[Python-Dev] generator vs iterator etc. (was: How assignment should work with generators?) (original) (raw)

Koos Zevenhoven k7hoven at gmail.com
Mon Nov 27 11:35:38 EST 2017

On Mon, Nov 27, 2017 at 3:55 PM, Steven D'Aprano <steve at pearwood.info> wrote:

On Mon, Nov 27, 2017 at 12:17:31PM +0300, Kirill Balunov wrote: ​​

> 2. Should this work only for generators or for any iterators? I don't understand why you are even considering singling out only generators. A generator is a particular implementation of an iterator. I can write: def gen(): yield 1; yield 2; yield 3 it = gen() or I can write: it = iter([1, 2, 3]) and the behaviour of it should be identical.

​I can see where this is coming from. The thing is that "iterator" and "generator" are mostly synonymous, except two things:

(1) Generators are iterators that are produced by a generator function

(2) Generator functions are sometimes referred to as just "generators"

The concept of "generator" thus overlaps with both "iterator" and "generator function".

Then there's also "iterator" and "iterable", which are two different things:

(3) If obj is an iterable, then it = iter(obj) is an iterator (over the contents of obj)

( ​4) ​Iterators yield values, for example on explicit calls to next(it).

Personally I have leaned towards keeping a clear distinction between "generator function" and "generator"​, which leads to the situation that "generator" and "iterator" are mostly synonymous for me. Sometimes, for convenience, I use the term "generator" to refer to "iterators" more generally. This further seems to have a minor benefit that "generators" and "iterables" are less easily confused with each other than "iterators" and "iterables".

I thought about this issue some time ago for the views package, which has a separation between sequences (seq) and other iterables (gen):

https://github.com/k7hoven/views

The functionality provided by views.gen is not that interesting—it's essentially a subset of itertools functionality, but with an API that parallels views.seq which works with sequences (iterable, sliceable, chainable, etc.). I used the name gen, because iterator/iterable variants of the functionality can be implemented with generator functions (although also with other kinds of iterators/iterables). Calling the thing iter would have conflicted with the builtin iter.

HOWEVER, this naming can be confusing for those that lean more towards using "generator" to also mean "generator function", and for those that are comfortable with the term "iterator" despite its resemblance to "iterable".

Now I'm actually seriously considering to consider renaming views.gen to views.iter when I have time. After all, there's already views.range which "conflicts" with the builtin range.

​Anyway, the point is that the naming is suboptimal.​

SOLUTION: Maybe (a) all iterators should be called iterators or (b) all iterators should be called generators, regardless of whether they are somehow a result of a generator function having been called in the past.

(I'm not going into the distinction between things that can receive values via send or any other possible distinctions between different types of iterators and iterables.)

​—Koos​

​(discussion originated from python-ideas, but cross-posted to python-dev in case there's more interest there)​

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