[Python-ideas] + operator on generators

[Serhiy Storchaka]

25.06.17 15:06, lucas via Python-ideas пише:
> I often use generators, and itertools.chain on them.
> What about providing something like the following:
> 
>      a = (n for n in range(2))
>      b = (n for n in range(2, 4))
>      tuple(a + b)  # -> 0 1 2 3
> 
> This, from user point of view, is just as how the
> __add__ operator works on lists and tuples.
> Making generators works the same way could be a great way to avoid calls
> to itertools.chain everywhere, and to limits the differences between
> generators and other "linear" collections.
> 
> I do not know exactly how to implement that (i'm not that good at C, nor
> CPython source itself), but by seeing the sources,
> i imagine that i could do something like the list_concat function at
> Objects/listobject.c:473, but in the Objects/genobject.c file,
> where instead of copying elements i'm creating and initializing a new
> chainobject as described at Modules/itertoolsmodule.c:1792.
> 
> (In pure python, the implementation would be something like `def
> __add__(self, othr): return itertools.chain(self, othr)`)

It would be weird if the addition is only supported for instances of the 
generator class, but not for other iterators. Why (n for n in range(2)) 
+ (n for n in range(2, 4)) works, but iter(range(2)) + iter(range(2, 4)) 
and iter([0, 1]) + iter((2, 3)) don't? itertools.chain() supports 
arbitrary iterators. Therefore you will need to implement the __add__ 
method for *all* iterators in the world.

However itertools.chain() accepts not just *iterators*. It works with 
*iterables*. Therefore you will need to implement the __add__ method 
also for all iterables in the world. But __add__ already is implemented 
for list and tuple, and many other sequences, and your definition 
conflicts with this.