type(C.from_iterable(['ab', 'cd']))
<class '__main__.C'>
The patch LGTM.

> I would just remove the decorator.
+1

>>> import itertools
>>> 
>>> class A(itertools.chain):
... def from_iter(arg):
... return A(iter(arg))
... 
>>> class B(A):
... pass
... 
>>> B('a', 'b')
<__main__.B object at 0x7f40116d7730>
>>> B.from_iter(['a', 'b'])
<__main__.A object at 0x7f40116d7780>
>>>
it should be B

changed iter(arg) to *arg
>>> import itertools
>>> 
>>> class A(itertools.chain):
... @classmethod
... def from_iter(cls, arg):
... return cls(*arg)
... 
>>> class B(A):
... pass
... 
>>> B('ab', 'cd')
<__main__.B object at 0x7fc280e93cd0>
>>> b = B.from_iter(['ab', 'cd'])
>>> b
<__main__.B object at 0x7fc280e93d20>
>>> next(b)
'a'
>>> next(b)
'b'
>>> next(b)
'c'
>>> next(b)
'd'
>>> next(b)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration
>>>

My counter proposal #18752 is that chain.from_iterable become a deprecated alias for a new function, chain_iterable. With '@classmethod' removed, the current Python equivalent would work for chain_iterable.

New changeset 29fa1f418796 by Raymond Hettinger in branch '3.3':
Issue 18301: The classmethod decorator didn't fit well with the rough-equivalent example code.
http://hg.python.org/cpython/rev/29fa1f418796