list(ZZ) --> infinite loop

[nathanncohen]

For some reason, list(something) breaks if __len__ raises a NotImplementedError, but a TypeError is ignored.

Python....

As len(NN) already raises a NotImplementedError, I just copied the behaviour.

Nathann

CC: @videlec

Component: basic arithmetic

Author: Nathann Cohen

Branch/Commit: 47cec14

Reviewer: Jean-Pierre Flori

Issue created by migration from https://trac.sagemath.org/ticket/18378

[nathanncohen]

Branch: public/18378

[sagetrac-git]

Commit: 9ba72b3

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

9ba72b3

trac #18378: list(ZZ) --> infinite loop

[nbruin]

comment:4

For posterity, the reason that TypeError gets caught is to make this work:

sage: l=(i for i in range(10))
sage: len(l)
TypeError: object of type 'generator' has no len()
sage: list(l)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

It may well be an accident that list checks len of its argument before it checks if it's iterable, so I would not consider changing the error type a reliable way to prevent an infinite loop. I don't think python officially supports a way of preventing list(<infinite iterable>) from going in an infinite loop. It works, though...

[nathanncohen]

comment:5

It may well be an accident that list checks len of its argument before it checks if it's iterable, so I would not consider changing the error type a reliable way to prevent an infinite loop. I don't think python officially supports a way of preventing list(<infinite iterable>) from going in an infinite loop.

Oh I see. Thanks ;-)

[sagetrac-git]

Changed commit from 9ba72b3 to 47cec14

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

47cec14

trac #18378: list(ZZ) --> infinite loop

[jpflori]

Reviewer: Jean-Pierre Flori

[jpflori]

comment:7

Looks good enough to me, especially if NN already behaves like this.

[nathanncohen]

comment:8

Thanks !

Nathann

[vbraun]

Changed branch from public/18378 to 47cec14