Backport next()

[birkenfeld]

BPO	2719
Nosy	@gvanrossum, @birkenfeld, @rhettinger, @abalkin
Files	nextbackport.diff: #1

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields: ```python assignee = 'https://github.com/gvanrossum' closed_at = created_at = labels = ['interpreter-core', 'type-bug'] title = 'Backport next()' updated_at = user = 'https://github.com/birkenfeld' ``` bugs.python.org fields: ```python activity = actor = 'georg.brandl' assignee = 'gvanrossum' closed = True closed_date = closer = 'georg.brandl' components = ['Interpreter Core'] creation = creator = 'georg.brandl' dependencies = [] files = ['10141'] hgrepos = [] issue_num = 2719 keywords = ['patch', '26backport'] message_count = 13.0 messages = ['65980', '65992', '65993', '65995', '66000', '66004', '66005', '66006', '66007', '66008', '66009', '66010', '66017'] nosy_count = 4.0 nosy_names = ['gvanrossum', 'georg.brandl', 'rhettinger', 'belopolsky'] pr_nums = [] priority = 'critical' resolution = 'accepted' stage = None status = 'closed' superseder = None type = 'behavior' url = 'https://bugs.python.org/issue2719' versions = ['Python 2.6'] ```

[birkenfeld]

Backporting 3.0's next() builtin.

There's no change w.r.t. __next__/next, that is tracked in bpo-2336.

[rhettinger]

ISTM, the only value added by next(g) is that it replaces g.next() with a more conventional spelling, g.__next__(). Since 2.6 still has g.next (),I don't see how this backport adds value. It does however create a second way to do it that will be confusing to some remaining in the 2.x world. I think we should avoid double spellings in 2.6 except in cases where the 2-to-3 converter would need help.

[birkenfeld]

IMO having next() in 2.6 helps since if you use it consistently you don't have to care about calling .next() or .__next__().

Also, I don't see how this is different from having e.g. reduce() and functools.reduce() in 2.6.

[rhettinger]

The problem is with the "if you use it consistently" premise. That will not hold in an environment with legacy code, multiple programmers, lots of code in ASPN recipes and published materials, and third-party modules. A patch like this dooms Py2.6 programmers to seeing both of these forms intermixed throughout the code base. This is *not* a win.

[gvanrossum]

I think it's important to make this available in 2.6; it will let people writing 3.0-oriented code in 2.6 (as several developers are planning to do) do the right thing for this syntax.

[abalkin]

I thought new code is supposed to use Py_TYPE macro instead of ->ob_type:

+ "%.200s object is not an iterator", it->ob_type-

tp_name); .. + res = (*it->ob_type->tp_iternext)(it);

Py3k branch has the same issue.

[abalkin]

One more question: What is the rationale for

+ res = (*it->ob_type->tp_iternext)(it); + if (res == NULL) { .. + PyErr_SetNone(PyExc_StopIteration); + return NULL;

?

I would think tp_iternext failing to set an exception should not be translated into stop iteration. Instead, builtin_next() should return NULL without an exception set and thus trigger a SystemError.

[gvanrossum]

I would think tp_iternext failing to set an exception should not be translated into stop iteration. Instead, builtin_next() should return NULL without an exception set and thus trigger a SystemError.

Wrong; the iternext slot is designed to return NULL without setting an exception. See e.g. listiter_next().

[gvanrossum]

+1 on this. I have a few nits about the code:

Line 1083: "%.200s object is not an iterator", it->ob_type->tp_name); Line is too long.

Line 1088: if (res == NULL) { How about

if (res != NULL) return res;

?

Line 1089: if (def) {
if (def != NULL) {

Line 1093: PyErr_Clear(); I would only call this if PyErr_Occurred() returns true.

[abalkin]

On Wed, Apr 30, 2008 at 12:41 PM, Guido van Rossum \report@bugs.python.org\ wrote:

Guido van Rossum \guido@python.org\ added the comment:

> .. builtin_next() should return > NULL without an exception set and thus trigger a SystemError.

Wrong; the iternext slot is designed to return NULL without setting an exception. See e.g. listiter_next().

I did not know that. Thanks for the explanation. In this case, wouldn't it be cleaner to call PyIter_Next which is documented to return NULL with no exception?

[abalkin]

On Wed, Apr 30, 2008 at 12:41 PM, Guido van Rossum \report@bugs.python.org\ wrote:

the iternext slot is designed to return NULL without setting an exception.

This is not what the documentation says:

""" iternextfunc PyTypeObject.tp_iternext An optional pointer to a function that returns the next item in an iterator, or raises StopIteration when the iterator is exhausted. """ \http://docs.python.org/dev/c-api/typeobj.html#tp_iternext\

It looks like documentation needs to be updated, but wouldn't it be odd to specify that setting StopIteration exception is optional? It's probably more logical to intercept StopIteration in slot_tp_iternext rather than at every place where tp_iternext is called.

[gvanrossum]

Feel free to submit a patch to fix the docs.

Changing the API is not an option -- it's been like this since the tp_iternext slot was added, and it's been designed like this for a reason: so that in the common case of iterating over a built-in sequence type no exception objects have to be created. In particular the for-loop code would just discard the StopIteration instance again.

The requirement that the exception is *optional is so that if you're calling a Python iterator that *does create the exception, the exception object (with whatever data the creator might have attached to it) doesn't get lost (or worse, have to be recreated).

Calling PyIter_Next() here instead of inlining it would not be advantageous; it would just slow things down since we'd have to make a redundant call to PyErr_Occurred() to distinguish the StopIteration case from other errors.

[birkenfeld]

Updated and committed as r62599. Also fixed your nits in the original 3k version in r62598.