Added is_iterator method and fixes sage to use it.

[nthiery]

The following mantra occurs at three places in Sage's code to test whether v is an iterator:

     if hasattr(v, 'next'):

This is not quite correct since some sage objects have a next method without being iterable, or with a different semantic.

Let me quote python's doc:

The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:

iterator.iter()::

 Return the iterator object itself. This is required to allow both containers and iterators to be used with the for and in statements. This method corresponds to the tp_iter slot of the type structure for Python objects in the Python/C API.

iterator.next()::

Return the next item from the container. If there are no further items, raise the StopIteration exception. This method corresponds to the tp_iternext slot of the type structure for Python objects in the Python/C API.

Therefore here is the good way to test if an element is an iterator:

    try:
        return it is iter(it)
    except:
        return False

Note: it is not sufficient to check for the existence of the methods since some sage object implement __iter__ to raise a NotImplemented exception !

Florent

CC: @sagetrac-sage-combinat @williamstein

Component: misc

Keywords: iterators, itertools

Author: Florent Hivert

Reviewer: Nicolas M. Thiéry

Merged: sage-4.2.1.rc0

Issue created by migration from https://trac.sagemath.org/ticket/7398

[hivert]

comment:2

Attachment: trac_7398_iter_detect-fh.patch.gz

[nthiery]

Attachment: trac_7398_iter_detect-fh.2.patch.gz

Updated patch after review by Nicolas (uses itertools to simplify further sage.server.interact.list_of_first_n)

[nthiery]

comment:4

William: this makes a small edit to sage.server.notebook.interact.list_of_first_n

You may want to check/backport to the notebook code

[nthiery]

Changed keywords from iterators to iterators, itertools

[hivert]

comment:5

The given patch actually breaks somme code... I'm uploading a new one.

[hivert]

comment:6

Attachment: trac_7398_iter_detect-fh.3.patch.gz

Nicolas : can you re-review this patch...
I commented out some line saying that it was never user and actually is was quite a lot... I don't understand why we didn't caught it by the tests. Anyway, Massena did.

Sorry for the mess,

Florent

[hivert]

Attachment: trac_7398_combclass_set_compat-fh.patch.gz

[hivert]

comment:7

The patch trac_7398_iter_detect-fh.3.patch broke something in graph_generators.
the patch trac_7398_combclass_set_compat-fh.patch should fix it.

Apply those two patches in that order.

Cheers,

Florent

[mwhansen]

Merged: sage-4.2.1.rc0