The way iterators work in Python, there is the following (potentially surprising) behavior: >>> l = [1, 2, 3] >>> g = (x for x in l) >>> list(g) [1, 2, 3] >>> list(g) [] That is, iterators are “used up” during the iteration and will not return items again which have already been iterated over. Un…

[image] lmtzx9h4qqnt: But is this really always the case? I think there are a few exceptions for cases that involve iterating over streaming data. e.g. the eachline iterator is documented as closing the file at the end of iteration, so it is isempty if you try to iterate a second time.

Also iterating over a Channel will take from it, but that is probably expected.

Also see Iterators.Stateful , which turns any iterator into a “stateful” iterator (i.e. an iterator which behaves more like a Channel or a Python iterator). For example: julia> l = [1, 2, 3] 3-element Vector{Int64}: 1 2 3 a generator is stateless (as is the vector l itself) julia> g = (x for x…

Indeed, “proof by counter-example” :smiley: julia> s = "a b c d" "a\nb\nc\nd" julia> el = eachline(IOBuffer(s)) Base.EachLine{IOBuffer}(IOBuffer(data=UInt8[...], readable=true, writable=false, seekable=true, append=false, size=7, maxsize=Inf, ptr=1, mark=-1), Base.var"#408#411…

[image] lmtzx9h4qqnt: focus more on how to define new iterators, not on what the guarantees on the behavior should be There are no guarantees, because iterators can be inifinite and/or stateful, and the only way to find out is read the docs/source or experiment. The good news is that (as far…

Well, it’s definitely possible to document whatever guarantees there should be as part of the interface specification. The way the documentation is structured, it just didn’t seem clear to me one way or the other. Infinite iterators don’t cause problems here in principle – since their values obviou…

[image] lmtzx9h4qqnt: Well, it’s definitely possible to document whatever guarantees there should be as part of the interface specification. There are indeed proposals to formalize the guarantees that iterators provide via traits. See for example this issue and the links therein: …

A relevant previous discussion: [image] Iterators.Stateful is not an iterator? New to Julia I’m trying to understand iterators better because I’m trying to decide whether I should, for my specific application, define iterate for my type or define a constructor of Iterators…

Is it always safe to “re-use” an iterator/generator?

General Usage

lmtzx9h4qqnt March 15, 2023, 2:21am 10

A relevant previous discussion:

Topic		Replies	Views
Iterators.Stateful is not an iterator? New to Julia	8	1919	March 1, 2022
From generator (with Channel) to iterator General Usage	6	2659	April 24, 2018
Yield in iterator General Usage	5	1383	November 28, 2023
Does length(itr) need to be invariant? (stateful iterator) General Usage question , iterators	4	217	September 24, 2024
`isempty(enumerate(Iterators.Stateful(...)))` advances the stateful iterator? General Usage	3	483	November 16, 2021

Is it always safe to “re-use” an iterator/generator?

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